Πειραιάς 2014

Σημειώσεις του σεμιναρίου με τίτλο:

Τροπικοί ΚυκλώνεςΑνάλυση – Πρόγνωση - Αποφυγή

Δρ. Νίκος ΜαζαράκηςΦυσικός - Μετεωρολόγος

Τροπικοί ΚυκλώνεςΑνάλυση – Πρόγνωση - Αποφυγή

Δρ. Νίκος ΜαζαράκηςΦυσικός - Μετεωρολόγος

ΠεριεχόμεναΕισαγωγή………………………………………………………………………………...........…7Κεφάλαιο 1ο

Τροπικοί κυκλώνες …………………………………………………………………….………9Τροπικοί κυκλώνες – γενικά, διαβαθμίσεις τ.κ………………………………...11Ονοματολογία τ.κ., Κλίμακα ανέμου Saffir – Simpson……….……………..12Δημιουργία ενός τ.κ………………………………………………………………………….13Εμπειρικοί κανόνες………………………………………………………………………..…14Εξασθένηση ενός τ.κ., Δομή ενός τ.κ…………………………………………….....15Το μάτι του τ.κ., το τοίχος του ματιού του τ.κ …………………………………..17Οι σπειροειδής ζώνες βροχόπτωσης του τ.κ……………………………………..18Εποχές και τροχιές…………………………………………………………………………….19Παρατήρηση……………………………………………………………………………………..20Επικίνδυνο και πλεύσιμο ημικύκλιο………………………………………………….21Rule 1-2-3………………………………………………………………………………………….23Κεφάλαιο 2ο

Μετεωρολογικοί χάρτες και Τροπικοί Κυκλώνες……………………………… 27Ιαπωνικοί χάρτες…………………………………………………………………..………….29Λήψη Ιαπωνικών χαρτών μέσω email……………………………………………….32Αμερικάνικοι χάρτες………………………………………………………………..……….34Λήψη Αμερικάνικων χαρτών μέσω email………………………………………….35Πηγή λήψης μηνυμάτων από τη ΝΟΑΑ και το NHC…………………………..37Πρόγνωση τροχιάς με βάση τον χάρτη των 500 mb………………………….40Κεφάλαιο 3ο

Ασκήσεις ………………………………………………………………………………………..…47Άσκηση 1………………………………………………………………………………………..…48Άσκηση 2…………………………………………………………………………………………..50Άσκηση 3…………………………………………………………………………………………..52Άσκηση 4…………………………………………………………………………………………..54Άσκηση 5…………………………………………………………………………………………..56Άσκηση 6…………………………………………………………………………………………..58

Παράρτημα Ι………………………………..……………………………………….………….61Αποσπάσματα από το βιβλίο “Modern Marine Weather” του David BurchΠαράρτημα ΙΙ…………………………………………………………………………….………85Αποσπάσματα από το βιβλίο “Ναυτική Μετεωρολογία” της Αικ. ΨύχαΒιβλιογραφία…………………………………………………………………………….………109

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ΕισαγωγήΑγαπητοί πλοίαρχοι οι παρακάτω σημειώσεις αποτελούν τα βασικότερα σημείατης παρουσίασης που γίνεται στα πλαίσια του μαθήματος ΝαυτικήςΜετεωρολογίας στον Α’ κύκλο σπουδών των υποψηφίων Α΄ Πλοιάρχων, με τίτλο«Τροπικοί Κυκλώνες: Ανάλυση – Πρόγνωση – Αποφυγή». Το συγκεκριμένομάθημα εντάσσεται στο μάθημα της Ναυτιλίας. Οι σημειώσεις αυτές μαζί με τιςασκήσεις που περιλαμβάνονται, έχουν σαν στόχο να συνοψίσουν τα κυριότερασημεία του μαθήματος.

Στο 1ο κεφάλαιο των σημειώσεων παρουσιάζονται επιγραμματικά τα κυριότερασημεία του μαθήματος, όπου δίνεται έμφαση στη θεωρία που συνοδεύει τηδιδασκαλία του τόσο σημαντικού μετεωρολογικού αυτού φαινομένου. Στο 2ο

κεφάλαιο παρουσιάζονται οι μέθοδοι που μπορούμε να αξιοποιήσουμε ώστε ναλάβουμε μέσω e-mail στο καράβι, όλους τους χάρτες και τις προειδοποιήσεις πουαφορούν τους τροπικούς κυκλώνες τόσο από την Ιαπωνική όσο και από τηνΑμερικάνικη μετεωρολογική υπηρεσία. Τέλος, στο 3ο κεφάλαιο περιλαμβάνονταιοι ασκήσεις που θα κάνουμε μαζί στο μάθημα και τέλος.

Οι σημειώσεις συνεχίζονται με το παράρτημα Ι, όπου παρατίθενταιαποσπάσματα από το εξαιρετικό βιβλίο του David Burch με τίτλο “Modern MarineWeather”. Πιο συγκεκριμένα παρουσιάζεται το κεφάλαιο που αφορά τουςτροπικούς κυκλώνες. Στο παράρτημα ΙΙ παρατίθεται το κεφάλαιο για τουςτροπικούς κυκλώνες από το βιβλίο με τίτλο «Ναυτική Μετεωρολογία» τηςΑικατερίνης Ψύχα.

Αν στο μέλλον σάς δημιουργηθούν απορίες ή έχετε κάποιο σχόλιο, παρακαλώπολύ στείλτε μου ηλεκτρονικό μήνυμα στο nikosmazarakis@yahoo.gr . Επίσηςμπορείτε να επισκεφτείτε τη σελίδα μου www.meteomarine.gr όπου μπορείτενα αντλήσετε πλήθος εκπαιδευτικών πληροφοριών.

Σας εύχομαι καλά και ασφαλή ταξίδια.

Δρ. Νίκος ΜαζαράκηςΦυσικός – Μετεωρολόγος

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Τροπικοί Κυκλώνες Κεφάλαιο 1ο

Διαβαθμίσεις τροπικών χαμηλών

Στον Ατλαντικό ονομάζονται “Hurricanes” ενώ στον Βορειοδυτικό Ειρηνικόονομάζονται “Typhoons”. Σε κάθε περίπτωση κάθε χρόνο ευθύνονται γιατην απώλεια δεκάδων ανθρώπινων ζωών αλλά και σημαντικώνκαταστροφών. Όπως παρατηρούμε και από τα παρακάτω διαγράμματα,παρά το γεγονός ότι τα επιστημονικά εργαλεία πρόγνωσης αλλά και ταμέσα παρατήρησης τα τελευταία χρόνια, έχουν βελτιωθεί σημαντικά, οισυνέπειες των τροπικών κυκλώνων τη νέα χιλιετία είναι σε σημαντικόβαθμό αυξημένες, συγκριτικά με τις προηγούμενες δεκαετίες. Κάτι πουίσως να υποδεικνύει την επίδραση των ανθρώπινων δραστηριοτήτων στηνκλιματική αλλαγή και υπερθέρμανση του πλανήτη μας.

• Tropical Depression: Μέγιστη ταχύτητα ανέμου 33 Knots, μία ή δύοκλειστές ισοβαρείς,70% να εξελιχθεί στην επόμενη κατηγορία

• Tropical Storm: Μέγιστη ταχύτητα ανέμου 34 - 63 Knots, 100 τροπικέςκαταιγίδες κάθε χρόνο, 70% να εξελιχθεί στην επόμενη κατηγορία

• Hurricane: Μέγιστη ταχύτητα ανέμου > 64 Knots

• Major Hurricane ή Super Typhoons: Μέγιστη ταχύτητα ανέμου > 96 Knots

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Τ ρ ο π ι κ ο ί κ υ κ λ ώ ν ε ς – γ ε ν ι κ ά

Ένα χαμηλό το οποίο σχηματίζεται κοντά στον ισημερινό, έως ότουεξελιχθεί σε τροπικό κυκλώνα, περνάει από διάφορα εξελικτικά στάδια ταοποία είναι τα εξής:

Το όνομά τους το παίρνουν με βάση το Αγγλικό αλφάβητο (εξαιρούνταιτα γράμματα Q,U,X,Y και Ζ) στον Βόρειο Ατλαντικό, ενώ όταν ο αριθμόςτους ξεπεράσει τους 21 χρησιμοποιούνται τα πρώτα γράμματα τουΕλληνικού αλφάβητου. Πάντα η λίστα αποτελείται εναλλάξ από έναανδρικό και ένα γυναικείο όνομα. Κάθε χρόνο υπάρχει και μίαδιαφορετική λίστα ανάλογα με τη γεωγραφική περιοχή, η οποίαεπαναλαμβάνεται έπειτα από την πάροδο 7 ετών. Όταν όμως έναςτροπικός κυκλώνας προξενήσει μεγάλες καταστροφές ή είναι ιδιαίτεραισχυρός, το όνομά του αφαιρείται από τη λίστα και μπαίνει στο πάνθεοντης ιστορίας. Τη λίστα για το τρέχον έτος καθώς επίσης και για τα 5επόμενα έτη μπορείτε να τη δείτε στο σύνδεσμο που ακολουθεί:

http://www.nhc.noaa.gov/aboutnames.shtml

Οι γενικότερες ονομασίες είναι οι εξής: στον Ατλαντικό και στον ΒόρειοΑνατολικό Ειρηνικό ονομάζονται Hurricanes, στο Ανατολικό Ινδικό και στηΘάλασσα της Ιαπωνίας Typhoons, στον Ινδικό Ωκεανό Cyclones και στηΘάλασσα της Αυστραλίας willy-willies.

Ονοματολογία τροπικών κυκλώνων

Κλίμακα ανέμου Saffir - Simpson

Κατηγορία Άνεμος

1 64-83 Knots

2 84-95 Knots

3 96-113 Knots

4 114-134 Knots

5 >135 Knots

Ανάλογα με τη μέγιστη ένταση των ανέμων που παρατηρείται στο εσωτερικότους, οι τροπικοί κυκλώνες διαιρούνται σε 5 κατηγορίες (κλίμακα ανέμουSaffir-Simpson), σύμφωνα με τον πίνακα που ακολουθεί:

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Οι τροπικοί κυκλώνες σχηματίζονται πάνω από ωκεάνια ύδατα τωνοποίων η επιφανειακή θερμοκρασία είναι μεγαλύτερη από 26,5 βαθμούςΚελσίου και στον Ατλαντικό ωκεανό ξεκινούν ως «τροπικά κύματα»(Easterly Waves ή Tropical Waves) από τις ακτές της Δυτικής Αφρικής.

Δημιουργία ενός Τροπικού Κυκλώνα

Οι ευνοϊκές συνθήκες» για τη δημιουργία ενός τροπικού κυκλώνα είναι:

1. Θερμά ωκεάνια νερά με θερμοκρασία μεγαλύτερη από 26.5 βαθμούςσε βάθος έως 50 μέτρα.

2. Ασθενείς άνεμοι στην επιφάνεια και καθ’ ύψος.

3. Απόσταση από τον ισημερινό μεγαλύτερη από 500 km.

4. Χαμηλές θερμοκρασίες στη μέση και ανώτερη ατμόσφαιρα

5. Υψηλές τιμές της υγρασίας για την τροφοδοσία των καταιγίδων

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1. Η ένταση ενός τροπικού κυκλώνα αυξάνεται όταν αυξάνεται ηεπιφανειακή θερμοκρασία του ωκεανού.

2. Επειδή οι τροπικοί κυκλώνες «προτιμούν» τα θερμά ύδατα, έναςχάρτης επιφανειακής θερμοκρασίας του ωκεανού μπορεί να μαςπροϊδεάσει για την κίνηση ενός τ.κ.

3. Η μέγιστη ένταση των ανέμων που συνοδεύουν ένα τ.κ. μπορεί ναεκτιμηθεί από τη σχέση:

Max Wind = 16 kts x Τετραγωνική ρίζα του dP

όπου dP είναι η διαφορά πίεσης μεταξύ της περιφέρειας και του κέντρουτου τ.κ.

4. Η μέση διάρκεια ζωής ενός τ.κ. είναι οι 10 ημέρες. Μπορεί νασυνυπάρχουν περισσότεροι από έναν τ.κ. την ίδια χρονική στιγμή.Όταν πλησιάσει ο ένας τον άλλον μπορούν να περιστρέφονται οένας γύρω από τον άλλον, με αυτόν που έχει το μικρότερο μέγεθοςνα κινείται γρηγορότερα (φαινόμενο Fujiwara).

Ε μ π ε ι ρ ι κ ο ί κ α ν ό ν ε ς

Παγκόσμιος χάρτης που απεικονίζει τις περιοχές που δημιουργούνται οι τροπικοί κυκλώνες και το όνομά

τους σε κάθε περιοχή.

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Ένας τροπικός κυκλώνας διαλύεται ή εξασθενεί όταν:

1. Κινηθεί πάνω από ψυχρότερα νερά.

2. Όταν ενισχυθεί ο άνεμος στα ανώτερα στρώματα

3. Όταν γίνει ηπειρωτικός

Εξασθένηση ενός Τροπικού Κυκλώνα

Δομή ενός Τροπικού Κυκλώνα

Οι κύριες περιοχές σε ένα τροπικό κυκλώνα είναι το μάτι, το τείχος τουματιού και οι σπείρες βροχόπτωσης (rainbands).

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Εγκάρσια τομή ενός τροπικού κυκλώνα. Παρατηρείστε τη σπειροειδή τροχιά που ακολουθεί ο άνεμος γύρω από το μάτι

και την ανοδική κίνησή του στο τοίχος του ματιού.

Κατακόρυφη τομή ενός τροπικού κυκλώνα και μεταβολή βαρομετρικής πίεσης και ταχύτητας ανέμου, σε συνάρτηση με την απόσταση από το μάτι.

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Το μάτι είναι μία περιοχή στην οποία επικρατούν ασθενείς άνεμοι μεεξαιρετικά όμως υψηλό κυματισμό. Η διάμετρος του ματιού κυμαίνεταιμεταξύ 10 – 20 nmi, ενώ σε κάποιες περιπτώσεις μπορεί να είναι μόλις 4nmi ή να φτάνει ακόμα και τα 45 nmi.

Μεταβολή της διαμέτρου του ματιού του τροπικού κυκλώνα Wilma (αριστερά: 19/10/2005, δεξιά: 24/10/2005)

Το μάτι του Τροπικού Κυκλώνα

Η πιο επικίνδυνη περιοχή είναι το τοίχος του ματιού όπου όλα ταμετεωρολογικά φαινόμενα έχουν τη μέγιστη ισχύς τους. Το σύνηθεςπλάτος του τοίχους του ματιού κυμαίνεται από 5 έως 30 nmi.

Το τοίχος του ματιού του Τροπικού Κυκλώνα

Το τοίχος του ματιού του Τροπικού Κυκλώνα Katrina (2005)

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Οι σπειροειδής ζώνες βροχόπτωσης εκτείνονται μετρικές εκατοντάδεςμίλια μακριά από το μάτι του τροπικού κυκλώνα. Το πάχος τους είναικυμαινόμενο από λίγα μίλια έως μερικές δεκάδες ενώ εκτείνονταισυνήθως έως και τα 250 nmi km μακριά από το μάτι. Η απόσταση όμως,όπως φαίνεται και στις παρακάτω εικόνες μπορεί να διαφέρεισημαντικά από περίπτωση σε περίπτωση. Έτσι στον Floyd οι σπείρεςεκτείνονταν στα 300 nmi, ενώ στον Andrew μόλις στα 80 nmi.

Οι σπειροειδής ζώνες βροχόπτωσης του Τροπικού Κυκλώνα

Στις δύο παραπάνω δορυφορικές εικόνες είναι αισθητή η διαφορά μεγέθους μεταξύ των τροπικών κυκλώνων Floyd και Andrew.

Tο μέγεθος των τροπικών κυκλώνων ποικίλει από περίπτωση σε περίπτωση,ενώ δεν καθορίζει και την ένταση του τ.κ. Για παράδειγμα ο Κατρίνα (2005)όταν «χτύπησε» τη Λουιζιάνα ήταν κατηγορίας 3 και η διάμετρός τουξεπερνούσε τα 650 km. Αντίθετα ο Charley (2004) ήταν κατηγορίας 4 αλλάη διάμετρός του δεν ξεπερνούσε τα 240 km, ενώ ήταν μετά τον Andrew οδεύτερος καταστροφικότερος τυφώνας.

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Οι τ.κ. εμφανίζονται συνήθως αρχές καλοκαιριού έως τέλη φθινοπώρουπου η θάλασσα έχει τη μέγιστη θερμοκρασία της. Οι μήνες λοιπόν με τημεγαλύτερη συχνότητα εμφάνισης είναι στο Β.Η. ο Αύγουστος και οΣεπτέμβριος και αντίστοιχα στο Ν.Η. ο Ιανουάριος – Φεβρουάριος καιΜάρτιος.

Εξαίρεση αποτελεί ο κόλπος της Βεγγάζης και η Αραβική Θάλασσα όπουλόγω των Μουσώνων παρουσιάζουν ένα μέγιστο αργά την Άνοιξη καιένα δεύτερο μέγιστο αργά το φθινόπωρο.

Οι περισσότεροι τ.κ. δημιουργούνται μεταξύ 5 και 25 μοιρών γ.π. καισυχνότερα στην περιοχή 6 με 10 μοίρες. Δε δημιουργούνται ποτέ κάτωαπό τις 3 μοίρες, επειδή η δύναμη Coriolis είναι εξαιρετικά μικρή.

Ε π ο χ έ ς κ α ι τ ρ ο χ ι έ ς

Οι τ.κ. έχουν την τάση έως τις 30 μοίρες να κινούνται προς τα δυτικά –βορειοδυτικά και μετά τις 30 μοίρες να κινούνται βορειοανατολικά.Αυτό οφείλεται στο γεγονός ότι από τις 10 έως τις 30 μοίρεςκυριαρχούν στις ωκεάνιες περιοχές οι Αληγείς άνεμοι, ενώ από τις 30έως και τις 60 μοίρες οι Δυτικοί άνεμοι. Ωστόσο μόνο το 50% των τ.κ.διαγράφουν παραβολική τροχιά και οι υπόλοιποι είτε έχουνακανόνιστη τροχιά είτε κινούνται μόνο δυτικά.

Όταν η διεύθυνσή του γίνεται πολική, πριν δηλαδή καμπυλώσουν τηντροχιά τους προς τα ανατολικά, συνήθως η ταχύτητα τους πέφτεισημαντικά στους 2 έως 8 KT.

Συνήθεις τροχιές και ταχύτητες των τροπικών κυκλώνων που δημιουργούνται στο Β.Η. (αριστερή εικόνα) και στο Ν.Η. (δεξιά εικόνα)

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Στην Αμερική όταν ο τ.κ. περάσει τον μεσημβρινό των 55 μοιρών δυτικά,τότε απογειώνονται τα ειδικά αεροσκάφη της ΑμερικανικήςΜετεωρολογικής Υπηρεσίας τα οποία εκτελούν επιτόπιες παρατηρήσειςκαι εισέρχονται και στο «μάτι» του κυκλώνα.

Τα μέσα παρακολούθησης και μέτρησης των διαφόρων μετεωρολογικώνπαραμέτρων σε έναν τ.κ. είναι πολλά. Έτσι όταν ένας τ.κ. είναι μακριάαπό την ακτή παρακολουθείται με τη βοήθεια των μετεωρολογικώνδορυφόρων.

Π α ρ α τ ή ρ η σ η

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Ε π ι κ ί ν δ υ ν ο κ α ι π λ ε ύ σ ι μ ο η μ ι κ ύ κ λ ι ο

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Ένας τροπικός κυκλώνας διαιρείται σε δύο ημικύκλια σε σχέση με τηδιεύθυνση κίνησής του. Έτσι στο Βόρειο Ημισφαίριο της γης, το δεξίημικύκλιο θεωρείται το επικίνδυνο (dangerous) και το αριστερόημικύκλιο θεωρείται το πλεύσιμο (navigable). Αυτό συμβαίνει για τουςεξής λόγους:

• όταν ένα πλοίο βρίσκεται στο δεξί ημικύκλιο, σε πιθανή ανακαμπύλωσητης τροχιάς του τροπικού κυκλώνα, το πλοίο θα τον βρει μπροστά του.

• στους ανέμους που προκαλεί ο ίδιος ο τροπικός κυκλώνας στο δεξίημικύκλιο προστίθεται και η ταχύτητα κίνησης ολόκληρου τουσυστήματος, καθώς στο δεξί ημικύκλιο έχουν την ίδια φορά. Έτσι σε αυτότο ημικύκλιο η ένταση των ανέμων είναι μεγαλύτερη από ότι στοαριστερό.

• η διεύθυνση και η φορά των ανέμων στο δεξί ημικύκλιο, τείνουν τοπλοίο να το φέρουν πάνω στην διεύθυνση κίνησης του τροπικούκυκλώνα, μπροστά από το μάτι.

Το επικίνδυνο και το πλεύσιμο ημικύκλιο σε έναν τροπικό κυκλώνα, σε κάθε ημισφαίριο της Γης (σχήμα από το βιβλίο Ναυτική Μετεωρολογία, Γ. Κασιμίδης)

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ειδικότερα το 1ο δεξί τεταρτοκύκλιο, θεωρείται ότι είναι το πιοεπικίνδυνο καθώς οι Αληγείς άνεμοι που «σπρώχνουν» τους τροπικούςκυκλώνες προς τα δυτικά, όπως βλέπουμε στο παραπάνω σχήμα, έχουντην ίδια φορά με τους άνεμους που δημιουργεί ο τροπικός κυκλώνας.

Στο Νότιο Ημισφαίριο της γης ισχύουν ακριβώς τα ίδια πράγματα, με τημόνη διαφορά ότι πρέπει να αλλάξουμε τις λέξεις αριστερά και δεξιά.Έτσι με αυτή τη λογική, στον νότιο ημισφαίριο της γης, το επικίνδυνοημικύκλιο είναι το αριστερό ενώ το πλεύσιμο είναι το δεξί.

Σε κάθε περίπτωση όπως εξηγείται και παρακάτω στον κανόνα Rule 1-2-3,ο πλοίαρχος δεν θα πρέπει σε καμία περίπτωση να πλησιάζει έναντροπικό κυκλώνα, σε απόσταση μικρότερη από αυτή όπου οι άνεμοιξεπερνούν σε ένταση τους 34 κόμβους.

Όταν σε μία περιοχή υπάρχει ένας τροπικός κυκλώνας ή μία τροπικήκαταιγίδα, είναι ζωτικής σημασίας να αποφύγουμε την περιοχή γύρω απότην διαταραχή που η ένταση του ανέμου ξεπερνάει τους 34 KT (8 BF). Είναιλοιπόν πολύ σημαντικό να γνωρίζουμε τη διάμετρο ή καλύτερα τα όριααυτής της περιοχής ή ζώνης που ο άνεμος ξεπερνάει τους 34 ΚΤ. Ηδιάμετρος αυτής της ζώνης διαφέρει σε κάθε διαταραχή και δεν είναιΠΟΤΕ σταθερή.

Ο κανόνας Rule 1-2-3 αναφέρεται για 1η φορά στις αρχές της δεκαετίας του‘70, στην εκπαιδευτική ταινία του Αμερικάνικου Ναυτικού (US Navy) μετίτλο “A Time for All measures”. Από τη δεκαετία του ‘90 στον κανόνα έχειενσωματωθεί η λεγόμενη “Danger Zone” η οποία βρίσκεται προσθέτονταςγια κάθε 24ωρη πρόγνωση, το δεκαετή μέσο όρο των σφαλμάτωνπρόγνωσης στη διάμετρο της ζώνης των 34 ΚΤ.

Τα σφάλματα αυτά έως και πριν από λίγα χρόνια θεωρούνταν ότι είναι γιατο 1ο 24ωρο πρόγνωσης τα 100 nmi, για το 2ο 24ώρο (48h forecast) τα 200nmi και για το 3ο 24ωρο (72h forecast) τα 300 nmi.

R u l e 1 - 2 - 3

Η εξέλιξη των σφαλμάτων πρόγνωσης από το 1970 έως και τις μέρες μας

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Τα τελευταία όμως χρόνια, η πρόγνωση της μελλοντικής θέσης μίαςτροπικής διαταραχής, έχει παρουσιάσει σημαντική βελτίωση. Έτσισύμφωνα με τον βετεράνο μετεωρολόγο της NOAA, Lee Chesneau, ηαπόσταση που προσθέτουμε στη διάμετρο της ζώνης των 34 ΚΤ είναι:

• Για τις 24h forecast τα 60 nmi

• Για τις 48h forecast τα 120 nmi

• Για τις 72h forecast τα 180 nmi

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Σχηματική απεικόνιση του κανόνα Rule 1-2-3

25

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Μετεωρολογικοί χάρτες και Τροπικοί Κυκλώνες

Κεφάλαιο 2ο

Ιαπωνικοί χάρτες

Οι Γιαπωνέζικοι χάρτες θεωρούνται από τους πιο αξιόπιστους για ένανναυτικό που ταξιδεύει στον Δυτικό Ειρηνικό Ωκεανό.

Όταν στη γεωγραφική περιοχή που καλύπτει ο χάρτης, υπάρχει έναςτροπικός κυκλώνας, τότε πάνω στο χάρτη αναφέρονται κάποιες επιπλέονπληροφορίες, όπως φαίνεται και στο παραπάνω σχήμα, για τον τροπικόκυκλώνα Tembin.

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Ακολουθεί η ανάλυση αυτών των πληροφοριών:

T 1214 TEMBIN (1214)Το “T” σημαίνει Typhoon. Μπορείτε επίσης να δείτε τα γράμματα “TS” πουσημαίνουν Tropical Storm ή “STS” που σημαίνουν Severe Tropical Storm πουαντιστοιχεί σε τροπικό κυκλώνα κατηγορίας 1, σύμφωνα με την κλίμακαSaffir – Simpson (βλ. και σχετικό πίνακα στο τέλος των σημειώσεων). Οαριθμός 1214 είναι ο αύξον αριθμός του Τροπικού Κυκλώνα και Tembinείναι το όνομά του.

965 hPaΕίναι η βαρομετρική πίεση στο μάτι του τροπικού κυκλώνα η στο κέντρο τηςτροπικής καταιγίδας. Σημειώνουμε ότι 1 hPa = 1 mb.

22.4N 124.5E PSN GOODΕίναι το στίγμα του ματιού κατά την ώρα ισχύος του χάρτη (Valid Time). Ηφράση PSN GOOD (PSN = Position) σημαίνει ότι η χωροθέτηση του ματιούείναι με μεγάλη ακρίβεια, και αυτό συμβαίνει γενικά, όταν ο τροπικόςκυκλώνας έχει δημιουργήσει ξεκάθαρο μάτι, το οποίο φαίνεται καθαρά στιςδορυφορικές εικόνες.

NW SLOWLYΠΡΟΣΟΧΗ: Η παραπάνω φράση ΔΕΝ ΕΙΝΑΙ ΠΡΟΓΝΩΣΗ αλλά είναι ηδιεύθυνση και η ταχύτητα κίνησης ολόκληρης της ατμοσφαιρικήςδιαταραχής, την ώρα που ισχύει ο χάρτης.

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MAX WIND 70 KT NEAR CENTERGUST 100 KTΜε τη φράση “near center” εννοούν το δακτυλίδι του ματιού, όπου οιάνεμοι έχουν τη μέγιστη έντασή τους, που στο συγκεκριμένο παράδειγμα,φτάνουν του 70 κόμβους. Η λέξη Gust σημαίνει ριπαίος άνεμος.

EXPECTED MAX WINDS 75 KT NEAR CENTERFOR NEXT 24 HOURSEXPECTED GUST 105 KTΕάν για τις επόμενες 24 ώρες υπάρχει κάποια αλλαγή στην πρόγνωση (είτευποβάθμιση είτε αναβάθμιση της διαταραχής), πάντοτε θα αναφέρεται στοσυγκεκριμένο σημείο. Έτσι στο συγκεκριμένο παράδειγμα, η ένταση τωνανέμων μέσα στις επόμενες 24 ώρες από την ώρα ισχύος του χάρτη, θαενισχυθούν από τους 70 στους 75 κόμβους.

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OVER 50 KT WITHIN 45 NMΜας λέει μέχρι ποια απόσταση από το κέντρο της διαταραχής οι άνεμοιξεπερνούν σε ένταση τους 50 κόμβους.

OVER 30 KT WITHIN 150 NM NE-SEMICIRCLE120 NM ELSEWHERE

Ίσως το σημαντικότερο τμήμα των πληροφοριών, καθώς μας λέει μέχρι ποιααπόσταση από το κέντρο της διαταραχής οι άνεμοι ξεπερνούν τους 30 ΚΤ.Οι Ιάπωνες διαιρούν μία τροπική διαταραχή σε δύο ημικύκλια, σύμφωνα μετο σχήμα που ακολουθεί:

Ν

S

SW

ΝE

SE

ΝW

W E

Έτσι στο συγκεκριμένο παράδειγμα ο τροπικός κυκλώνας είναι πιο ισχυρόςστο βορειοανατολικό ημικύκλιο, καθώς η ζώνη των 8 bf διευρύνεται έως και150 NM μακριά από το κέντρο του τροπικού κυκλώνα. Στο αντιδιαμετρικόημικύκλιο η αντίστοιχη απόσταση φτάνει τα 120 ΝΜ.

Λήψη Ιαπωνικών χαρτών μέσω emailΠέρα από την χρήση της συσκευής facsimile με τα γνωστά προβλήματαλήψης και ποιότητας των μετεωρολογικών χαρτών, υπάρχουνεναλλακτικές μέθοδοι, που μπορούμε χρησιμοποιώντας το email τουπλοίου, να κατεβάσουμε εύκολα, γρήγορα και απλά όλους τουςδιαθέσιμους μετεωρολογικούς χάρτες, σε απαράμιλλη ποιότητα.Περισσότερες πληροφορίες και οδηγίες μπορείτε να κατεβάσετεδωρεάν από τη σελίδα www.meteomarine.gr.

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Η πρόγνωση για την μελλοντική πορεία ενός τροπικού κυκλώνα, στουςΓιαπωνέζικους χάρτες παριστάνεται με κύκλους οι οποίοι αφορούν τοεπόμενο 12ωρο ή το επόμενο 24ωρο από την ώρα ισχύος του χάρτη.Στο παραπάνω παράδειγμα η Valid Time του χάρτη είναι 22AUG, 00z(δεν φαίνεται στον χάρτη). Στο εσωτερικό του 1ου κύκλου θα είναι γιατις επόμενες 12 ώρες το κέντρο της διαταραχής. Η πιο πιθανή θέσηείναι το κέντρο του κύκλου. Κάτω από τον μικρό κύκλο υπάρχει ηφράση “221200Z” που ουσιαστικά μας δίνει τη δυνατότητα νακαταλάβουμε για πιο χρονικό διάστημα ισχύει ο παρών προγνωστικόςκύκλος. Αντίστοιχα υπάρχει και ο μεγαλύτερος κύκλος ο οποίος καιισχύει για τις 230000Ζ,δηλαδή για το επόμενο 24ωρο.

Ώρα ισχύος χάρτη URLAnalysis 00z http://www.jma.go.jp/jp/g3/images/asia/YYMMDD09.pngAnalysis 06z http://www.jma.go.jp/jp/g3/images/asia/YYMMDD15.pngAnalysis 12z http://www.jma.go.jp/jp/g3/images/asia/YYMMDD21.pngAnalysis 18z http://www.jma.go.jp/jp/g3/images/asia/YYMMDD03.pngForecast 24h, 00z http://www.jma.go.jp/jp/g3/images/24h/YYMMDD09.pngForecast 24h, 12z http://www.jma.go.jp/jp/g3/images/24h/YYMMDD21.pngForecast 48h, 00z http://www.jma.go.jp/jp/g3/images/48h/YYMMDD09.pngForecast 48h, 12z http://www.jma.go.jp/jp/g3/images/48h/YYMMDD21.pngAnalysis 500mb, 00z http://www.jma.go.jp/jmh/wmapimgs/sml_00_auas50.pngAnalysis 500mb, 12z http://www.jma.go.jp/jmh/wmapimgs/sml_12_auas50.png

Όπου YY το έτος (π.χ. 12), MM ο μήνας (π.χ. 09) και DD η ημέρα (π.χ. 22)Για παράδειγμα αν θέλουμε να λάβουμε με το email μας την ανάλυση των 00z για τις 22 Σεπτεμβρίου του 2013 θα γράψουμε το url:

http://www.jma.go.jp/jp/g3/images/asia/13092209.png

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Ακολουθεί η περιγραφή του τρόπου που μπορούμε να χρησιμοποιήσουμεγια να κατεβάσουμε μέσω email τους Γιαπωνέζικους χάρτες. Στέλνουμε το urlτου χάρτη με email στη διεύθυνση www4mail@wm.ictp.trieste.it (σεπερίπτωση που ο server δεν απαντήσει δοκιμάστε τα παρακάτω εναλλακτικάemails: www4mail@collaborium.org, www4mail@ftp.uni-stuttgart.de,www4mail@kabissa.org, www4mail@web.bellanet.org).Στο τίτλο του email γράφουμε ότι θέλουμε ή το αφήνουμε κενό.

Στο κείμενο του e-mail γράφουμε το url που επιθυμούμε σύμφωνα με τονπίνακα που ακολουθεί παρακάτω:

Οι Αμερικάνικοι χάρτες είναι εξαιρετικά χρήσιμοι και αξιόπιστοι για τοναυτικό που ταξιδεύει στον Βόρειο Ατλαντικό και στο Βόρειο Ειρηνικόωκεανό. Η Αμερικανική μετεωρολογική υπηρεσία ΝΟΑΑ, μέσω των τριώνκέντρων της OPC, NHC και HPC καθημερινά εκδίδει χάρτες καιπροειδοποιήσεις για τους ναυτιλλόμενους.

Αμερικάνικοι χάρτες

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Στον παρακάτω χάρτη μπορούμε να δούμε ένα παράδειγμα από τον τροπικόκυκλώνα ILEANA ο οποίος σχηματίστηκε στον Ανατολικό Ειρηνικό Ωκεανό τονΑύγουστο του 2012.

Όνομα κυκλώνα

Θέση Τ.Κ. την ώρα έκδοσης του χάρτη

Κίνηση Τ.Κ. την ώρα έκδοσης του χάρτη

Ώρα έκδοσης του χάρτηIssued Time

Ώρα ισχύος του χάρτηValid Time

Οι αμερικανοί έχουν αναπτύξει ένα δικό τους αυτοματοποιημένο σύστημαλήψης των μετεωρολογικών χαρτών μέσω email που εκδίδει ημετεωρολογική του υπηρεσία. Το σύστημα αυτό ονομάζεται FTP MAILService. Για περισσότερες πληροφορίες επισκεφτείτε τη σελίδαwww.meteomarine.gr .

Στους πίνακες της επόμενης σελίδας, παρουσιάζονται συνοπτικά τα ονόματατων χαρτών που μπορούμε να λάβουμε μέσω της υπηρεσίας FTP Mail Serviceτης NOAA, για τον Βόρειο Ατλαντικό και τον Βόρειο Ανατολικό Ειρηνικό.

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Λήψη Αμερικάνικων χαρτών μέσω email

7 0 W 5 5 W

W e s tE a s tS y n o p t i c

T i m e s

P Y E B 8 6 0 0 z

0 6 z

1 2 z

1 8 z

M o s t c u r r e n t

P Y E B 8 7

P Y E B 8 5

P Y E B 8 8

P Y E B 1 1

P Y E A 8 6

P Y E A 8 7

P Y E A 8 5

P Y E A 8 8

P Y E A 1 1

Τα ονόματα των χαρτών που εκδίδονται από την Αμερικανική Μετεωρολογική Υπηρεσία (NOAA) για τον Ατλαντικό και τον Ειρηνικό Ωκεανό, ανάλογα με την

ώρα ισχύος

Περιγραφή χάρτη για Ατλαντικό Όνομα

U.S./Tropical Surface Analysis (W Half) PYEB11.TIF

Tropical Surface Analysis (E Half) PYEA11.TIF

24H Tropical Surface Forecast PYEE10.TIF

48H Tropical Surface Forecast PYEI10.TIF

72H Tropical Surface Forecast PYEK10.TIF

Tropical Cyclone Danger Area PWEK11.TIF

Tropical Surface Analysis QYFA99.TIF

U.S./Tropical Surface Analysis PYEB11.TIF

72 HR Tropical Cyclone Danger Area PWFK12.TIF

Περιγραφή χάρτη για Ειρηνικό Όνομα

East Pacific Surface Analysis PYFA90.TIF

U.S./Tropical Surface Analysis PYEB11.TIF

24HR Tropical Surface Forecast PYFE10.TIF

48HR Tropical Surface Forecast PYFI10.TIF

72HR Tropical Surface Forecast PYFK10.TIF

72 HR Tropical Cyclone Danger Area PWFK11.TIF

Pacific SST Chart 40N-53N, E of 136W PTBA88.TIF

Pacific SST Chart 23N-42N, E of 150W PTBA89.TIF

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Πηγή λήψης μηνυμάτων από τη ΝΟΑΑ και το NHCΓια να λαμβάνουμε αυτόματα κάθε 6 ώρες τα τελευταία advisories καιdiscussions για τον Ατλαντικό και για τον Βόρειο Ανατολικό ΕιρηνικόΩκεανό θα πρέπει να γραφτούμε στην υπηρεσία Govdelivery στοακόλουθο link:

https://public.govdelivery.com/accounts/USNWS/subscriber/new?preferences=true

Αφού εγγραφείτε στην υπηρεσία θα πρέπει να τσεκάρετε τις επιλογές γιατον Ατλαντικό και τον Ειρηνικό που ονομάζονται Advisories και discussion.Με αυτό τον τρόπο καθημερινά θα λαμβάνετε αυτόματα στο e-mail σαςόλες τις προειδοποιήσεις για τους τροπικούς κυκλώνες που υπάρχουν στονΒόρειο Ατλαντικό και τον Βόρειο Ανατολικό Ειρηνικό (βλ. παρακάτω σχήμα)

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Είναι πολύ σημαντικό όταν πλέουμε την εποχή των τροπικών κυκλώνωνστον Βόρειο Ατλαντικό και το Βόρειο Ειρηνικό Ωκεανό να λαμβάνουμε όλεςτις τελευταίες προειδοποιήσεις (advisories) καθώς, σε συνδυασμό με τουςχάρτες, έχουμε ένα ισχυρότατο εργαλείο που θα μας βοηθήσει ναχαράξουμε την πορεία μας με ασφάλεια και σιγουριά.

Ένα advisory διαιρείται σε τρία μέρη. Στο 1ο μέρος αναφέρονται όλες οιτρέχουσες πληροφορίες για έναν τροπικό κυκλώνα, στο 2ο μέρος (αρχίζει μετη λέξη “repeat”) αναφέρεται αναλυτικά η πρόγνωση των 3 ημερώνμπροστά και στο 3ο μέρος (αρχίζει με τη φράση “extended outlook”)αναφέρεται η πρόγνωση της 4ης και 5ης μέρας μπροστά. Στο παρακάτωσχήμα βλέπετε ένα από τα πολλά advisories που είχε εκδοθεί για τοντροπικό κυκλώνα Katrina με ισχύ στις 25 Αυγούστου του 2005 και ώρα0900z.

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1o μέρος

2o μέρος

39

2o μέρος(συνέχεια)

3o μέρος

Για να καμπυλώσει την τροχιά του ένας τροπικός κυκλώνας θα πρέπει ναισχύουν οι παρακάτω δύο προϋποθέσεις:

1. Η απόσταση, κατά γεωγραφικό πλάτος, μεταξύ του κέντρου τουτροπικού κυκλώνα και του νοτιότερου άκρου της ζώνης των ισχυρώννοτιοδυτικών ανέμων, θα πρέπει να μην ξεπερνά τα 1000 km (ή τις 9.3μοίρες κατά γ.π.). Το νοτιότερο άκρο της ζώνης των ισχυρώννοτιοδυτικών ανέμων καθορίζεται από την νοτιότερη ισοϋψή καμπύληστα 500 mb που περιβάλλει το Βόρειο Πόλο.

2. Το κέντρο του τροπικού κυκλώνα θα πρέπει να βρίσκεται ανατολικά τουάξονα της trough στα 500 mb. Σε διαφορετική περίπτωση η τροχιά τουτροπικού κυκλώνα δε θα καμπυλωθεί.

Η μεθοδολογία που ακολουθούμε για να βρούμε αν θα καμπυλώσει τηντροχιά του ένας τροπικός κυκλώνας, με τη βοήθεια του χάρτη των 500 mb,συνίσταται στα ακόλουθα βήματα:

αρχικά, εντοπίζουμε τη νοτιότερη ισοϋψή καμπύλη η οποία περιβάλλει το Βόρειο Πόλο και τη σχεδιάζουμε με κόκκινο χρώμα.

στη συνέχεια, χαράσσουμε με το μολύβι μας μία γραμμή παράλληλη με την νοτιότερη ισοϋψή, με τέτοιο τρόπο ώστε να απέχει 1000 km ή 9.3 μοίρες από αυτή, προς το Νότο.

τέλος, προεκτείνουμε αν είναι απαραίτητο τον άξονα της κοντινότερης trough προς νότο, και διαιρούμε την περιοχή σε τέσσερα τεταρτημόρια.

Π ρ ό γ ν ω σ η τ ρ ο χ ι ά ς μ ε β ά σ η τ ο ν χ ά ρ τ η τ ω ν 5 0 0 m b

1

2

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Με βάση την προηγούμενη μέθοδο ισχύουν οι παρακάτω κανόνες:

1. Εάν ένας τροπικός κυκλώνας βρεθεί μέσα στο 1ο τεταρτημόριο, τότε ητροχιά του θα καμπυλωθεί και θα γίνει παραβολική. Εάν όμως βρεθείστα υπόλοιπα τρία τεταρτημόρια θα διατηρήσει την τροχιά και τηνταχύτητά του αμετάβλητες.

2. Όταν ένας τροπικός κυκλώνας βρεθεί στο 2ο,3ο και 4ο τεταρτημόριο,τότε αποκτά τα χαρακτηριστικά των βαρομετρικών χαμηλών πουσυναντάμε στα μέσα γεωγραφικά πλάτη, τόσο στην ταχύτητα όσο καιστη διεύθυνση. Η ταχύτητα του τροπικού κυκλώνα στην επιφάνειαείναι το 30 – 50 % της ταχύτητας του ανέμου στα 500 mb.

3. Όταν ένας τροπικός κυκλώνας βρεθεί στο 1ο τεταρτημόριο για ναβρούμε πως θα κινηθεί ακολουθούμε τους τρεις κανόνες πουακολουθούν:

(i) Όταν ένας τροπικός κυκλώνας συναντάει στα βόρεια ένα ridge τότε ητροχιά του θα ακολουθήσει τη γενική ροή που υπάρχει στα 500 mb.Εάν όμως υπάρχουν δύο ridge, στα βορειοανατολικά και σταβορειοδυτικά, τότε ο τροπικός κυκλώνας θα κινηθεί ανάμεσά τουςκαι θα κατευθυνθεί προς τον βορρά.

(ii) Οι τροπικοί κυκλώνες έχουν την τάση να κινούνται μεταξύ τωνισοϋψών των 5820 και 5880 μέτρων.

(iii) Όταν σε έναν χάρτη επιφανείας, το κέντρο του τροπικού κυκλώνααπέχει απόσταση 5 μοιρών από ένα ψυχρό μέτωπο, τότε ο τροπικόςκυκλώνας θα ακολουθήσει την τροχιά κίνησης του ψυχρούμετώπου.

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Ο τροπικός κυκλώνας Ivy γεννήθηκε σαν τροπικό χαμηλό στην περιοχή των νησιώνCaroline ενώ από τροπική καταιγίδα αναβαθμίστηκε σε τροπικό κυκλώνα όταν είχεφτάσει στην περιοχή Northern Marianna Islands. Το μέγιστο της έντασής τουπαρατηρήθηκε στις 7/9 λίγο πριν καμπυλωθεί η τροχιά του, φτάνοντας οι άνεμοιπου τον συνόδευαν σε ένταση τα 210 km/h.

Π α ρ ά δ ε ι γ μ α π ρ ό γ ν ω σ η ς τ ρ ο χ ι ά ς ( I V Y )

(Κέντρο στα 500 mb του τροπικού κυκλώνα)

Άξονας Χ

Άξονας Υ (trough)Το νότιο άκρο της ζώνης των ισχυρών νοτιοδυτικών ανέμων

9.3 μοίρες

Σ χ η μ α τ ι κ ή α ν α π α ρ ά σ τ α σ η τ η ς μ ε θ ό δ ο υ τ ω ν M a - L i C h e n κ α ι L e e C h e s n e a u

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7 Σεπτεμβρίου

6 Σεπτεμβρίου

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8 Σεπτεμβρίου

9 Σεπτεμβρίου

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10 Σεπτεμβρίου

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ΑσκήσειςΚεφάλαιο 3ο

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Άσκηση 1

Με βάση τον Ιαπωνικό χάρτη επιφανείας της σελίδας που ακολουθεί να απαντήσετε στα παρακάτω ερωτήματα:α) Τι σημαίνει το γράμμα “Τ” αριστερά του ονόματος MUIFA και τι σημαίνουν τα γράμματα STS αριστερά του ονόματος MERBOK

β) Ποια είναι η ώρα και η ημερομηνία ισχύος του χάρτη.

γ) Να συμπληρώσετε τους παρακάτω δύο πίνακες με το στίγμα των διαταραχών τις ώρες που αναφέρονται στην αριστερή στήλη:

MUIFA LAT LON

06AUG0000UTC

06AUG1200UTC

07AUG0000UTC

MERBOK LAT LON

06AUG0000UTC

07AUG0000UTC

δ) Να χαράξετε με το μολύβι σας πάνω στον χάρτη, τα όρια της περιοχής των 30 ΚΤ, γύρω από τη MUIFA και γύρω από τον MERBOK (υπενθυμίζουμε ότι 1 μοίρα = 60 nmi).

ε) Στις πληροφορίες του MERBOK τι σημαίνει στην 4η γραμμή η φράση: “NW 06 KT”;

στ) Με βάση τη σχέση Max Wind = 16 kts x Τετραγωνική ρίζα του dP να υπολογίσετε τη μέγιστη ταχύτητα των ανέμων στη MUIFΑ.

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Άσκηση 2

Θεωρείστε ότι το τελευταίο advisory που λαμβάνετε από το NHC περιέχει τις παρακάτω πληροφορίες:

TROPICAL STORM CENTER LOCATED NEAR 22.0N 55.0 W AT 22/0000ZPOSITION ACCURATE WITHIN 10 NM

FORECAST VALID 23/0000Z 24.0N 60.0WMAX WIND 55 KT...GUSTS 75 KT.34 KT…175ΝΕ 150SE 150SW 150NW.

FORECAST VALID 24/0000Z 26.0N 66.0WMAX WIND 70 KT...GUSTS 105 KT.34 KT... 180ΝΕ 160SE 160SW 160NW.

FORECAST VALID 25/0000Z 29.0N 68.0WMAX WIND 50 KT...GUSTS 75 KT.34 KT... 170ΝΕ 150SE 150SW 150NW.

Στον base map της επόμενης σελίδας να εφαρμόσετε τον Rule 1-2-3 εντοπίζοντας την “Danger Zone”.

Σημειώστε: Πηγή Base Maps http://www.opc.ncep.noaa.gov/basemaps

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Άσκηση 3

Το κείμενο που ακολουθεί αποτελεί το advisory no. 14 που εκδόθηκε από το NHC στις 26 Αυγούστου 2005 για τον τροπικό κυκλώνα KATRINA. Εργαστείτε όπως στην άσκηση 2 πάνω στον base map της επόμενης σελίδας

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Άσκηση 4

Με βάση τον χάρτη επιφανείας της σελίδας που ακολουθεί να απαντήσετε στα παρακάτω ερωτήματα:

α) ποια θα είναι η θέση της Τροπικής Καταιγίδας Nadine στις 0900 UTC;

β) ποια είναι η βαρομετρική πίεση στο κέντρο της Nadine, ποια η κίνησή της και με πόση ταχύτητα κινείται;

γ) ποια θα είναι η αυριανή της θέση στις 0600 UTC;

δ) σε ποια «κουτάκια» γεωγραφικού μήκους και πλάτους υπάρχει πιθανότητα εμφάνισης τροπικής καταιγίδας το επόμενο 24ωρο;

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Άσκηση 5

Με βάση τον χάρτη επιφανείας της σελίδας που ακολουθεί να απαντήσετε στα παρακάτω ερωτήματα:

α) ποια θα είναι η θέση του Τροπικού Κυκλώνα Sandy στις 0300 UTC;

β) ποια είναι η βαρομετρική πίεση στο κέντρο της Sandy και ποια ήταν η πορεία της το προηγούμενο εξάωρο καθώς επίσης και η ταχύτητα κίνησης του ματιού;

γ) ποια θα είναι η αυριανή της θέση στις 0000 UTC;

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Άσκηση 6: Να εφαρμόσετε τη μέθοδο των Lee Chesneau και Ma-Li Chen

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Στις επόμενες σελίδες που ακολουθούν παρατίθενταιαποσπάσματα από το βιβλίο Modern Marine Weatherτου David Burch, Starpath Edition (2008).

Παράρτημα Ι

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4.5 Tropical Storms and Hurricanes

Executive summaryIf your goal is to sail in a hurricane, you can indeed do just that. They are frequent enough, last longenough, and are located well enough that you could look up when and where they are, and then gothere and sail into one, or wait for one to come to you. These data are presented in Figure 4.5-1 andTable 4.5-1.

On the other hand, if your goal is to not sail in a hurricane, then you can equally well succeed at that.Look up the same information you did before, but then don't go there.

In one sense it is that simple, but in another it isn't. Hurricane distribution in time and space is asta­tistical one, and they do wander out of the norms. To strictly rule out the interaction withhurricanes would rule out a vast amount of ocean for many months. So if we are to do any sailing in ornear these zones, we are obligated to know the basics of their behav­ior in order to safely managetheir appearance in our space which is to imply we did our homework and have not willfully oraccidentally ventured into their space.

We are much assisted in this by the great amount of data that has been collected on these things overthe years, as well as by the large number of trained people whose very occupation it is to watch them(via satellite and other special observations), forecast their behavior, and report it over the radio.With any SW re­ceiver we can get current and forecasted data on their location, intensity, andmotion. The National Hurri­cane Center (NHC) is the main resource for this data in waters adjacent tothe US, and they also list the Regional Centers responsible for the other hurricane zones.

But we are still left with the task of interpreting this information usefully and taking proper actions.Remember that even when the thing itself is in clear sight visually, there is often still the chance thatprop-er maneuvers will greatly reduce its impact. And when it is still far off and approaching, theproper maneu-ver can have an even larger effect on minimizing the interaction.

Remember, too, that 150-knot winds and chaotic 50-foot seas are not the sort of thing you survivesolely with good seamanship, intelligence, and will power. It also takes a great deal of luck, which asprudent navigators we are reluctant to rely upon. It is the grav­est mistake to not maneuver awayfrom them when you have the knowledge and ability to do so. Shipboard forecasting of tropicalstorms is cov­ered in Section 9.3. Storm avoidance maneuvering is covered in Section 4.8. A relatedarticle on the danger-ous and navigable sides of a tropical storm is Section 4.7.

Tropical CyclonesThe term "cyclone" refers to any area within which the atmospheric pressure is low relative to itssur-roundings. Such systems are generally circular and exhibit counterclockwise circulation in theNorthern Hemisphere (clockwise in the Southern Hemisphere). Tropical cyclones are cyclonic stormswhich originate in the tropics and subtropics of both hemispheres. First appearing as minoratmospheric disturbances over warm oceanic waters, they can develop into the most destructivestorms on earth. Fully mature tropi­cal cyclones are relatively rare compared with other types ofstorms, but they are exceedingly violent and must be regarded with the utmost respect. Avoiding atropical cyclone should be the primary consideration for any mariner planning or conducting a voyagethrough areas where such storms occur.

Mature tropical cyclones are known by various names throughout the world: in the Atlantic andEast­ern North Pacific they are called hurricanes; in East Indian and Japanese waters they are calledtyphoons; in the Indian Ocean they are called cyclones; off Aus­tralia, they are called willy-willies; andoff the Phil­ippines, baguios. In this section, we generally refer to them as tropical cyclones, orhurricanes. Regional names may be used when referring to a specific locality.

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Figure 4.5-2. Formation of a tropical depression. Topshows normal trade wind flow meeting at theIntertropical convergence zone (ITCZ), wh ich istypically just north of the equator , but close enoughthat no Coriolis effect occurs the Coriolis force is zeroat the equator and increases with strength at higherlatitudes. This normal flow can be distorted two ways.A longitudinal trough moving west called aneasterly wave can sh ift the ΝΕ wind to the Ε and SEas shown. They bring clouds and rain for 2 to 3 daysas they pass, travelling at some 9 to 18 kts. Gustywinds can occur as the trough line goes by Some60% of trop ical cyclones can be traced to theinfluence of an easterly wave, but as many as 85%of strong hurricanes start th is way. Easterly wavesare often visible on satellite photos and alwaysclearly marked on trop ical maps. They appear aswaves in the trade winds with a wavelength(distance between them) of about 1,000 nmi.Another distortion of trade wind flow can come fromthe random oscillation of the ITCZ (doldrums). It doesnot remain at a fixed latitude but waves about inirregular patterns in response to local disturbances.When th is region bulges far enough north, as shown,the Coriolis force starts to kick in and bend the windto the right.When an easterly wave meets one of these northernexcursions of the ITCZ conditions are set up forformation of a circular low, as shown on the bottom.Once th is Low starts circulating, if the water is warm(80°F or more), and the air unstable enough, withthe right winds aloft, conditions are right for thenewly formed Low to deepen into a tropical storm andperhaps hurricane.

Tropical cyclones are one variety of cyclone. The other major group of cyclones includes those whichoriginate outside of tropical and subtropical regions and hence are called "extratropical cyclones."The familiar "Lows" of temperate regions, such as the winter storms of the North Pacific and NorthAtlantic Oceans, are exatropical cyclones. While both varieties of storm share the traits of low centralpressure and cyclonic winds, there are important differences between them. Properties of each arecompared in Table 4.4-1. The most important distinguishing feature of tropical cyclones is theconcentration of large amounts of en-ergy into a relatively small area. For the mariner, this translatesinto very high winds and heavy seas.

Origin and Development of Tropical CyclonesThere is at present no single theory that adequately accounts for the complex origin and behavior oftropical storms. There are, however, large amounts of observational data that scientists and sailorshave used to determine the general conditions un-der which tropical storms develop. Mature tropicalstorms require very specific atmospheric and oceanic conditions. Even when the proper conditionsexist, such storms, more often than not, do not form. Only 10 percent of tropical disturbances, thefirst stage of a tropical cyclone, eventually develop hurricane force winds.

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Hurricanes form only over warm water. A goodrule of thumb is the water must be at least 80°Ε. The intensity of hurricanes increases as thewater temper­ature increases. Hurricanesderive their energy from the latent heat ofcondensation released as water vaporcondenses. As long as the center remains overwarm water, a hurricane has a limitless supplyof heat energy. This heat energy is convertedinto the kinetic energy of motion, which iswhat drives the winds. Since hurricane pathsoften follow that of warm water, weather mapsshowing water temperature could be valuableguides to predicting their paths.

Hurricanes form most commonly in latesummer and early autumn when tropicalwaters are at their warmest. When planningvoyages, remember not to overlook the basicfact that the seasons are reversed in theSouthern Hemisphere.

Most hurricanes form between 5 and 25o latitude, and even more particularly between 6 to 10o

latitude. Hurricanes do not form within 3° of the equator be­cause they require the Coriolis effectto initiate cy­clonic circulation and the Coriolis effect is insignifi­cant close to the equator.

Hurricanes always develop from a pre-existing dis­turbance in the tropical atmosphere. Thisdisturbance may be moving, but only slowly, less than 10 knots or so. Obviously when traveling inthese waters we should be alert to any such disturbance to the east of us. The probability is lowthat it evolves into a storm or hurricane, but it does have that potential at certain times of theyear.

Tropical storms generally develop in a region of below normal pressure (less than 1004 mb) in lowlatitudes. Heavy rain or rain showers are usually pres­ent.

The Intertropical Convergence Zone (ITCZ) is that belt of light winds, showery squalls, andascending air currents which lies between the converging ΝΕ and SE trade winds near the equator.Commonly known to mariners as the doldrums, the ITCZ exerts a primary influence on all weatherin the tropics, and is an important element of tropical storm formation. When the ITCZ migratesacross the equator, the trade winds follow it. Those winds are then subject to the Coriolis effectwhich can transfer sufficient spin to the converging winds to cause the formations of a largevortex. If such vortices are reinforced and sustained, they can develop into storms and hurricanes.

Figure 4.5-4. The 1-2-3 Rule refers to the NHC forecastposition uncertainties of 60-120-180 nautical miles at 24-48-72 hours, respectively. The danger area to be avoided isthe gray area inside the indicated cone, which is definedby the forecasted radii of winds over 34 kt, extended by asafety margin derived from ten years of forecasted trackerror analysis. This is similar to the uncertainty pattern theNHC prints on their projected storm tracks on theirwebsite, but is not quite as conservative. They use 100, 200,and 300 nmi, based on evaluations from 1989 to 1999.Looking at their achievements since then, the predictionsare better, hence this new form of the Rule (Lee Chesneau,private communication). It is easy to understand and toplot the boundaries from wireless predicted positionreports.

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One of the situations in which a hurricane is most likely to develop is the conjunction of an easterlywave with an ITCZ that bulges across the equator, as shown in Figure 4.5-2. Easterly waves are troughsof low pressure moving west with the trade winds. They can impact a cyclonic pattern to the ΝΕ trades,which can be reinforced and closed by the deflected trades. A closed cyclonic circulation is the crucialstep in or-ganizing a tropical storm.

Hurricane formation also requires the wind to be vertically uniform throughout the atmosphere. In theeasterly trades, hurricanes do not form where winds aloft are westerly. Even when they are easterly, ifthey are very strong they will not be uniform with the surface flow. Near Hawaii, for example, thetrades are northeasterly, but the winds aloft are typically westerly, so this is an unlikely place forhurricane formation. Hence with maps of the winds aloft available, one could probably improve on thestatistical estimates of the probability of hurricane formation from tropical disturbances. The steadinessof the winds, and hence hurricane formation, is enhanced where the air temperature near sea level isrelatively constant over a wide area.

Tracks and MovementNo two tropical cyclones tracks have ever been identical. They do, however, tend to follow trends whichmay help in predicting their future position. Typical tracks are shown in Figure 4.5-1. The best source ofrecent tracks for specific storms, regions, or seasons is the history section of the NHC website. A printedsummary (including extratropical systems) is given in the Mariner's Weather Log.

After formation, while still in lower latitudes, the tracks of tropical storms tend to be sinusoidal. That is,they swerve back and forth, while maintaining a generally westward (and somewhat poleward)direc­tion. During this period, their speed is slow, some 4 to 13 knots, and erratic. They may stop andstart in fits, and may even go backwards for short intervals. See Figure 4.5-3.

At some point in their westward progress, tropi­cal storms usually "curve" poleward. After they curvepoleward, they tend to "recurve" on around to the east that is, to a poleward track with an easterlycomponent. This happens when they come under the influence of the westerly winds aloft. During thepro­cess of recurvature, the speed of the storm is usually very slow (2 to 8 knots) and erratic.

After recurvature, the track of a tropical cyclone is much straighter and its speed increases dramatically- some 15 to 20 knots typically, but speeds as high as 40 or 50 knots have been observed. Since it ismoving poleward, it may rapidly leave the area of warm seas from which it draws it energy. The stormwill then begin to decay, though it can continue to carry potent winds and precipitation. At this stage itcan evolve into an extratropical cyclone.

Figure 4.5-5. Cross section of an intense hurr icane. Notwo hurricanes or typh oons are th e same, but th ey dohave similar general properties. Th is pic-ture isinten ded to repr esent average values for a largesystem. Note th e correlation in clouds, squalls, wind,and barometer wh ich are sh own here verysch ematically.This one shows winds at 100 miles off of some 50 to 70knots; in smaller such sys-tems, wh ich may be moretypi-cal (peak wind 100 kts), the winds could be as lowas 40 knots or so at 80 to 100 miles off, wh en th esystems are still below some 20° latitude. At high erlatitudes after recurvature th e region of strong windsoften expands considerably.The very strong winds are concentrated near the wall cloudwithin some 50 miles of the center. keep alert early and putas much distance as possible between you and the center.The time scale on the pressure trace assumes a storm motion of some 300 miles in 24 hours (a relative speed of 12.5 knots). The dotted linealong the clouds represents roughly the outline that would appear as the cloud bar on the horizon. This could be in clear sight while conditionsat your position are still relatively normal, with still adequate time to maneuver out of the path of the most intense winds and seas. Withinthis schematic picture, once the barometer starts down (5 mb below the normal for the region, season, and time of day, after perhaps a slightrise) then it is clear sign of the presence of a tropical cyclone, even if a cloud bar is not discernible.

When attempting to project the future course of a tropical cyclone, keep in mind that superimposed onthe general behavior discussed above, there are more than a dozen generic patterns to these tracks thathave been identified over the years, including many twists, zigzags, loops, and so forth. It is unlikely onecould out-guess them. Wireless reports of forecasted positions from the NHC are the best bet. These getmore accurate every year, but they still strongly recommend a11 mariners keep in mind what they callthe 1-2-3 Rule for estimating the uncertainty in the forecasted positions. See Figure 4.5-4.

The only thing one might count on when plan-ning routes around these, or to minimize likely contactshould one develop, is the bigger picture that they move generally west and then curve poleward. Theyrarely move toward the equator.

Classification of Tropical CyclonesIn successive stages of development, the tropical cyclone may be classified by form and intensity. Thefour primary stages are known successively as tropical disturbance, tropical depression, tropical storm,and hurricane or typhoon.

Tropical disturbanceA tropical disturbance is a migrating system of convection, generally 100 to 300 miles in diameter. Itdoes not have fronts and may or may not alter the local prevailing winds. It does not have strong windsor closed isobars i.e., isobars that completely enclose the low. To be classified as a tropical disturbance,a system must have maintained its identity for 24 hours or more. Only 10 percent of tropicaldisturbances develop into mature hurricanes.

Figure 4.5-6 P h otograp h of Hurricane Klausin 1984, cour-tesy of Ch arles Brown, wh owas tak ing part in NOAA's VoluntaryObserving Ship pro-gram at th e time he tookthis ph oto less th an 200 mni from th e wall ofth e storm, uniquely visible in th e picturecentered beh ind th e headland on th e left.On th e righ t are several squalls. Notice howcalm th e water is at th is distance off andhow clear th e sk y. Klaus Lasted from Νον 5 to13, with p eak wind s of 80 kts at 980 mb onΝον 9.

Tropical depressionThe next stage in the development, a tropical de­pression, has one or more closed isobars, some rotarycirculation at the surface, and sustained wind speeds Lip to 33 knots. They can be spotted in satellitephotos of clouds, and sometimes by QuikSCAT wind patterns even when not noticeable in the clouds.

Tropical depressions are assigned a number to aid in tracking and disseminating information aboutin-dividual cyclones. The regional hurricane center as-signs the numbers, beginning with the number"01" at the start of each new calendar year. The label may also include a letter. The label is used onadvisories until the depression develops into a tropical storm, at which time it gets a real name and thenumber is no longer used on maps. A tropical depression has a roughly 70 percent chance ofdeveloping into a tropical storm, but there is only about a 30 percent chance that a tropical depressionwill develop into a hurricane.

Tropical stormA tropical storm has closed isobars and a distinct rotary circulation. Sustained winds over 34 knots butstill less than 63 knots. There are about 100 tropical storms formed on a typical year worldwide. All

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tropical storms are assigned a name. A tropical storm has about a 70 percent chance of maturinginto a full fledged hurricane.

Hurricane or TyphoonA hurricane has dosed isobars, strong and very pronounced rotary circulation, and sustained

wind speeds of 64 knots or higher. Hurricanes are the most well-structured products of the earth'satmosphere. They are, in essence, huge heat engines that convert the thermal energy of warmwater into the kinetic energy of wind and waves. And despite the fact that they are a major threatto coastal towns and to unfortunate mariners that happen into their path, they are in fact a crucialelement of the earth's heat distribution process. Though always different in details, they sharecertain common features, described below.

WindTo be classified as a hurricane, a tropical storm must have maximum sustained winds of over 64

knots. Many hurricanes, however, have much higher winds as do, by the way, many extratropicalstorms. Super typhoons of the Western North Pacific have sustained winds as high as 140 to 150knots at some time during their lifetimes. To make matters worse, gusts in a hurricane may exceedthe sustained winds by as much as 30 to 50 percent. Gusts in a 150-knot hurricane could conceivablyreach 225 knots.

The wind in a hurricane crosses the isobars at an average angle of 20-30 degrees, with the angledecreasing nearer the center. At the wall cloud of the eye, there no longer any inflow and the windsare tangen­tial to the isobars. Typical wind patterns in an intense cyclone are shown in Figure 4.5-5.

In the Northern Hemisphere, wind speeds are typically greatest on the right side of a tropicalcy­clone looking in the direction of movement. In this sector, the forward movement of the storm isadded to its internal wind velocity, resulting in a higher net wind. On the left side the forwardmovement of the storm is subtracted from the observed wind velocity. In the Southern Hemisphere,the left side of the storm has the more powerful winds. The distinctions between the right and leftsides of a cyclone, however, are much more significant with more complex origins than coveredhere. See Section 4.7 for details on the sides of a storm.

PressureThe high winds of a hurricane are the result of very low pressure at the center of the storm

compared with that in surrounding areas. In simplest terms, this low pressure results as warm airrises out of the center of a hurricane. The intense winds occur as outlying cooler air rushes inwardto replace the warmer ascending air. Section 2.1 on wind has a more specific discussion of wind flowin response to pressure patterns. There is also a detailed discussion of pressures near tropicalstorms in the onboard forecasting discussion of Sec­tion 9.3.

The barometric pressure at the center of a typical hurricane may be 6O or 70 millibars below themean value well away from the storm. Very intense storms can have pressures 100 millibars lower.Because the wind strength is related to the difference in pressure over a horizontal distance (thepressure gradient), the pressure at the center of a storm is a measure of its intensity.

A formula for estimating the maximum sustained winds of an intense tropical cyclone is:Μax Wind = 16 kts x Square root [dP],

where dP is the pressure difference in mb from the periphery to the center of the storm. Ahypothetical barograph trace for a typical hurricane is shown in Figure 4.5-5. In that sample, theperiphery is at 1010 mb and the central pressure (Pc) is 930, so dP = 80 mb, which would give a maxwind of 143 knots. Another way to express this result is 15 kts x Square root [ 1010-Pc], where justthe central pressure is used along with an average. This gives 134 kts.

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The EyeThe center of a hurricane possesses a feature found in no other meteorological phenomena on earth,the "eye" of the storm. The eye is a circular or elliptical area, 10 to 20 miles in diameter, of relativelylight winds and clearing skies. The eye is often depicted as a welcome brief respite for those caught ina hurricane, but this is a seriously flawed conception. Waves within the eye are mountainous and veryconfused, as they are converging from a11 surrounding parts of the storm. As the eye passes, thewinds return with their previous ferocity, but from the opposite direction.

Wall cloudSurrounding the eye of a hurricane is an immense structure of rapidly ascending and rotating air calledthe wall cloud. The wall cloud is the area of most violent winds and heaviest precipitation in the entirehurricane. Depending on eye diameter, the wall cloud zone extends some 5 to 30 miles beyond theeye.

Size of hurricanesBased upon the extent of their associated clouds, winds and barometric pressure perturbations,tropical cyclones average 400 to 500 miles in diameter. This is the size of the cirrus pattern that wouldbe seen in a satellite photo. In weather discussions and publications, however, the diameter of ahurricane can be expressed in several ways:

(1) the area of hurricane force winds (> 64 Knots)

(2) the area of tropical storm winds (> 34 knots); or

(3) the diameter of the outermost closed isobar.The latter is well defined and clear in surface mapsand roughly coincident with the circular pattern ofcirrus clouds seen in satellite photos. The forecastdescriptions of tropical storms, on the other hand,provided in voice and text forecasts, are very specificand unambiguous. They tell the winds in the fourquadrants about the storm along with the radii of thevarious quadrants. Samples are given in Section 7.3on reading weather maps.

When a storm first reaches hurricane intensity, thediameter of 64-knot winds is relatively small (120miles). But as the storm ages, it expands laterally. Atypical Atlantic hurricane has an area of hurricaneforce winds of about 100 miles in diameter, with galeforce (34 to 64 knots) winds over an area up to some400 miles in diameter. Super typhoons of theWestern Pacific may have hurricane force winds to aradius of some 300 nautical miles or more, with galeforce winds spanning an area over 1500 miles indiameter. Tropical storms in other oceans, however,are generally much smaller than this. Record sizeswere illustrated in Figure 4.1-1. A cross section οftypical hurricane dimensions is shown above inFigure 4.5-5. Remember that "tropical storm winds"(34 to 63 kts) is a different definition than "stormwinds" (48 to 63) used outside the tropics.

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Eventually, the storms slide further poleward and out of the trades. The path of any individualstorm is influenced not only by the trade winds and westerlies, but also by the circulation aloft, whichitself is related to the presence of high or low pressure areas in the region. A strong High in the NorthAtlantic, for example, may block the recurvature of an Atlantic hurricane to the ΝΕ and force it ontothe North American mainland.

The average life span of a hurricane is about 10 days, from its recognition as a tropical disturbanceto it death. A hurricane dies after it leaves the region of warm waters and moves over land or overcolder waters. If the hurricane remains in tropical waters, it can persist indefinitely; a few hurricaneshave lasted over a month. But sooner or later they get blown away from the tropical waters. Section4.4 discusses the life of tropical storm (briefly a hurricane) Alberto that traveled all the way from theYucatan Channel to the UK in June (10 to 19) of 2006.

There are sometimes two or more hurricanes pres­ent simultaneously in one region. If the twohurricanes approach each other, they may revolve around one another, with the smaller hurricanemoving more rapidly known as the Fujiwara effect.

4.7 Sides of a Tropical StormMost storms are asymmetric to some extent, having conditions more severe on one side than the

other. But when it comes to practical matters of what we are going to do about it, the subject ofassymetry is pretty much restricted to dealing with tropical storms. Tropical systems are relativelysmall and well local­ized, with relatively well predicted short term tracks, so in many cases they can bemaneuvered around to great advantage, because the left right distinctions are much morepronounced in tropical systems. It would be rare that any smaller vessel could maneuver to one sideor the other of an extratropical storm spread a thousand miles across the ocean.

On the other hand, modern weather routing ser­vices with good long range forecasting andknowl-edge of individual vessel performance data can direct the routes of fast deep sea vessels toavoid the worst conditions, even in extratropical systems. This type of ship routing can even be moregenerally useful with these larger extratropical storms than they are with tropical storms thoughusually the gain is mainly from getting to the side that puts the wind and seas aft or reduces them,and not so much an issue of the distinctions in severity on the two sides. Weather routing is discussedChapter 9.

Figure 4.7-1. Sides of a storm. Winds and seas are big-ger on the right hand side, and first winds of the storm as they approach tend topush you into the path of the storm. The Buys-Ballot law locates the low center. back to the surface wind, left arm out, and slightlyforward, to point to the Low center

ReviewHurricanes are steered by the winds of the upper atmosphere. As they develop, they move slowlyaway from the equator, curving toward the pole when they enter the easterly trade wind belts. Whilein the tropics, their path can be very erratic. They may even stop and reverse direction. Occasionally,a vessel may be struck twice by the same hurricane.

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Thus the discussion of this section is essentially limited to tropical systems. 1η the NorthernHemisphere, wind and sea conditions on the right hand side of storms (called the "dangerous" side orsemicircle) are more severe than those on their left hand sides (called the "navigable" semicircle). Inthe Southern Hemisphere, the situation is reversed. Right and left are defined as when facing in thedirection of motion of the storm center as shown in Figure 4.7-1. There are several reasons that leadto conditions be-ing different on the two sides, having to do with wind speed, wind direction relativeto the storm's path, the sea state produced, and to its general location on the globe. In this sectionwe look at these factors, and how you determine what side you are on. The next section outlinesways to increase your distance from a storm center.

In the Northern Hemisphere, storm winds cir­culate counterclockwise around the center of a Low.Thus, when facing in the same direction the storm is moving, the left side has wind blowing aft, theright side has wind blowing forward (Figure 4.7-1). If the storm were stationary, the wind speed oneither side would be about the same if we sailed into it. Once the storm starts to move, however, thisis no longer true.

Consider a cylinder of still air, the size of the storm, moving east at 20 knots. If this moving airpassed over you, you feel a west wind of 20 knots, regardless of what side of the cylinder you are on.A 20-knot west­erly, after all, is just air moving east at 20 knots.

Now consider this cylinder of air a revolving storm with winds circulating counterclockwise at 30kts. While the storm remained stationary, there would be a 30-kt easterly north of the center, with a30-kt westerly south of the center. When the storm starts to move east at 20 kts, however, its ownmotion adds to the surface winds it carries with it. South of the center, we get 30 + 20, or 50 knots ofwesterly, whereas north of the center we get 20 knots of westerly + 30 knots of easterly, whichleaves a 10-kt easterly. In this idealized example there is dramatic difference in wind between rightand left side of the storm, especially when we recall that the force of the wind is proportional to thewind speed squared-the force of the wind is 25 times stronger on the dangerous side in this(idealized) example.

Thus, in the Northern Hemisphere, a storm's mo­tion adds wind to the right side (dangeroussemicircle) and reduces the wind on the left side (navigable semicircle) the vector addition of winds,however, is never quite so complete as indicated in this example at the peak location right on thebeam of the storm. The overall flow pattern of the wind is more complex and under other influences,so the distinctions, though still very important, are not this drastic.

Another independent factor can enhance wind speed on the right side in some cases. Tropicalstorms tend to move westward through the northern tropics (contrary to the direction of midlatitudestorms that move easterly), which leaves the subtropical Highs on their right hand side. As the stormsapproach the isobar pattern around a near stationary High, their own isobars are compressed givingrise to stronger winds on the side closest to the High (Figure 4.7-2). A simi-lar type of isobarcompression takes place when a High moves up against a stationary thermal low over land. Aprominent example on the California coast was mentioned in Section 4.1. Thus there is thisadditional factor that can contribute to winds on the right in some cases. The situation is exactlysymmetric in the southern Hemisphere.

The name "dangerous side," however, usually has more to do with wind direction than windspeed. When a storm approaches so as to leave you on its dangerous side, its increasing winds tendto push you into the path of the storm (north or south of the equator), whereas on the navigable sidethe build-ing wind pushes you away from the path of a storm. Keeping in mind that tropical stormsand hurricanes are often relatively small in size, this wind direction can be the dominant factor inyour overall experience of the event. Recall that just some 80 to 100 miles from a typical hurricanecenter that might have 100 kts of wind, could be down to 40 knots or so. Sailing against the wind inbig seas on the dangerous side can mean no progress at all, versus accelerated progress away from iton the navigable side.

Still another factor is tied to typical paths of the storms themselves. They move generally westwardin both hemispheres, and the tend to curve poleward rarely do they move toward the equatoronce formed. Hence the dangerous side is also the side the storm is likely to turn toward as itprogresses. It is, however, difficult to stress any tactical advantage to this propensity. First, themotions are erratic and only statistically go poleward, and second it could wrongly imply somevirtue in crossing over an anticipated path based on its likelihood of curving to the right.Remember, when it comes to actually meeting the storm, distance from its center is the key issue.It would be hard to reckon giving up good separation for being on one side or the other when itactually passes. Most experts do not recommend crossing the path of a tropical storm if there isany risk in losing CPA (closest point of approach). See Section 4.8. Forecasted behavior and theNHC's 1-2-3 Rule for predicting the storm po­sition uncertainty are key components to the analysis(Figure 4.5-4).

Finally there is the all important state of the sea. Winds on the dangerous side are stronger, so theymake bigger waves. They are also bigger on this side because those made in the aft right quarterare the least affected by the changing wind direction as the rest of the storm moves over them. Ineffect, waves on the dangerous side have a larger fetch. Waves built by tropical storms andhurricanes reach phenomenal heights, so any maneuver to avoid the worst of them by staying onthe navigable side, is crucial to the best handling of the situation.

How you might be able to take any of this knowl-edge into account in negotiating specificsituations, however, is another matter. This clearly depends on where you are relative to thestorm, how big it is, and most important how fast you can move in the prevailing wind and seas.Section 4.8 does not offer any more advice on this, but it does show how to maximize theseparation once you make your choice.

Figure 4.7-2. How an interaction of atropical storm with a subtropical Highcan increase the gradient on thedan-gerous side in either hemisphere.The same effect can persist when atropical storm heads north along theGulf Stream with the Atlantic High onits right.

A key issue in choosing your best route will al-ways beknowing precisely where you are relative to the storm'spresent location and to its forecast track. Storm data areavailable hourly on even the most ru-dimentary shortwaveradios (Section 1.6) improve reception with a long wire foran external antenna. The storm position is given indecimal decrees along with its true course and speed. It'sbest to keep an ongoing plot of the storm location. Use asmall scale plot to show both you and the storm, and thenmaybe a larger scale plot of the storm region so you canplot the course of the storm and then subsequentpositions to see if it is doing as predicted or to watch howits course might be changing.

If you get caught in the fringes of the storm circulation orhave lost your contact to the forecasts, then you have theclouds, barometer, and wind to watch (Section 9.3). Theclouds and barometer and maybe sea state can tell you itis coming, but it is the wind that tells what side you areon, or more specifically' how the wind shifts. In theNorthern Hemisphere, on the right hand side it shifts tothe right, and on the left it shifts to the left. But these arenot jingles to remember and apply. There is too much atstake to make a mistake (that's a better jingle). Draw acircular low and mark its direction of motion and thendraw in the wind arrows to be sure it all makes sense. Notethe wind arrows on the leading edge the Low. The side withwind pushing you into the path is the dangerous side. 72

Figure 4.7-3. Sides of a storm. Winds and seas are bigger on the right hand side and the first winds of the storm as they approachtend to push you into the path of the storm. The wind shift history (veer or backing) establishes your location within the circulationpattern. Must use true wind only (stopped or correction for your motion) and away from squalls to record the trend.

If you are on the dangerous side, the wind veers as shown in Figure 4.7-3. If it is not clear which shiftis α veer, put the arrow heads together (as shown) and you will see if the shift is to the right or left.Remember these shifts all refer to true wind direc­tions, so it is crucial to convert the apparent windto true wind to correct for your motions (Chapter 2), or stop completely to get good data on the truewind direction and speed. Also these are expected shifts when within the organized circular flowaround the fringes of the Low. Outside of that range, the wind and wind shifts could be doinganything, so the plot of positions is the main source of relative locations. Also bear in mind that if it isα well developed hurricane or intense tropical storm, then that system will be circled by rings ofsqualls, or squall lines, and these bring typical squall winds related to the squalls themselves, so youhave to be well away from these to evaluate the wind of the main system. If you have to startrunning before a local squall, keep in mind what direction you would ideally be going to keep awayfrom the main system. You may have the opportunity to benefit from such α squall, or may be betteroff to heave to and avoid losing ground on the direction you want to go.

If the wind direction remains steady, but the ba­rometer keeps dropping and the wind keepsbuilding, then you are more or less right in line with the storm's path. This is a crucial observation. Itis time to make way as best you can toward the navigable side as out­lined in the next section.

4.8 Storm Avoidance ManeuveringIf you know a tropical storm is posing a risk of col­lision some days or distance off and you are notyet within its circulation, then the safest thing is to go out of your way to avoid the encounter. TheNHC and other Regional Specialized Meteorological Centers (RSMC) give storm track predictions thatcan be used to guide your choice of route, which will change as the storm's path changes.

Besides all of the standard weather sources, tropi­cal storm location, course, and speed, are alsogiven in the storm warnings broadcast each hour at h+8m on WWV (male voice) and h+48m onWWVH (female voice), available on portable shortwave radios (see sidebar at the end of Section1.6).

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Figure 4.8-1. CPA on perpendicular course (left side) versus optimum course (right side). A vessel traveling at half the speed of thestorm has an optimum course that is 30° forward of the perpendicular (Figure 4.8-2). In this case the CPA occurs as the stormcrosses its stern, at time Τ4, whereas on a perpendicular course (left side) the CPA occurs much sooner (Τ3) and it is smaller than onthe optimum course. Storm is moving straight up the diagram. The vessel departs from the storm track (at ΤΟ) at the place wherethe storm will be at Τ4.

If you do get caught on the fringes of its circula­tion, then the goal is to act quickly to set a course thattakes you as far away as possible from the storm cen­ter as it passes. It is important to do this earlywhile the wind and seas let you make good progress. A first step is to carefully establish where You arerelative to the storm center and what side of its projected path you are on. A running plot of the storm'slocation is key to keeping track of its motion.

Figure 4.8-2. Storm avoidance Maneuvering. To putth e most distance between yourself and anapproach ing trop ical storm or to ach ieve maximumCP A with another vessel or squall h ead forward ofth e perpendicular to the target's true path by anangle alph a (a), wh ere a depends on th e ratio ( R ) ofyour speed in present conditions (SOG) to th e stormspeed . If your speed is 12 kts and th e squall's speed is19 kts (R = 0.63), ch oose a = 37°, or interpolate for amore accurate value of 39°, or if you have a trigcalculator compute the angle alph a: a = arcsin (yourspeed/ storm speed).DL and TL are the distance lost (in CP A) and time lost(in time to CP A) if we choose a perpendicular cour seversus the optimum diagonal course. In the examp leabove, our CP A would be 17 per cent smaller and thetime to the CP A would be 36 per cent shorter if wewent perp endicularto th e path rather than forward by the optimumamount. W h en on the op timum heading, the CP Aoccurs when the squall crosses your ster n, wh ich isnot th e case on any oth er heading.

Figure 4.8-3. Storm avoidance plot. A storm isrep orted 200 nmi off in direction 170, moving towar d320 at 19 kts. Your max speed is 12 kts. Findmaximum CP A and time to CP A. This is the examp legiven in Bowditch that they solve by maneuveringboar d meth od.The optimum cour se is 39° forward of th eperpendicular to the storm track (Figure 4.8-2),wh ich is 320+90 = 050 and 050-39 = 0ll. Plot thestorm location at 200 nmi off (A-Β) using anyconvenient miles scale. Draw in th e storm track (8-C)and proj ect your course back (A-D) to see where thestorm crosses your stern. Time to CP A is how long ittakes the stor m to get fr om 8 to D or Τ = (8-D)/19k ts.The value of the CP A is distance D-A plus the distanceyou travel in th at time, or CP A = (D-A) + 12 x Τ Plotth is out at a scale of your ch oice to lear n that CPA =187 nmi and time to CPA = 4h 52m.

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The best route to take to maximize your separation depends on the storm's speed and yourmaximum speed and, of course, on the storm track, which is likely to change, maybe even frequently.The optimum course can be determined from a plotting tech­nique using a maneuvering board that isexplained in Bowditch (Pub. 9, American Practical Navigator), but it is not a very intuitive procedure,unless you work a lot with relative motion diagrams in radar.

To simplify the solution we have made a table and procedure that tells you the course that willmaximize your distance from the storm as it goes by. It is easy to use. It also tells you the benefits ofchoosing the optimum course.

Guidelines to keep in mind are if you are faster than the storm your best route is just to run awayfrom it on a reciprocal course; but if you are very slow compared to the storm, the best route is oneperpen­dicular to the storm's track. If you are faster you have more freedom to choose yourpositioning; much less so when you are slower.

For any speed lower than the storm speed, the op­timum course is somewhere forward of theperpen­dicular route by an angle that depends on the ratio of your two speeds. By heading forward ofthe per­pendicular you give yourself more time to increase the separation from the track of thestorm. If you take the optimum course, the closest point of approach (CPA) occurs when the stormcrosses your stern (Fig-ure 4.8-1). On a perpendicular course, the CPA occurs well before the stormcrosses your stern, so you give up much in both the CPA and the time to CPA.

To figure optimum course, estimate your speed in present wind and sea conditions (SOG) and dividethat 6γ the speed of the storm, and call that ratio R. Then use the table in Figure 4.8-2 to look up thevalue of the angle a, which is how much you point forward of the perpendicular.

The table in Figure 4.8-2 also shows the advantages in time and CPA of choosing the optimum course.Ev-ery mile counts when interacting with well localized tropical storms and hurricanes. Also note thateven though the CPA of the optimum course is not always increased a lot over the perpendicularcourse, the time to CPA is increased significantly. The more time you have, the more the chances arethe storm will deviate to a course that gives you more separation, besides giving more time to prepare.Notice that the closer your speed is to the speed of the storm, the more you might benefit from anoptimum diagonal course.

This shortcut solution gives the optimum course to take but does not offer numerical values of the CPAand time to CPA. The maneuvering board solution does offer this data if done properly, but it seems abetter solution to plot the routes on a chart or universal plotting sheet and just measure the valuesyou want directly, as outlined in Figure 4.8-3.

This plot should be ongoing in any event so you can update your choice of course and monitor yourprogress. An e-chart program is a convenient way to make the plot and analyze it. Draw the lines asroutes between waypoints, if the program does not offer a specific way to draw range and bearinglines.

Needless to say, every circumstance in such conditions will be unique and no fixed proposal is theanswer to all the challenges that might arise. This pro­cedure just provides a way to figure the routethat would offer the best separa­tion if that is what it boils down to. There are numerous books onheavy weather seamanship that describe more specific actions to consider in these conditions.

Remember this maneuver only works when done early enough that you have some choice in route.Once the storm gets closer, your options for increasing separation and minimizing contact with thesystem are more limited.

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Figure 4.8.4. Five-day path and wind speed projections for Hurricane Katrina and tropical storm Isaac respectively, from the nhc.noaa.gov.

NHC ProjectionsThe National Hurricane Center of-fers remarkably accurate prediction for the locations and windspeeds of tropical systems. (Ι am sure they would not put it that way and they are continually striv­ingfor more improvement, but the fact is the predictions are usually very good and they are very easy touse and interpret.) The data are available from the website nhc.noaa.gov. As soon as clouds startmoving in a circle or QuikSCAT winds show some organization, they are right on top of it. Thedisturbance is marked on a map and easily followed. They are also updated every 6 hours or so, ofnot more frequently. Samples of the predictions are shown in Figure 4.8.4.

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9.3 Onboard Forecasting of Tropical StormsTropical storms and hurricanes are certainly the best forecasted weather systems afloat, and the

main re­source for this data is the National Hurricane Cen­ter (NHC) in Miami, FL (nhc.noaa.gov). Assoon as clouds or wind starts moving in a circle (apparent in satellite photos or QuikSCAT data), there isa team of scientists watching its every movement and feeding these observations into varioushurricane computer models to predict its behavior. The models may not be as precise as one mightlike, but from a mariner's point of view, we are told every hour or so where the system is located and abest guess of what it is likely to do next. These predictions get better and better each year.

Figure 4.5-4 shows the "1-2-3 Rule" from the NWS, which is the key to prudent application of theirforecasts. Even if we do not have the forecasts, we should be aware of this diagram and how theposi­tion uncertainty increases. Again, however, it would be rare to not have the forecasts. Even aninexpensive shortwave radio brings the storm location, course and speed every hour.

Furthermore, these storms occur in specific places during specific seasons, which are welldocumented and readily available. The NHC also has links to the agencies (RSMC) that watch the otherhurricane zones around the world. Nevertheless, as with all weather phenomena, their occurrence andbehavior are spread over some statistical range of space and time, so despite precautions we mightend up in the neighborhood of one. And despite other precautions, we might also end up withoutaccess to the official reports, which puts us onto the topic at hand of doing the best we can to detectsuch systems and to monitor their progress relative to our own position and maneuvers.

Even with these good official reports readily avail-able to us when all our equipment is functional,we are dealing here with the most severe systems mari-ners might encounter, clearly life threateningto any vessel. But the appearance of such a system in our proximity does not at all foretell inevitabledoom. These are usually small systems that we can indeed maneuver away from. Just getting some100 miles away from the center can make a tremendous differ­ence in the force of the wind and seas.And since they are so, well tracked, we are, with prudence, more likely to get away from it than to getcaught right in it.

Considering such an encounter with a tropical storm after our training on how to avoid them, wenote that if luck comes in units that are statistically distributed between good and bad, then we are infor good luck when it comes to escaping it, since we have already had one unit of bad luck. That is, wewere prudent in our choice of route and season (based on our training), but still encountered ahurricane. Thus we should be lucky in our efforts to maneuver around it. On the other hand, if wewere not so prudent about where we are and when, and in what type of vessel, then we are notdealing with luck at all, we are deal-ing with the process of learning to respect the sea.

The key point to avoiding hurricanes within a hurricane zone is to treat each tropical disturbance asa potential hurricane although only some 10% of such disturbances actually develop to that extent.Start your reckoning immediately as to your chances of encountering it on your present course, basedon known behavior of the systems and then if you have any chance at all of running into it,immediately maneuver to avoid it as discussed in the next section, or go back if that is an option, or goto some other destination that might offer some shelter.

Keep in mind also that if you are anchored in some partial shelter (none are complete shelter forsuch systems), then your biggest hazard might well be other anchored boats breaking loose, crashinginto you, and tearing you lose. It is not uncommon to maintain power into the wind to relieve stress onthe anchor cable during the encounter. It is prudent to look around to the extent you can to see thatothers are being as careful as you are.

Once the thing has developed into a tropical storm or hurricane, it does not move around verysecretively. The very strong winds are tightly located, but the signs of these usually extend far ahead ofthe system. 77

The goal is to get as much distance as possible be­tween you and the passing storm and to go for thenavigable side if you have that choice. Proper action, however, cannot be taken until you know where thestorm is relative to you, and which way it is moving' For tropical storms, predicting the location andmotion of the storm are the only goals of forecasting you do not have to guess whether or not it will bebad. You also have to have some feeling of what progress you might be able tο make in the given andanticipated winds and sea state.

Some or all of the following signs will be evident as a tropical storm approaches.

SwellsAs mentioned for all storms above, this is even more prominent for tropical systems. The onset of a

new long low swell coming from the direction of the storm, provided you are in deep water, could be thefirst sign. Long swells are deep swells, so in shoal water they can be refracted from the slope of thebottom and change direction. This swell is typically the earliest natural sign, although the sea state you dohave might mask this in some cases. If you happen to be somewhere in deep water within some 1,000miles of a hurricane look for this sign as a learning experience, even when there is no threat from thatparticular storm. Alternatively, you might try virtual research by following the NHC Internet site to spotthe development of a tropical storm that they believe is going to become a hurricane, and then watch thewave and swell maps to see if you can detect any indication of the effect of the storm.

You might also find it interesting to zoom in on tropical island groups on Google Earth to see howprevailing smells refract around the islands though this is rather another topic.

BarometerHurricanes occur in the tropics, and often even more specifically in the region of the doldrums.

Careful barometer use in this region has an important difference from use at higher latitudes because ofthe relatively large diurnal (twice daily) variation of the pressure due to an atmospheric tidal action. Thetides are caused by solar heating and cooling, which creates 3 cycles having periods of 4, 12, and 24hours. The 12-hr cycle has the largest amplitude being in the range of 1 to 2 mb.

This daily pressure variation also takes place at higher latitudes, but it is weaker and usually maskedby normal variations in the pressure that accompany moving weather systems. Even in the tropics, it isstrongest near the equator and grows weaker with increasing latitude. The effect is stronger over landthan over water.

The peaks are usually at about 10 AΜ and PM local time, with minimums at about 4 AΜ and PM.(Local time of 10 AΜ means 2 hours before local apparent noon, when the sun is at its peak height in thesky. Four PM local time is 4 hr after local noon.) The effect of the 24-hr and 8-hr cycles is to make themorning peaks slightly larger than the nighttime peaks, while the afternoon minimum is slightly lowerthan the early morning minimum. Since the highs and lows are caused by solar heating, the range of thiseffect can depend on the local cloud cover and humidity.

The diurnal variation must be taken into account when monitoring pressures in low latitudes to watchfor approaching systems. A sample is shown in Figure 9.3-1. These days this type of data can be readilyob­tained from the NDBC.

Barometric pressure drops near an approaching hurricane are shown in Figure 9.3-2. See also thepressure profile shown in Figure 4.5-5, which extends out some 300 nmi. The following are signs thatmight be evident while still far enough from the storm to avoid its worst impact with a lot of assumptionshere about average speeds of storm motion and your motion, average changes in the actual centralpressure of the storm, etc.

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Figure 9.3-1 Diurnal pres-sure variation in thetrop-ics at Lime Tree Bay, Vl, 17.70Ν, 64.75W. Tofigure the proper local time, take the longitudedivided by 15 to get the time zone and then subtractthat from GMT In this case, 64.75/15 = 4.3 so Localtime = GMT - 4. Note they show it at 3h, whichmeans they must be on daylight saving time. Thepredicted local times are shown on the plot.Halfway points between the full range on each cycleshows how the actual ambient pressure is changingover these 5 days-slowly rising from about 1016.5to 1019.0 mb, with an indication of a down turntaking place. Note that the expected differencebetween morning and night time peaks (about 0.5mb) is masked on Apr 2 by the rise

in the ambient pressure over that period. To watchfor an approaching tropical storm, we need toextract an actual pres-sure drop from a pattern likeThis one. Knowing ahead of time how much of a change is due to diurnal fluctuations, lets you discover the real drop, which could be masked during its initial hours unless you correct for this. Using the NCDC resource explained at the end of Section 8.2, we can learn that the average for March at this location is 1016.7 with a standard deviation (SD) of 1.7mb. Thus this pressure of 1019 is about 2 SD higher than normal, making it a rare occur-rence (about 3% probability). There is related discussion at the end of this section.

Within the tropics one should understand the diurnal variation. With the exception of this dailyvariation, pressures in typical hurricane regions are generally fairly stable. Hence any deviation fromthe average pressure should be regarded with suspicion. With an accurate barometer and knowledgeof the average pressure for your month and location, along with the standard deviation of that averagepressure, you can be very specific about your observations of the pressures and what they mightimply, as explained in detail at the end of this section-which could be very valuable if you have lostwireless contact with your forecast sources.

The swells mentioned earlier might show up in the range of well over 500 miles off. At about 500miles away, we might expect the disappearance or ir­regularity in the diurnal variation of barometerread-ings. Also a slight rise in average pressure might be detected while still in fair weather somewherenear the edge of the cirrus clouds.

At about 300-150 miles away, we might detect a slow fall increasing from 1 to 2 mb/hr with perhapsthe diurnal variation still evident but fading.

Warning: once the barometer gets down to, say, 5 or 6 mb below the normal for the region, season,and time of day, (after perhaps a slight rise) then you have a clear sign of the likely presence of atropical cyclone, even if a cloud bar (discussed below) is not discern­ible. In other words, when sailing inhurricane zones, it is crucial that you record the barometer even in fair weather so that you can indeedlearn what is normal behavior and average values. Five mb down from av-erage is something like 2 SD,which means you have about 97% (100% - 3%) of this coming from a storm and not from typicalstatistical pressure variations.

Paying careful attention to the natural signs around you including your barometer, you are still in aposition to increase the closest point of approach significantly using the methods outlined in Section 4.8.Proper action at this stage could be crucial.

At about 120 to 60 miles away, a typical drop rate might increase to about 5 to 6 mb/hr. The role offorecasting is over, though you may still be able to run some, depending on where you are. At somepoint soon however, the barometer will undergo a real plummet of some 10 to 30 mb/hr. At this stageor even earlier, running from the system is no longer practical and the goal is to take whatever courseyou can to move away from its center and back away from its direction of motion. Your main goal now istry to not get closer and minimize the interaction time by sailing in the opposite direction it is moving.

Clouds, sky, and rainThe first cloud signs are similar to those of a warm front or extratropical Low. First signs are the

invasion of cirrus into a clear sky or high above the normal trade-wind cumulus of the tropics. Thesecirrus, how­ever, might very early show distinct bands radiating from the distant horizon.

If the storm actually comes your way, the cirrus are followed by cirrostratus, cirrocumulus and thenaltocumulus with a bank of black cloud (bar of the storm) on the horizon. The bar is made up of thicknimbostratus and regions of intense cumulonimbus squalls. See section 5.4 on clouds for samplepictures and descriptions.

Note that the whole thing, cirrus down to bar, might be clearly evident on the horizon while you arestill in fairly light air and moderate or calm seas. Such might be the case if one is passing you some 300miles off. See Figure 9.3-3 for a very schematic rep­resentation of the difference of cloud motions whenone is passing versus coming at you. Before the wall, which is when you enter heavy squalls andtorren­tial rain, you might have sporadic spells of rain, but could have otherwise reasonable weatherwith at least patches of clear sky. Figure 4.5-6 showed an example of calm water with a hurricaneclearly in view on the horizon.

WindCareful wind observations are also crucial to forecasting the location and motion of tropical storms.

These storms bring with them well defined, tightly circulating wind fields so the direction and change indirection tell us very clearly what side of its path we are on.

Warning: an increase in speed or change in direction of the trade winds, especially with an unsteadyor dropping barometer, is good indication that a tropical storm is approaching.

To encounter a hurricane, you will most likely be sailing in trade wind conditions before hand-E to ΝΕwinds in northern tropics, E to SE in the southern tropics. In the northern latitudes, the storm will likelybe coming out of the east bringing with it counterclockwise circulating winds. Its path can be erratic andthe actual approach to you could be from any direction, hence it is not dear what direction the firstwinds of the storm will actually be. But there will likely be 3 stages to the wind. The present normaltrade winds, a transition period with shifting winds or even calms, and the entrance into the actualcirculating wind field of the cyclone. Just building trade winds may be the first wind sign, but there areother transitions possible.

Figure 9.3-2 Typ ical pressure drops with an app roach ing hurr icane. The "4-5-6" means a drop of 4 or 5 mb in 6 hr.

Figure 9.3-3 (Very) schematic appearanceof a storm bar, coming toward you, left,and going by you, right.

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If you had access to real reports or maps, you could anticipate the behavior of the transitionwinds, but if making all observations unaided, this transition is uncertain. It will take placesomewhere about 300 to 200 miles from the storm center. In other words, typically at 200 miles off,you will be in the circulating winds of the storm still, in many cases, with plenty of time to get out ofits worst conditions.

To figure the direction to the storm, you must be within the circulating wind field of the storm, inthe new wind. You should get the new wind well before you get into the squalls. The first step is torecognize that you are in the new wind (the barometer would be starting a slow steady drop), andthen use this wind direction to figure the direction to the storm.

Use the Buys Ballot's law, fine tuned for tropical cyclones meaning with back to the wind and leftarm out, rotate it 22' forward to point to the low. In other words, the standard recommendation forinitial contact with cyclones is the storm center is 10 points (ll2') to the right of the wind direction. Ifthe wind is northerly at say 010, the center of the storm is lo­cated in direction 010 + 112, or 122°.

For confirmation, this is the direction from which we might have first seen swells appear (orroughly that direction), and this would be the focal point of the radiating bands of cirrus clouds, and ifa bar were visible, also the center of the black bar of clouds on the horizon.

Note that if you are already into the region of squalls, then you should use 9 points instead of 10(rotate left arm forward just 11°), and you would have to be very careful in evaluating the true winddi­rection since this is strongly influenced by individual squalls pulling wind into newly formed ones,and blasting it down and out from mature ones.

The next step is figure if you are to the right, left, or center of the storm's track toward you. Υοuneed this information to determine which way to go. This is determined by which way this new windshifts as the storm moves towards you, or past you.

Figure 4.7-3 showed these wind shifts. In the NH, if the new wind veers (i.e. 045 goes to 050 goesto 055) then you are on the right side of the storm. This is the dangerous side, which will ultimatelyhave the strongest winds and seas with winds tending to push you onto the path of the storm.

If the new wind backs (i.e. 045 goes to 040 goes to 035), then you are on the "navigable" left sideof the storm. On this side, getting away from the storm's track will be a downwind course.

Naturally, if you have good long range radar (ship class only, yacht radars will not help with this),and are close enough to see at least the squalls in the bar, then you might be able to get furtherindication of the storm's location and motion. The squalls the rain of which will show up clearly onthe radar screen will tend to move parallel to the isobars around the storm, so the storm location willbe perpendicular to that path. You must carry out a careful radar maneuvering plot, however, toextract this information. One way to do this is to do direct plotting using your GPS position and therange and bearing to the center of the squall. You may have 2 or 3 squalls that you can follow thisway.

Record your position, the time, and the range and bearings to all well defined squalls. Thensometime later, after they have moved or you have moved, do this again and plot all positions on auniversal plot­ting sheet or chart. Figure the course and speed of each squall from their first andsecond positions. Note that the squalls will be moving very rapidly, much faster than the storm itself.

Again, all these observations are of most help when combined with an official report of theloca­tion, speed, and course of the storm. The goal here has been to outline what might be donewithout this outside assistance.

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Another Good use for an Accurate BarometerAtmospheric pressures in the low latitudes where tropical storms form and travel for most of their

lives are significantly more stable than corresponding pressures at higher latitudes. At higherlatitudes there are more systems coming by with larger and more varied pressure changes takingplace. At Long Island Sound, NY, for example, in the month of March, the average pressure is 1016.3mb with a standard deviation of 9.7 mb, based on an average of 8,ll1 observations throughout theday at one station over the past 10 years. These data were obtained from the NCDC as outlined at theend of Section 8.2. For comparison, in San Juan, Puerto Rico in September, peak of the hurricaneseason, the average pressure is 1013.5 mb with a standard deviation of 1.7 mb.

The key factor here is the standard deviation. Studies show that an extended set of pressuremeasurements for the same location follows what mathemati­cians call the normal distribution (bellcurve). This Curve describes much of what we see in nature that stems from more or less randomdistinctions. The heights of American women, for example, between ages of 18 and 24 follows anormal distribution with a mean value of 65.5 inches and a standard deviation (SD) of 2.5 inches.

Figure 9.3-4 shows three examples. Atmospheric pressures in the tropics are like the tall narrowexample with small SD, whereas at higher latitudes the pressure distributions are more like the lower,spread out curve. In fact the difference between New York and Puerto Rico mentioned above is evenmore dra­matic than shown in the figure.

Figure 9.3-4 Three "normal" distributions of (say) 100 measurements with 3 different standard deviations (SD). When SD is small, the values are all near the mean value (like monthly pressures in the tropics); when it is large they are more spread out (like pressures at high latitudes). To find the SD of a set of numbers, first find the average, then make a list of all the differences between each value and the average, then square each difference (to cancel what side of the average if is on), then find the average of these squared differences, and then take the square root of that. In this sense, the SD is the average difference from the mean for the full set.

In normal distributions it is easy to predict the probability of values that differ from the mean (average)value of the data set, as shown in Figure 9.3-5. From this we can make Table 9.3-1 that tells us about thepressures we might observe and what they can mean. For example, if the mean pressure within a tropicalstorm zone is 1014 for the time we are there, with an SD of 2 mb, and we observe a pressure of 1010 mb,then that is 2 SD below the average. Then just that modest pressure drop means a lot. It has only a 3%chance of being part of the normal pressure fluc­tuation at this location, and more like a 97% chance ofbeing a tropical system approaching.

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With tropical SDs of just 2 or 3 mb, you do not have to be much below average before the (mental) flag should be upto alert you. Note this is another valuable application of a calibrated barometer that you can trust to give the rightpressure.

This behavior, and even this approach to tropical storm warning, is not newly recognized, but 1 ven­ture to guessthat it has not been used much, at least 6γ small craft mariners. The reason is they have not had accurate barometerstill the recent advent of elec­tronic units, and they have not had the crucial data accurate average values and thestandard deviations.

Figure 9.3-5 Probabilities related to any normal distribu-tion of observations. In a normal distribution, 68% (2χ34) of the values are within 1SD of the mean, half above and half below; 95% are within 2 SD of the mean; and 99.7% fall within 3 SD of the mean. If we carefully figuredall the areas under the sections, we would learn that the probability of a value being less than the mean by 1 SD is 16%; less than 1.5 SD is 7%; and less than 2 SD is about 2 or 3%, which is summarized in Table 9.3-1.This is all possible now, and it is something easy to watch when sailing in tropical waters without wire-less contact that would give properwarnings of such systems.The pressure distributions (standard deviations) show why this is useful knowledge in the tropics, but not at higher latitudes. If the pressuredrops to 4 mb below the average in, say, San Francisco or Annapolis, it does not tell us anything at all about the weather, but this identicaldrop on St. Croix or Mauritius is essentially a red light, turned on to warn us to pay attention.A good barometer (needed for this analysis) is easy to come by and inexpensive. It is also easy to confirm that you have it set to the rightpressure (Section 2.2). The average monthly pressures themselves for a particular region in a hurricane zone can often be found from a PilotChart as shown in Figure 9.3-6, if one does not have access to the Internet to use NCDC. Notice from the figure, however, that you need touse the right month for the analysis, because the monthly averages vary from month to month by more than the SD for each individualmonth.The third item needed for this is the standard de­viation, but if you do not have this data, and cannot access the NCDC to get it, you canassume the SD = 2.5 mb, which is likely a good average for most hur­ricane zones in hurricane seasons.Again, we are describing an observation that is most valuable when you have lost wireless contact with your forecast sources. Otherwise youknow a1-most hourly where the storms are located and how they are moving.

Figure 9.3-6 Pressures in part of the SW Atlantic hur-ricanezone for three months taken from the Pilot Charts. Thesevalues compare reasonably well with the more accurate datawe can obtain from the NCDC (see end of Section 8.2), whichfor San Juan Airport in Puerto Rico (18Ν, 66W) are: Aug(1015.5, SD 1.6), Sept (1013.5, SD 1. 7), and Oct (10 12.9, SD 2.0). Accurate mean monthly pressures and standard deviationsworldwide are now available in The Barometer Handbook fromStarpath Publications. See starpath.com/barometers.

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Στις σελίδες που ακολουθούν παρατίθενταιαποσπάσματα από το βιβλίο Ναυτική Μετεωρολογία τηςΑικατερίνης Γ. Ψύχα και του Μιχάλη Π. Μηνόγιαννη,εκδόσεις Ίδρυμα Ευγενίδου.

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• Chen Ma-Li and Lee S. Chesneau 2008. Heavy weather avoidance androute design. Concepts and applications of the 500 mb charts. Paradise CayPublications, INC. Arcata, CA, USA.

• Vasquez Tim, 2003. Weather map handbook, a guide to the internet,modern forecasting and weather technology. Weather Graphics Technologies,USA.

• http://www.opc.ncep.noaa.gov/UGbegin.shtml : Radiofacsimile user’s guide

• http://www.marineweatherbylee.com/ : Lee Chesneau’s web page

• http://www.meteomarine.gr

• Κασιμίδης Ε. Γεώργιος, 2001. Ναυτική Μετεωρολογία, εκδότης: Γ.Κασιμίδης.

• Burch David 2008. Modern Marine Weather. Starpath, Seattle,Washington

• Ahrens Donald, 2007. Meteorology Today, an introduction to weather,climate and environment. Thomson Higher Education.

Βιβλιογραφία

Χρήσιμο πληροφοριακό υλικό για τις μεθόδους λήψης των μετεωρολογικών πληροφοριών στο καράβι

Οδηγίες για τον τρόπο που διαβάζονται οι αμερικάνικοι μετεωρολογικοί χάρτες

Χρήσιμη σελίδα με πλήθος πληροφοριών από τον βετεράνο μετεωρολόγο της OPC, Lee Chesneau

• http://weather.noaa.gov/fax/marine.shtml

• http://www.opc.ncep.noaa.gov/shtml/briefing.shtml

Τα επίσημα Marine RadioFaxes των Αμερικανών

Τα επίσημα Marine RadioFaxes των Αμερικανών που εκδίδονται από την OPC

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