Clouds (Gavins Part of Meteo Project 2008-2009)

8/19/2019 Clouds (Gavins Part of Meteo Project 2008-2009)

1/23

Stratus

Distribution: Worldwide, most common near coast andmountains.

Height: 0 to 6500 feet.

Cause: Lifting of a large air mass, followed bycondensation.

ssociated Weather: Dri!!le or slight rain, or snow in sub"!ero tem#eratures.

Ha!ard Warning: Can be an a$iation ha!ard as it sometimes mas%s terrain.

&tratus clouds form in sheets or layers 'stratus is the Latin word for layer( and occur when relati$ely largeareas of moist air rise gently in a stable atmos#here to a le$el where condensation occurs. )ormally, thelifting of the air mass is a result of an incoming frontal system or wind encountering a large landmass suchas a mountain range. slightly different form of stratus may occur when a layer of fog that has de$elo#ed atground le$el starts to rise as it is warmed by the &un. *his formation is %nown as fog stratus.

&tratus is the lowest altitude cloud formation, with condensation occurring any where between ground le$eland about 6,500 feet. *y#ically, stratus has a ragged. gray a##earance, and $aries in thic%ness from a semi"trans#arent sheet of a few feet to a dec% of around +,500 feet.

ts hori!ontal s#read is usually far greater and may co$er hundreds of s-uare miles. )ormally, there is nosignificant weather associated with this cloud, although light dri!!le or rain, or light snow in sub"!erotem#eratures, may fall if the dec% is sufficiently thic%. &tratus that gi$es rise to #reci#itation is often %nownas nimbostratus.

When it forms close to the ground, stratus can mas% the surrounding terrain, #articularly in mountainousareas, and this has been the cause of many a$iation accidents. ortunately, radar e-ui#ment fitted to mostmodern aircraft is reducing the dangers associated with this cloud.


2/23

Stratocumulus

Distribution: Worldwide.

Height: /000 to 6500 feet.

Cause: Lifting of a large air mass, followed bycondensation combined with relati$ely wea% instability atcloud le$el.

ssociated Weather: )ormally none, but may #roduce light #reci#itation if cloud issufficiently thic%.

0ne of the most common clouds worldwide, stratocumulus is a good indicator of moisture in the lowerle$els of the atmos#here. t usually occurs between /,000 and 6,500 feet. &tratocumulus usually has aragged a##earance along its u##er surface, but can ha$e a well"defined and flattish base. t tends to form incom#arati$ely shallow layers, sometimes se$eral hundred miles wide. *he color of the cloud may $ary fromwhite to dar% gray, de#ending on the light conditions and the thic%ness of the dec%. ts somewhat lum#ya##earance, indicati$e of con$ection within the cloud, is what distinguishes stratocumulus from stratus.

*wo #rocesses may gi$e rise to strato"cumulus, either se#arately or in combination. n the first, a large,moist air mass is lifted by a frontal system or a landmass to a le$el where condensation occurs slightinstability at cloud le$el then creates the cloud1s cumuliform sha#e.

*he second #rocess in$ol$es #oc%ets of warm air rising from the ground as a result of wea% con$ection,gi$ing rise to condensation at the same le$el o$er a wide area.

n this case, the clouds may subse-uently de$elo# into cumulus humilis , and some of these may e$ende$elo# into cumulonimbus clouds.

)ormally, if stratocumulus has not de$elo#ed $ertically by mid"afternoon, the time of ma2imum groundtem#erature, it will tend to dissi#ate, resulting in a clear e$ening s%y. f it is sufficiently thic%, stratocumulusmay #roduce light dri!!le, or snow in sub"!ero tem#eratures, but this is not common.

Orographic Stratus

Distribution: 3$er hills and mountain rangesworldwide.

Height: 0 to +000 feet abo$e ground le$el.

http://www.theairlinepilots.com/met/clouds.htm#cumulus%20humilis%23cumulus%20humilishttp://www.theairlinepilots.com/met/clouds.htm#cumulus%20humilis%23cumulus%20humilis


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5/23

Cumulus Mediocris

Distribution: Worldwide,e2ce#t ntarctica.

Height: /000 to ;000 feet.

Cause: Wea% to moderate con$ection.

ssociated Weather: )one

Cumulus mediocris is generated by slightly stronger con$ection than that which gi$es rise to cumulus humilis. *his #roduces a cumulus cloud as tall as it iswide, as estimated by an obser$er on the ground.

*he base of this medium"si!ed cloud 'mediocris means moderate in Latin( can begin to form from /,000feet u#ward, de#ending on the surrounding humidity. Cumulus mediocris is normally white or light gray,and has a com#arati$ely flat base.

*his cloud is often a transitional stage between the lesser humilis and the more de$elo#ed congestus #hase.4ediocris clouds are more common in the late morning or early afternoon, after the ground has warmedenough to generate con$ection. *hey are not large enough to #roduce #reci#itation. During aircraft

#enetration of the cloud, they can #roduce slight and short"li$ed turbulence.

f mediocris clouds occur at le$els where strong winds are blowing, the winds may shred the clouds intohori!ontal fragments, which then s#eed across the s%y. *his $ariation is %nown as cumulus mediocrisfractus.

4ediocris occurs o$er most oceans and landmasses when con" ditions are right, with the e2ce#tion ofntarctica, where the cold surface tem#eratures generally inhibit con$ection.

Cumulus Congestus


Height: /000 to /0,000 feet.

Cause:Con$ection, enhanced by atmos#heric instability.

ssociated Weather:Can #roduce moderate to hea$y showers.

http://www.theairlinepilots.com/met/clouds.htm#cumulus%20congestus%23cumulus%20congestushttp://www.theairlinepilots.com/met/clouds.htm#cumulus%20congestus%23cumulus%20congestus


6/23

Ha!ard Warning: 4oderate turbulence at cloud le$el.

Cumulus congestus re#resents the ne2t stage in the $ertical de$elo#ment of a cumulus cloud after cumulusmediocris


7/23

the fire. *he bases of these clouds are often difficult to discern, as they are usually hidden by the smo%efrom the wildfire, but the cloud to#s are normally situated well abo$e the smo%escreen.


8/23

$irga.

*he #owerful con$ecti$e u#drafts associated with cal$us clouds can #roduce significant turbulence.Howe$er, the #reci#itation associated with the cloud can usually be located by onboard aircraft radar ande$asi$e action ta%en.

Cumulonimbus with Pileus


Height: /0,000 to 0,000 feet.

Cause: &trong con$ection lifting and e$entually absorbinga layer of air.

ssociated Weather: )one from #ileus, but cal$us cloudwill #roduce #reci#itation.

Ha!ard Warning: &ignificant turbulence at cloud le$el.

3nce a cloud has reached the cal$us stage, and if con$ection is still occurring and is enhanced by instabilityin the surrounding air mass, the cloud will continue to grow $ertically. When the air is ascending -uitera#idly "" s#eeds of /0 to 0 miles #er hour directly u#ward are #ossible "" a rather curious #henomenonmay ta%e #lace.

*he strong u#draft associated with the cal$us cloud #ic%s u# a slab of air and thrusts it u#ward. *his causesthe water $a#or in the slab to condense, and a smooth, elongated, ca#"li%e formation, %nown as a #ileuscloud '#ileus is the Latin word for felt ca#( a##ears abo$e the rising mass of the cal$us. s the cal$us cloudcontinues to rise, it gradually catches u# with the #ileus cloud. When the two clouds meet, some of the

#ileus cloud to##les off the sides of the rising cal$us. 9$entually, the ascending cloud o$erta%es the #ileus,which remains dra#ed o$er the #ea% of the cal$us until the two clouds merge com#letely.


9/23

Cumulonimbus Incus

Distribution: Worldwide, e2ce#t ntarcticacommon in the tro#ics.

Height: /000 to 5,000 feet.

Cause: ing of Clouds7, a mighty mountain of moisture oftenconsiderably taller than 4ount 9$erest, and sometimes reaching 60,000 feet in tro#ical and subtro#icalareas.

n its full magnificence, it is crowned with a huge, wedge"sha#ed mass of high cloud resembling a blac%smith1s an$il 'incus is the Latin word for an$il(. *his #art of the cloud, often referred to as ahammerhead or thunderhead, is a clear sign of a fully fledged thunderstorm.

Cumulonimbus incus can begin early in the morning as cumulus humilis , then #roceed through the stages ofmediocris and congestus . or the cloud to continue de$elo#ing at this #oint, the con$ecti$e #rocess mustcombine with atmos#heric instability to #roduce a #owerful u#draft.

s long as the air in the $icinity of the u#draft remains unstable, the cloud continues to rise and e2#and.9$entually the cumulonimbus cloud reaches the to# of the tro#os#here, where the air tem#erature le$els offand begins to increase with altitude. *his change in tem#erature has the effect of #lacing a lid on the u#draft,and the cloud can rise no further. Howe$er, the momentum of the air below continues to #ush u#ward, ands#reads the cloud out in a radial fashion at the tro#o#ause, forming the characteristic an$il sha#e. *he

#osition of this formation therefore indicates the height of the tro#os#here in the area.

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10/23

8ecause the an$il is situated well abo$e the le$el wherethe air tem#erature dro#s below free!ing, this #art of thecloud is com#osed of ice crystals, which forms a crown of cirrus abo$e the main cloud mass. *hese ice crystals may

be blown about by strong, high"le$el winds, #roducing astrea%y a##earance.

Warning &igns

n rare cases, the u#draft associated with the cloud is so #owerfulthat it #unches through the tro#o#ause and carries a #arcel of cloud into the lower le$els of the stratos#here, before losingmomentum and falling bac%. *his #roduces an u#ward bulge on

the otherwise flat u##er surface of the an$il "" a good indicator of a #articularly se$ere storm, which may #roduce hail, strong windgusts, and e$en tornadoes.

cumulonimbus incus formation must always be regarded as asignificant a$iation ha!ard because of the #owerful air currents in$ol$ed in its formation and the #otentiallydamaging effect of the large hailstones it may #roduce. ortunately, because of the #reci#itation generated,the cloud is easily located by onboard and ground"based radar, and elaborate #rocedures are in #lace to steer aircraft around any such acti$ity.


11/23

Cumulonimbus with Mammatus

Distribution: Worldwide, e2ce#t ntarctica common in the tro#ics.

Height: +5,000 to /5,000 feet.

Cause: &trong con$ection followed by re$erse"direction con$ection.

ssociated Weather: Hea$y rain, wild s-ualls, hail.

Ha!ard Warning: &e$ere turbulence in cloud, strong winds, hail, lightningand ris% of tornadoes.

4ammatus is one of the most s#ectacular and distincti$e of all cloud formations, ma%ing it a fa$orite withweather"watchers and #hotogra#hers ali%e. t consists of #endulous globules of cloud 'mamma is the Latinword for breast( that hang from the underside of the an$il of a thundercloud. 4ammatus is alwaysassociated with mature cumulonimbus clouds and is therefore an indicator of se$ere weather conditions.

*he formation occurs as a result of a #rocess that can be described as re$erse"direction con$ection. During athunderstorm, warm, moist u#drafts rise to the to# of the tro#os#here. Here the tem#erature le$els off andthe air stabili!es. *his causes the rising cloud to e2#and hori!ontally o$er areas of cooler, cloud"free air. *hetem#erature difference between the two air masses creates instability under the an$il, which causes #oc%etsof warm, moist air in the cloud to con$ect downward. *his re$erse"direction con$ection is enhanced by theeffects of gra$ity and by #reci#itation from the cloud. *he #rocess #roduces near"symmetrical #rotuberanceson the underside of the an$il, %nown as the mammatus, which may co$er large areas.

Tornado Alert

s the an$il of a mature cumulonimbus cloud can s#read out o$er hundreds of s-uare miles, the center ofthe storm may be some distance away from the mammatus formation. Howe$er, mammatus normally occurssoon after the cumulonimbus cloud has reached ma2imum growth and intensity and is usually a sign of a

#articularly $igorous thunderstorm. n the nited &tates, this formation is regarded as a clear warning signof #ossible tornado de$elo#ment.

irline #ilots will normally ta%e action to a$oid any cumulonimbus clouds, but #articularly those bearingmammatus formations, as these indicate es#ecially se$ere turbulence within the cumulonimbus. 4ammatusmay be obser$ed where$er cumulonimbus clouds occur, but it is #articularly common in areas wherethunderstorms are se$ere, such as tro#ical and subtro#ical areas. 8ecause the formation is associated withmature cumulonimbus clouds, it is most li%ely to be seen from mid"afternoon to early e$ening, when groundheating and associated con$ecti$e acti$ity ha$e reached a ma2imum.


12/23

Altostratus

Distribution: Worldwide,common in middle latitudes.

Height: 6500 to +6,500 feet.

Cause: Lifting of a large air mass followed by condensation.

ssociated Weather: 92tensi$e areas of rain and snow.

Ha!ard Warning: ce accretion on aircraft.

*his cloud is found in the middle le$els of the atmos#here and is always a sign of the #resence of significantamounts of moisture in those layers. t is ty#ically featureless, ranging from a thin, white $eil of cloudthrough which the &un is #lainly $isible, to a dense, gray mantle that may bloc% out the &un com#letely.

ltostratus is the result of the lifting and condensation of a large air mass, usually by an incoming frontalsystem. *his can result in an e2tensi$e dec% of cloud, which may e2tend o$er thousands of s-uare miles. fsufficiently thic%, altostratus can #roduce rain or snow o$er a wide area.

When stratus cloud co$ers the entire s%y, it can be difficult to determine whether it is a low or middle"le$elformation. s a rough guide, if you can discern a te2ture in the cloud dec%, it is more li%ely to be low"le$elstratus if it a##ears smooth and structureless, it is more li%ely to be an altostratus formation.

or #ilots, a thic% dec% of altostratus can be a cause for concern if tem#eratures within the cloud are belowfree!ing, because ice may build u# on #arts of the aircraft as it #asses through the cloud, altering the #lane1saerodynamics. ortunately, most aircraft are e-ui##ed with de"icing de$ices that eliminate this #roblem.

Altostratus Undulatus



Cause: Lifting of a large air mass, followed bycondensation combined with wind shear at cloud le$el.


ltostratus undulatus usually occurs in a thin layer of altostratus, and its distincti$e, undulating a##earance is due to wa$e motion in the air mass. *his motion isnormally a result of wind shear, which occurs when one layer of air slides o$er another layer mo$ing at adifferent s#eed or in a different direction 'or both(. *his creates $ertical eddies, or wa$es, of air between the


13/23

layers, and if sufficient moisture is #resent, cloud will form where the wa$e rises and dissi#ate where it falls.

De#ending on the moisture content of the air mass and the degree of wind shear, undulatus may occur asfairly continuous wa$es across the s%y with thin cloud connections at the base, or it may brea% u# intounconnected wa$e #ea%s.

ndulatus #roduces no significant weather, but, because the cloud is #roduced by wind shear, it is regardedas a sign of local turbulence. n most cases, howe$er, this turbulence would be only slight and would not bea concern for aircraft #ilots.

Altocumulus



Cause: Lifting of a large air mass, followed bycondensation combined with instability.

ssociated Weather: Light rain, if cloud is thic% mayindicate an a##roaching front.

Ha!ard Warning: ce accretion on aircraft.

While altostratus is often flat and featureless, altocumulus usually creates interesting and $aried s%ies. nsome cases, many thousands of small altocumulus clouds will be strung together across the s%y ins#ectacular formations.

s with altostratus, altocumulus normally occurs when a large air mass is lifted to middle le$els by alandmass or an incoming frontal system, and condensation occurs o$er a wide area. *he #rinci#al difference

between the two formations is that altocumulus is affected by instability in the surrounding atmos#here.*his gi$es rise to its distincti$e cumuliform te2ture.

n isolation, altocumulus does not ha$e great significance for the weather"watcher, although it can #roducelight #reci#itation if the dec% is sufficiently thic%. Howe$er, if the e2tent of an altocumulus formationa##ears to be increasing during the course of a day, this may be a sign of an a##roaching frontal system.

3ften altocumulus and altostratus a##ear together in a mi2ed s%y. &atellite #hotogra#hy re$eals that mi2edaltocumulus"altostratus formations may e2tend o$er thousands of s-uare miles, #articularly when associatedwith a frontal system.

f altocumulus combines with a thic% dec% of altostratus at a le$el where the tem#erature is below free!ing,significant airframe icing may affect the aerodynamics of aircraft flying through the cloud. 3therwise, slightto moderate turbulence will be the only concern for the #ilot. ltocumulus formations are often more


14/23

distinct and dramatic at sunrise and sunset.

Altocumulus Undulatus



Cause: Lifting of a large air mass, followed bycondensation combined with instability and wind shear atcloud le$el.

ssociated Weather:Light rain, if cloud is thic% may indicate an a##roachingfront.

ltocumulus undulatus occurs when a layer of altocumulus cloud is affected by wind shear. *he mechanismis the same as that which gi$es rise to altostratus undulatus . *he altocumulus $ariation consists of #arallel

bands of cumulus clouds. *hese may form in #atches or e2tend o$er a wide area of the $isible s%y. When the bands form close together, they often resemble ri##les on the surface of a #ond.

ltocumulus undulatus is distinguished from altostratus undulatus by its discernible cumuliform te2ture. swith all cumulus clouds, this is a result of a certain amount of instability at cloud le$el, which gi$es rise tofurther u#lift at $arious #oints within the cloud. *hese clouds always indicate the #resence of significantamounts of moisture at middle le$els, and, if on the increase, may signal the a##roach of a frontal system. fthe cloud dec% is sufficiently thic%, this formation can #roduce rain, or snow in sub"!ero tem#eratures.3ften, altocumulus undulatus occurs together with altostratus formations in a mi2ed s%y. n such cases, itmay be difficult to discern which cloud formation is #roducing the #reci#itation.

ll undulatus clouds are regarded by those in the a$iation industry as a sign of turbulence. Howe$er,altocumulus undulatus seldom gi$es rise to anything more than slight to moderate turbulence and is

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therefore not regarded as a danger by #ilots.

Altocumulus Mac erel S y



Cause: Lifting of a large air mass, followed bycondensation combined with instability and wind shear.

ssociated Weather: 4ay indicate an a##roachingfrontal system.

*he mac%erel s%y $ariation of altocumulus is named for itsresemblance to the scales of a fish. t is almost certain thatthe name originated among early mariners, and it may ha$e considerable anti-uity.

s with altocumulus formations in general, mac%erel s%y is #roduced by the lifting of a large, moist airmass, usually by an a##roaching cold front, combined with instability at cloud le$el. *he e2act causes ofthis #attern ha$e not been firmly established, but it is li%ely that a form of wind shear, similar to that which

#roduces undulatus formations, is the cause. n this $ariation, the wind shear gi$es rise to a more intricate #attern of small wa$es, which #roduces the much finer te2ture of mac%erel s%y.

8ecause it is often created by an a##roaching frontal system, mac%erel s%y has long been associated infol%lore with deteriorating weather conditions. 4ore often than not, this formation, li%e other middle"le$elclouds, is a good indicator of changing weather, although, as in all such cases, the front may #ass somedistance away from the obser$er, resulting in little change in local conditions.

http://www.theairlinepilots.com/met/clouds.htm#altostratus%20undulatus%23altostratus%20undulatushttp://www.theairlinepilots.com/met/clouds.htm#altostratus%20undulatus%23altostratus%20undulatus


16/23

Altocumulus Castellanus



Cause: Lifting of a large air mass followed bycondensation combined with instability.

ssociated Weather:4ay #oint to thunderstorm de$elo#ment later in the day.

*his cloud is named for the turret"li%e #rotruberancesthat grow from its main dec%, resembling the battlements of a medie$al castle. While not a s#ectacularformation, it is significant in that it indicates instability in the middle layers of the atmos#here and maytherefore #oint to thunderstorm de$elo#ment later in the day.

Castellanus generally occurs when a layer of colder air slides across an area of altocumulus cloud. *hiscreates instability, and locali!ed bubbles of air start to rise from the cloud dec%. Condensation within these

#oc%ets of air creates the castellanus effect.

ny subse-uent con$ection from the ground will be enhanced by this middle"le$el instability. Cumulusclouds forming in these areas are therefore more li%ely to de$elo# into cumulonimbus clouds.

or this reason, meteorologists are always alert to re#orts of castellanus. f se$eral ground obser$ers ha$enoted castellanus de$elo#ment by the middle of the day, then there is a greater li%elihood of thunderstormacti$ity later in the afternoon, following further ground heating.

&ince the formation of altocumulus castellanus in$ol$es $ertical air currents, #ilots can e2#ect to e2#erienceslight to moderate turbulence as they #ass through these clouds, but this will #ose no danger to aircraftsafety.

Altocumulus !enticularis

Distribution:Common o$er mountain ranges worldwide.


Cause: ir mass forced to rise to


17/23

condensation le$el by landmass.

ssociated Weather: Light rain or snow high winds.

Ha!ard Warning: 4oderate turbulence at cloud le$el.

ltocumulus lenticularis clouds are named for their smooth, round, lens"li%e sha#e 'lenticular means lens orlentil"li%e(. *hese middle"le$el clouds can form s#ectacular #atterns that delight weather"watchers and

#hotogra#hers and ha$e almost certainly been res#onsible for a number of 3 sightings o$er the years.

When wind blows across a mountain range, it tends to form air wa$es on the lee side of the mountains. *his #rocess, %nown as the mountain wa$e effect, is usually in$isible, but when moisture is #resent at the to# ofthese wa$es, lenticular clouds form where the wind rises and dissi#ate where it falls.

8ecause mountain ranges are nearly always of irregular sha#e and wind may mo$e at different s#eeds atdifferent le$els, the wa$es #roduced in this manner often ha$e $arying distances between their crests'referred to as the wa$elength( and the resulting clouds form an irregular #attern.

Howe$er, if the mountain range has a fairly regular sha#e and the wind is blowing at a steady s#eed ata##ro2imately right angles to the mountains, the wa$e crests, and any resulting clouds, will form a regular

#attern. urthermore, if alternate layers of moist and dry air are #resent abo$e the mountains, the cloudsmay #ile u# on to# of each other li%e stac%s of #lates. t is these distincti$e stac% formations that ha$e, onoccasion, been mista%en for 3s.

f the wind generating the wa$es has a fairly constant s#eed, the cloud #attern will be stable and longlasting, remaining $irtually stationary in the s%y for e2tended #eriods. enerally, no significant weather is

#roduced by altocumulus lenticularis, but, occasionally, if there is sufficient moisture in the surroundingatmos#here, these clouds can become thic% enough to generate light rain, or snow showers in sub"!erotem#eratures. 8ecause these formations are associated with high"s#eed winds in the middle layers, they may

be #recursors of windy conditions at ground le$el.

Sur"ing The #aves

*he roller"coaster motion of the air, made $isible by the formation of these clouds, can #roduce significantle$els of turbulence, and commercial airliners will try to a$oid such areas. High"le$el glider #ilots, howe$er,sometimes do the o##osite: they loo% for lenticular clouds as signs of a source of u#lift for the aircraft. *hegliders can surf along the mountain wa$es, maintaining altitude by remaining on the rising side of the wa$e


18/23

crest.

Lenticular clouds occur in most #arts of the world, and may form o$er -uite small mountains. oode2am#les occur when moist winds blow in off the


19/23

Cirrus


Height: bo$e +6,500 feet.

Cause: &aturation of air mass at u##er le$els.

ssociated Weather: f e2tensi$e, may indicate ana##roaching frontal system may also be a sign of adecayed thunderstorm.

*he Latin word cirrus, meaning wis# of hair, is the namegi$en to high"le$el clouds that stretch across the s%y in delicate strands. *hese formations indicate the

#resence of moisture at high le$els of the atmos#here. t these le$els, the tem#erature is normally belowfree!ing, and any air mass that cools to saturation will #roduce ice rather than water dro#lets.

Cirrus clouds therefore consist of many millions of ice crystals, and these are blown about by u##er"le$elwinds, #roducing characteristic white strea%s.

mong the most distincti$e cirrus sha#es are the irregular twists and tangles of cirrus intortus and the hoo%sha#es of cirrus uncinus . Less common, but e-ually dramatic, is cirrus radiatus, which forms in long,

#arallel lines that seem to radiate from a #oint on the hori!on.

Cirrus clouds may form in isolated #atches or co$er a wide area of the s%y, de#ending on the distribution ofmoisture. solated #atches rarely ha$e any great significance, but an e2tensi$e dec%, increasing from onedirection, may indicate an a##roaching front.

Cirrus may be the result of local thunderstorm acti$ity. *he an$il that often forms abo$e a cumulonimbuscloud is actually a cirrus cloud. t is generated as the thunderstorm #um#s moisture u# to the $ery to# of thetro#os#here, where it free!es into ice crystals. fter the storm has com#leted its life cycle, high"le$el windsmay dis#erse the an$il across the s%y, #roducing e2tensi$e cirrus formations far downwind.

Cirrus Uncinus



Cause: &aturation of air mass at u##er le$els, combinedwith strong wind immediately below cloud le$el.

ssociated Weather:4ay indicate an a##roaching frontal system.

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*his often s#ectacular $ariation of cirrus cloud is also %nown as hoo%ed cirrus 'uncinus is the Latin forhoo%(, or cirrus mares1 tails, a reference to the cloud1s resemblance to a horse1s tail.

Cirrus uncinus forms in much the same way as other cirrus formations. Howe$er, its distincti$e #attern offilaments is the result of a high"s#eed wind below the le$el at which the ice crystals form. s the crystalsdescend under the influence of gra$ity, this wind ra#idly smears them across the s%y, forming the distincti$e,elongated, hoo%ed sha#es.

Li%e other cirrus clouds, uncinus is a result of high"le$el moisture, and is therefore often associated with thea##roach of a frontal system. &ince it is also e$idence of a high"s#eed, high"le$el wind, it may indicate the

#resence of a ?et stream.

)ormally, cirrus uncinus #roduces no significant weather on the ground, although snow showers may be$isible immediately below cloud le$el. *hese usually e$a#orate well before reaching the ground and aretherefore classified as $irga.

s cirrus uncinus generally indicates the #resence of high" s#eed winds, #ilots often associate this cloudwith turbulence. n most cases, howe$er, the turbulence would cause little discomfort to #ilots or

#assengers.

Cirrus $elvin%Helmholt&



Cause: &aturation of an air mass at high le$els,combined with wind shear.


Ha!ard Warning: 4oderate to se$ere le$els of turbulence at cloud le$el.

##earing as a slender, hori!ontal s#iral of cloud, cirrus >el$in"Helmholt! is one of the most distincti$ecloud formations. Howe$er, it tends to dissi#ate only a minute or two after forming and, as a result, is rarelyobser$ed.

*he sha#e of this %ind of cirrus is the result of a #articular ty#e of wind shear. n general, wind shear occurswhen one layer of air slides across another layer mo$ing at a different s#eed or in a different direction 'or

both(. *his gi$es rise to $ertical eddies that #roduce a regular #attern of air wa$es.

n most cases, wind shear creates a series of gently undulating cloud formations along the to#s of the wa$es.n the case of the >el$in"Helmholt! formation, howe$er, the eddies are more #owerful, and carry the cloud


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o$er the #ea% and down the other side, so that the wa$es 7brea%7 in the manner of ocean wa$es brea%ing asthey a##roach the shore. s these wa$es com#lete a circulation, they create a distincti$e cor%screw #attern.

*his form of instability also occurs in fluids and in the 9arth1s outer atmos#here. t was first described in thelate nineteenth century by 8aron >el$in '+@/;"+A0B(, a &cottish #hysicist, and Hermann $on Helmholt!'+@/+"A;(, a erman #hysicist "" hence the name of the cloud.

>el$in"Helmholt! wa$es are #robably -uite common in the u##er tro#os#here, but generally there isinsufficient moisture #resent to generate cloud and render the #attern $isible.

*he #resence of this cloud indicates a degree of wind shear that is li%ely to #roduce moderate to se$ereturbulence at cloud le$el. n the absence of cloud, the same #rocess can be a ma?or source of clear airturbulence at high le$els. s this turbulence is in$isible and does not show u# on radar, aircraft mayencounter it une2#ectedly.

Cirrostratus



Cause: &aturation of a large air mass at high le$els.

ssociated Weather: f the cloud is thic%ening, it mayindicate the a##roach of a frontal system.

Cirrostratus is an e$en layer of cirrus that co$ers a wide areaof the s%y. s with other %inds of cirrus, it is formed when amoist air mass is lifted to a le$el where it cools to saturationand forms ice crystals. n the case of cirrostratus, this lifting occurs on a large scale.

4eteorologists distinguish $arious ty#es of cirrostratus. mong the most common are fibratus andnebulosis. *he former consists of long, thin filaments, %nown as striations, that s#read out across a wide areaof the s%y. *he e$en te2ture of this formation results from the ice crystals being blown by strong, steady,high"le$el winds.

n the case of cirrostratus nebulosis, the u#lift that gi$es rise to the cloud is $ery gentle, and the resultingice"crystal layer is e2tremely thin, with $ague edges that are difficult to discern and a lac% of the te2ture orfiber common to other cirrus clouds. re-uently, the only sign of cloud formation will be a slight diminutionof the intensity of sunlight.

When it forms in a $ery thin layer, cirrostratus nebulosis often gi$es rise to haloes, sun dogs, andiridescence.


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3ccasionally, snow showers fall from cirrostratus formations, but these usually e$a#orate before reachingthe ground and are classified as $irga. f a build"u# of cirrostratus from one direction is ta%ing #lace, this is asign of an increase in moisture at u##er le$els and may indicate the a##roach of a weather system such as acold front.

Cirrostratus formations may cause slight turbulence at cloud le$el, but this is unli%ely to affect aircrafto#erations or discomfort #assengers.

Cirrocumulus



Cause: &aturation of a large airmass at high le$els, combinedwith instability at cloud le$el.

ssociated Weather: ncreasing co$er may indicate ana##roaching frontal system.

Cirrocumulus, li%e cirrostratus , occurs when a large area of moist air at a high le$el of the atmos#herereaches saturation and forms ice crystals. What differentiates cirrocumulus from cirrostratus is the #resenceof instability at cloud le$el. *his gi$es the cloud its cumuliform a##earance.

n isolation, this formation does not normally ha$e any great significance. Howe$er, if there is a steadyincrease in this cloud o$er a #eriod of time, it may indicate the a##roach of a frontal system.

Cirrocumulus is one of the most attracti$e of all clouds, often forming s#ectacular #atterns that may stretchfor hundreds of miles across the s%y. 3ne dramatic form of cirrocumulus is cirrocumulus undulatus, whicha##ears as a fine, ri##led #attern in the s%y.

s with other undulatus forms, these ri##les are #roduced by atmos#heric wa$es generated by wind shear.Howe$er, in the case of cirrocumulus undulatus, the entire structure has a much finer a##earance. *his is

#artly due to the fact that atmos#heric wa$es formed at high altitudes tend to ha$e a shorter wa$elength thanthose formed in the middle layers, but it is also a result of the greater distance between the cloud formationand the obser$er on the ground.

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Contrails



Cause: ircraft o#erations at high le$els combined withsignificant moisture in the surrounding air.

ssociated Weather: f long"lasting, can indicate ana##roaching frontal system.

*his is an e2ce#tional formation in that it does not occur naturally. =ather, it is an artificial cirrus cloud #roduced by high"le$el aircraft o#erations.

*he term contrails is short for condensation trails.

ll aircraft engines emit water dro#lets from their e2hausts. When an air#lane flies through the u##er le$elsof the tro#os#here, where tem#eratures are normally well below / '0 C(, these dro#lets immediatelyfree!e to form ice crystals, creating an artificial cloud.

3ften, the surrounding air mass will contain little moisture, and the resulting cloud will be thin, short"li$ed,and in$isible to an obser$er on the ground. Howe$er, if the surrounding air mass is close to saturation, thecloud #roduced will be much broader and longer and may last for half an hour. t is this $isible formationthat is %nown as a contrail. or the weather" matcher, a long"lasting contrail can be a useful sign, as itre$eals the #resence of significant high"le$el moisture. *his, in turn, may indicate the a##roach of a frontalsystem.

Contrails also ha$e a military and strategic significance in that they re$eal the #resence and location of high"le$el aircraft that would normally be in$isible to the na%ed eye. *he enduring memory of many whowitnessed the 8attle of 8ritain, during the &econd World War, is the criss"crossing s#ider1s web of contrailsthat occurred day after day, high abo$e 9ngland, as the Luftwaffe and the =oyal ir orce engaged incombat in the u##er le$els of the tro#os#here.

Documents

Clouds (Gavins Part of Meteo Project 2008-2009)