FIS Handbook

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FIS HANDBOOK

VOLUME 5

METEOROLOGY FOR AVIATORS

INDEX

Chapter Title Page No.

1. Met services for aviation 1

2. Met Charts, Forecasts and Briefing 9

3. Met observations and elements 17

4. Structure of the Atmosphere 27

5. Atmospheric Pressure 31

6. Temperature 39

7. Atmospheric obscurity 43

8. Winds 51

9. Lapse rate, Stability and Instability 57

10. Clouds and Precipitation 67

11. Thunderstorms 83

12. Jet Streams 95

13. Mountain Waves 103

14. Clear Air Turbulence 109

15. Ice Accretion 115

16. Condensation Trails 127

17. General Circulation over the globe 133

18. Air Masses 141

19. Fronts and Depressions of Middle latitudes 147

20. Western Disturbances 155

21. Tropical Discontinuities & Convergence zones 159

22. Tropical Depressions 167

23. Tropical Cyclones 171

24. Microburst - Windshear 179

25. The Monsoons 185

26. Aviation Climatology of India 195

27 Surface Weather Charts 207

28 SPECI and Weather warning 217

29 Elements of weather forecasting 221

30 Pre-Flight, In-Flight and Post Flight Procedures 225

31 Meteorological requirements for aviation 231


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CHAPTER 1

MET SERVICES FOR AVIATION

Introduction

1. On seeing this book for the first time, you may ask yourself two questions: what is

Meteorology, and why must I learn something about it? By the time you have read this chapter, you

should be clear on these two points at least.

2. Meteorology includes the study of all the changing atmospheric conditions, such as fog, snow,

rain, thunderstorms and wind, which go to make up our weather. It is the branch of science whichdeals with the earth's atmosphere and the physical processes occurring in it.

3. Why do you need to study Meteorology? Well, an explorer needs a map to show him the

features of the terrain over which he will travel. A mariner navigating the oceans must be familiar with

the ways of the sea. Your aircraft operates in the earth's atmosphere, therefore to operate efficiently,

you as aircrew must understand the behaviour of the ocean of air in which you fly.

Aim and Scope of the Book

4. Some weather manifestations can be awe-inspiring as we feel uncertain when we encounter

phenomena that we cannot explain, but understanding breeds confidence. Moreover, the many and

varied facts we need to know about the behaviour of the atmosphere can be grasped more easily by

understanding the physical reasons underlying them. So this book sets out to do two things: to

explain weather phenomena of significance to aircrew in terms of simple physics, i.e. the laws of

motion, of heating and cooling, of condensation and evaporation, and so on, and to outline the

facilities that are available for obtaining weather information on the ground and in the air. The aim is

to help you to operate with maximum efficiency, safety, and confidence, in all types of weather.

Value of Weather Knowledge

5. At one time pilots thought that it would be possible to get above the weather by flying at about

20,000'. Nowadays it is realised that even above 40,000' certain weather features are still important.

These include wind, temperature, density, condensation trails, and sometimes even thunderstorms or

icing. Winds at about 30,000' to 40,000' often exceed 100 knots in a narrow belt, and aircraft caught

unprepared may be swept off their intended track or have insufficient fuel to return to base.

6. Aircraft with modern aids operate regularly in weather, which would once have been

considered too bad for flying. However, there are still minimum weather limits for safe flying. Some


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FIS Book 5: Meteorology for Aviators 2

weather hazards, such as thunderstorms, may be readily negotiated by a fully trained pilot, but may

involve a less experienced pilot in difficulties. In adverse conditions, knowledge of the weather and its

forecast development is of the utmost value in helping inexperienced aircrew to avoid hazards and

experienced aircrew to negotiate them confidently.

7. It is impossible to change the weather to suit your flight, except on a small scale, e.g. over

part of an aerodrome, fog may be dispersed by FIDO or similar heating equipment. However, it is

usually possible to plan your flight to suit the weather, by selection of route, altitude and time of

departure or arrival. A good example of the tactical use of weather operationally, was the escape of

the German cruisers Scharnhorst and Gneisenau through the English Channel in 1942 under cover of

a slow moving belt of low clouds and bad weather. Another example was the Bomber Command raid

on Milan in October 1942, when British aircraft used weather cover to cross France by day.

8. Your understanding of the significance of the various weather symbols will enable you to

recognise when a forecast is going wrong, and to decide on appropriate action based on your

appreciation of the weather situation obtained from the pre-flight weather briefing or forecast, and

from your own flight observations. In this connection you should remember that accurate weather

reports made during your flight are likely to be of value to other crews flying in the locality, particularly

if adverse conditions are unexpectedly met. Apart from helping the Meteorological Officer to check

his forecast, your reports may provide the first indication of unforeseen developments.

9. One of the best ways of acquiring a fund of useful weather information is to pay frequent visits

to your local meteorological office. By discussions with the forecaster you can clear up many

problems and also gain an insight into his difficulties.

10. Various components, which make up the weather, may have widely different meanings and

importance for different people. A pilot may be chiefly interested in the weather at base and

destination; the navigator may be more interested in winds and temperatures at various heights,

whereas the signaller is probably concerned with those areas where bad weather may interfere with

communications. The forecaster caters for all these varied requirements on request, but to use the

service efficiently you must know what facilities are available, as well as the limitations of the service,

and be able to understand weather charts and technical terms.

Importance of Meteorology in Aviation

11. Meteorology is the study of weather processes in the atmosphere. Since the atmosphere is

the medium through which an aircraft is flown, it is essential for the pilot to know this subject so as to

enable him to fly the aircraft efficiently, economically and safely.

12. Meteorological information is useful to aviation in the following ways:


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Met Services for Aviation 3

(a) It enables a proper selection of site for an airfield and correct orientation of runways.

(b) It helps engineers to design aircraft properly by giving them data of pressure, wind

temperature and turbulence characteristics of the atmosphere at various levels.

(c) It assists in accurate planning and correct execution of take-off, climb, cruise,

descent/diversion, approach and landing etc. of an aircraft.

World Meteorological Organisation

13. Atmospheric processes occur on a large scale. To understand them it is necessary to

observe the behaviour of the atmosphere simultaneously over a large part of the globe and transmit

these observations quickly to the users. This needs a well-knit organisation on an international scale.

14. The World Meteorological Organisation (WMO), which is a specialised agency of the United

Nations Organisation, co-ordinates and standardises meteorological practices all over the world. It

also helps in maintaining communication networks through which member countries exchange their

meteorological data quickly. Quick exchange is facilitated by using internationally agreed codes for

different types of messages. The WMO works in close collaboration with the Met division of the

International Civil Aviation Organisation (ICAO) in matters connected with meteorological

requirements for aviation.

India Meteorological Department

15 India is a member country of the WMO. Meteorological requirements in India are looked after

by the India Meteorological Department (IMD). The IMD works in a close collaboration with the Civil

Aviation Department as well as the Indian Air Force in matters affecting civil and military flying.

Function of IMD

16. The function of IMD in regard to aviation can be broadly classified as follows:

(a) Maintenance of IMD observatories for taking observations of meteorological

elements.

(b) Maintenance of IMD communications network to exchange meteorological data

quickly.

(c) Maintenance of forecasting offices at civil airfields.

(d) Collection and statistical analysis of meteorological data and their periodical


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publication in the form of climatic summaries, tables and atlases.

Observatories

17. The IMD maintains various types of observatories depending on the nature of meteorological

elements observed. The types of observatories which are of direct concern to aviation are:

(a) Surface Observatories. These observe meteorological elements from a ground

position at 0830 and 1730 hrs, every day. Some of the observatories take observation at

0230, 0530, 1130, 1430, 2030 and 2330 hrs also. All these observations and the coded

messages containing the data are known as “SYNOP”. The network of surface observatories

is fairly dense there being one or more such in almost every district.

(b) Current Weather Observatories. These are located at airfields and important

checkpoints on air routes. They take surface meteorological observations generally once in

an hour but their hours of watch may vary according to traffic requirements. The coded

messages from these observatories are known as “METAR”. These observatories also issue

special reports whenever meteorological elements deteriorate to certain specified minima,

which are considered hazardous for aviation. When the hazardous conditions cease, reports

of improvement are issued. Such special reports are known as “SPECI".

(c) Pilot Balloon Observatories. These observe and report the direction and speed of

winds at different levels at 0530, 1730 and 2330 hrs daily while a few take observations at

1130 hrs also. The observations are made by visual methods. The reports from these

observatories are known as “PILOT". In India, there are about 65 such observatories.

(d) RAWIN Observator ies. These measure the upper winds by radio methods twice a

day, i.e. at 0530 and 1730 hrs. The coded messages are known as "RAWIN".

(e) Radio Sonde Observatories. These observe the pressure, temperature and

humidity at higher levels daily at 0530 hrs and 1730 hrs. The coded messages are known as

"TEMP". Generally, RAWIN and radio sonde observatories are combined at one location.

(f) Weather Radar Observatories. These are located at the major civil airfields. The

radar sets are specially designed to detect thunderstorms, their distance, vertical extent etc.

The coded messages from the radar observatories are known as "RAREP".

Ship and Aircraft Observations

18. Apart from the static observatories mentioned above, the IMD has the benefit of


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meteorological reports from ships and buoys in Indian waters. Most ships of the merchant navy take

meteorological observations at the standard hours of observation and transmit them to coastal radio

stations by wireless. They also transmit special reports whenever they are in the field of depressions

or cyclonic storms.

19. Another important sources of meteorological observations are aircraft on routine or non-

routine flights. Reports from aircraft are known as "AIREP". They are of immense use in forecasting

for aviation purposes.

Rockets and Satellites

20. Nowadays, data on the upper layers of the atmosphere can be had by means of rockets and

weather satellites. Weather satellites are especially useful in observing cloud coverage over ocean

and collection of data from sparse areas and can give invaluable assistance in locating incipient

storms. Satellite data is exchanged on international basis with the help of Data collection platforms

(DCPs), which are installed in inaccessible and inhospitable terrain. These DCPs are automatic

weather stations, which record observation and transmit through satellites.

Communication Networks

21. Observatories transmit their reports to the nearest Regional Met Centre (RMC) by means of

telegrams bearing high precedence. There are six such centres located Delhi, Mumbai, Nagpur,

Guwahati, Kolkata and Chennai. These centres are connected to each other by landline teleprinter

through which the messages are exchanged. These centres are also connected to other aviation

forecasting offices by teleprinter channels.

22. At Delhi, there is a broadcasting station known as the All Indian Met Broadcast centre

(AIMBC) which transmits Met messages by radio teleprinter (RTP). These broadcasts also contain a

selection of messages from neighbouring countries. The broadcasts can be received by any

forecasting office, which has suitable equipment for reception. The AIMBC works on a 24-hour basis

and does almost continuous transmission of a very large volume of meteorological messages.

23. METARS issued by current weather observatories at airfields are exchanged on the Fixed

Aeronautical Communication Service (FACS) maintained by the Civil Aviation department. They are

also transmitted on ground to air W/T and R/T channels for aircraft in flight.

Met Information Centre

24. Aviation Met service is provided through forecasting offices. They are divided into different

categories on the basis of number of weather charts prepared, hours of forecasting, the type of watch


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they keep and the extent to which they can issue route forecasts independently. These are:

(a) Main Met Offices (MMO). These maintain 24 hours-forecast watch, prepare

necessary charts at all synoptic hours and issue forecast upto any distance, including

destinations outside India.

(b) Dependent Met Offices (DMO). These keep restricted hours of forecasting watch,

prepare necessary charts to issue the forecasts and issue route forecast upto any distance in

India.

(c) Subsidiary Met Offices (SMO). These have no forecasting watch. No charts or

forecasts are issued independently. All forecasts are obtained from respective MMOs.

25. Area Met Watch Off ices. In addition to the above offices, Area Watch Offices (AWMO) are

maintained at Chennai, Mumbai, Delhi and Kolkata to provide aircraft in flights advance notice of

actual or impending weather development or trends that are potentially hazardous for aviation. The

significant information, SIGMET INFORMATION, is issued for the occurrence or expected occurrence

of any one of the following phenomena within their jurisdiction by Met Watch Office.

(a) Active thunderstorm area.

(b) Severe line squall.

(c) Heavy hail.

(d) Severe turbulence.

(e) Severe icing.

(f) Marked mountain waves.

(g) Widespread dust storm.

Climatic Data

26. The IMD performs a very useful function in collection and statistical analysis of meteorological

data. It publishes these data periodically and also brings out climatic summary, tables and atlases.

These are of immense use in various planning tasks connected with aviation.

Met Organisation in the IAF

27. The IAF has its own set up for catering to meteorological requirements at AF airfields. This

set-up is independent of the IMD, from the administrative as well as functional points of view.

However, due to the nature of work, the Met organisation in the IAF maintains close liaison with units

of the IMD.


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28. At every flying station, there is a Met section, which has facilities fairly similar to DMOs in the

IMD. The section is under the control of the Air Officer Commanding (AOC) / Station Commander

through the Chief Operations Officer (COO),. The technical aspects of Met Sections in the IAF are

looked after by the Assistant Chief of the Air Staff (ACAS) of Meteorology at Air Headquarters through

Command Met Officers at Command Headquarters and closely correspond to those followed in the

IMD.

29. Met sections generally have three communication channels for receiving met messages.

(a) Land Line Teleprinter. This is connected to the nearest IMD Met Office which itself

is part of bigger network and has Duplex system.

(b) RTT Reception Channel. This is for receiving broadcasts from AIMBC.

(c) W/T Met Point to Point Channel. The exclusive channel serves to exchange

hourly METARS of Air Force airfields all over India.

30. The other met data is received through modern met equipment like: -

(a) SIRAVDS. It stands for Satellite Imageries Reception and Video Display System.

This is a ground based equipment by which met section receives INSAT satellite imageries

every three hours.

(b) DRS. It stands for Direct Reception System, which provides satellite communication

for exchange of meteorological data.

(c) MMHS. It stands for Meteorological Message Handling System. It is a computer

based system, which handles data from various channels.

(d) DMSS. It stands for Distributed Message Switching System. It is a dedicated net

work of IAF (Met), which is based on propagation of electromagnetic radiation's in

troposphere hence commonly referred to as 'Tropo network'.

SAMPLE QUESTIONS

1. Expand the following:

(a) WMO.

(b) RMC.

(c) AIMBC.

(d) RTP.


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(e) SIRAVDS.

(f) MMHS.

(g) DRS.

(h) RAWIN.

(j) FACS.

(k) DCP.

2. Choose the correct answer:

(a) IAF Met works under:

(i) IMD. (ii) ICAO.

(iii) Air HQ.

(b) IAF Met follows generally conventions laid down by

(i) WMO. (ii) Air HQ.

(iii) Guidelines are from WMO but specific requirements of IAF.

(c) Type of Met section depends upon

(i) Rank of S Met O. (ii) No of D Met O.

(iii) Duration of forecasting watch.

(d) Air Force Met section have their own equipment's for receiving

(i) Satellite pictures. (ii) Radar reports.

(iii) Temperature in upper air.


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9

CHAPTER 2

MET CHARTS, FORECASTS AND BRIEFING

Introduction

1. Simultaneous observations of the atmosphere are made at a large number of meteorological

stations. These observations are transcribed into coded messages, which are collected at the

respective MMOs and disseminated by the aid of radio or other means of telecommunication for

national and international use. These reports are received in large numbers at forecasting offices and

before they can be comprehensively viewed by the forecaster, they are required to be plotted on

suitable charts.

Weather Charts and Information

2. The following types of charts are used in plotting meteorological observations:

(a) Surface Weather Charts.

(b) Upper wind Charts.

(c) Constant Pressure Charts.

(d) Auxiliary Charts.

3. Surface Weather Chart. On this chart, the position

of each reporting station is marked by a small circle with its

three-letter station name code alongside. The coded message

is represented by entries in and around the appropriate station

circle, some in figures, some in symbols, but in a standard form

which is agreed internationally in order that charts may be

interpreted with equal facility by any nation. An inland station

model is shown in the Fig 2-1. Fig 2-1: Inland Station Model

4. Upper Wind Chart. One of the factors in air navigation is wind at flight altitude. The

observed winds for selected levels (viz. 1,000 ft, 2,000 ft, 3,000 ft, 5,000 ft and so on) are plotted on

separate upper wind charts using a system commonly referred to as the "Barb and Pennant" system.

Upper wind charts are useful in representing the general wind flow over an area at various levels.

They also show the flow of moisture and vertical and horizontal shear in the wind, which cause

atmospheric turbulence and bumpiness.

5. Constant Pressure Charts. No study of the weather situation can be complete unless it is

three-dimensional. The surface charts do take some account of this aspect of the problem since the


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plotted reports include features of weather e.g. cloud and rain which originate far above the surface

layers but a detailed analysis of observations of pressure, temperature, humidity and wind in the

upper air is an essential part of the diagnosis of any synoptic situation.

6. The most practical way of displaying the patterns of circulations in the free atmosphere is by

construction of contour charts for selected pressure levels. Data collected from radiosonde

observations are used to prepare constant pressure charts.

7. Auxi liary Charts . Elements of surface observations, which are not plotted on the main

synoptic charts and are plotted on separate charts, are called auxiliary charts.

Functions of Met Section

8. The principal functions of a Met Section are:

(a) To keep observational watch on the airfield for hours of watch and issue hourly

METAR and SPECI to ATC during watch hour.

(b) To receive METAR and SPECI of other airfields for smooth air operation.

(c) Reception and plotting of SYNOP, PILOT & TEMP messages as per requirement.

(d) Preparing charts of various types based on messages pertaining to surface and

upper air meteorological condition.

(e) Analysing these charts and issuing forecasts and adverse weather warnings on a

routine and non-routine basis during the hours of forecasting watch.

(f) Briefing aircrew on present state and anticipated trends in weather in relation to flying

activities.

Met Codes for Surface and Upper Air Reports

9. Following is a brief description of the met codes used for transmitting surface and upper air

reports:

(a) SYNOP Code. A surface observation reported in a form known as "SYNOP".

(b) PILOT Code. Upper winds observed by usual method are passed in a coded

message known as "PILOT". It consists of four parts namely A, B, C and D.


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Met Charts, Forecasts and Briefing 11

(c) TEMP Code. The code form used for transmission of radio sonde / RAWIN

observations of pressure, temperature, humidity and wind in the upper air is known as

"TEMP".

Met Codes for Current Weather and Forecasts

10. (a) METAR. Current weather recorded at hourly or half-hourly intervals at CWOs is

transmitted in a code known as METAR.

(b) SPECI/SPECIAL. This code form is used for messages indicating sudden

deterioration in any element and its subsequent improvement, as per laid down criteria.

(c) TAF. A forecast of terminal weather conditions is encoded in a form known as TAF

(Terminal Aerodrome Forecast).

(d) ROFOR. A weather forecast for a specified route is written on T-3/T-4 form is

known as ROFOR (Route Forecast).

The codes mentioned in para 9, follow a pre-determined pattern and use symbols, which are

interpreted with the help of code tables approved by the WMO.

Synoptic Meteorology

11. Study of weather over a large area by means of charts indicating various weather elements is

known as "synoptic meteorology". The various types of charts used in plotting weather are listed

below:

(a) Surface Weather Charts.

(b) Upper Wind Charts.

(c) Tephigram.

(d) Constant Pressure Charts.

(e) Thickness Charts.

(f) Other Special Charts (Auxillary charts).

12. Surface Weather Charts. The principal weather chart used by vast majority of aviators is

the 'Surface Chart', on which SYNOP messages are plotted. This is also known as ‘Synoptic Chart’.

It is usually prepared five times a day, corresponding to observation at 0000, 0300, 0600, 1200 and

1800 hrs UTC.

13. The chart is prepared on a base map covering India and immediate neighbourhood. The size

of the map is 3' X 2 ½' which has a scale of 1:10,000,000.


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14. The plotting of the surface chart is done according to a standard scheme involving of figures

as received in SYNOP message for elements like pressure, temperature etc. and of agreed standard

symbols such as type of cloud and kind of weather etc. The chart also shows Isobars, trough patterns,

lows & depressions.

15. Upper Wind Charts. While the surface chart is the principal chart from which the

information on the sea-level pressure as well as clouding and weather phenomena occurring over

different areas can be derived, the upper winds charts are used for wind speed, direction and upper

air pattern to arrive at the intensity of surface synoptic system and winds for navigation. The data is

collected by pilot balloon observations of various stations.

16. Tephigram. The term 'Tephigram' is given to the thermodynamic diagram in which height,

temperature and humidity are plotted against pressure in the atmosphere. The Tephigram indicates

the various stages of atmospheric stability (or instability), which helps a great deal in determining an

accurate forecast.

17. Constant Pressure Chart. We have seen in previous paragraph that the Tephigram gives

variation of height, temperature and humidity with pressure over a single station. However, due to

many reasons, at times it becomes necessary to know the distribution of these quantities at fixed

levels over a given number of stations for fixed height. The common practice is to prepare such

charts at fixed pressure levels instead of at fixed height.

18. Thickness Charts & Other Auxiliary Charts. These charts contain data, the details of

which are beyond the scope of this book.

Interpretation and Plotting o f Various Symbols

19. You have read earlier that the most commonly used chart by aviators is the surface chart

where plotting is done according to a standard scheme involving of figures as received in the SYNOP

message for such elements as pressure, temperature, visibility etc., and of agreed standard symbols

for elements such as kind of cloud and present weather. The arrangement of these symbols or

figures also follows a standard pattern.

Obtaining of Weather Forecast

20. At all civil airfields where routine or non-routine traffic exists, meteorological forecasting

offices are established for providing weather service for aviation purposes. The scale of such offices

depends upon the traffic requirements. In case of IAF, all flying stations have a Met Section, forproviding weather service. The IAF Met sections function in close liaison with IMD to be able to make

full use of the facilities provided by them.


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Types of forecasts

21. Met sections issue different types of forecasts. The main types are:

(a) Local Forecasts. These are for an area of radius 50 kms around the airfield. They

are valid for 6-12 hours and may give an "Outlook" for a further period of 6 hours.

(b) Route or Flight Forecasts. These are issued for a specific flight over a given route

and are written on standard forms like T-3 & T-4.

(c) Terminal Aerodrome Forecasts. These are forecasts for the required period in

respect of terminal or diversionary airfields. They are known as "TAFs" and written in T-10.

(d) Area Forecasts. These are forecasts covering specified area and these are issued

on request of such purpose as aerial survey, photoreconnaissance etc.

(e) Trend Forecasts. These are short-range local forecasts valid for 2 hours. They

specify only significant anticipated changes in the met conditions, and are appended to all

METARS. When no significant change is anticipated the word 'NOSIG' is appended.

(f) Weather Warning & Cautionary Met Reports. These are issued in anticipation of

imminent adverse weather for flying at the airfield or its neighbourhood. These are advisory

in nature and are issued with a notice of 1/2 hr to 1 hr.

Met Briefing

22. The procedure by which a Met Officer conveys information to a pilot is known as ‘Met

Briefing‘. For local flying of a flight(s) or Squadron(s), it is more convenient if a combined briefing is

done for all aircrew of the Flight or Squadron. This is known as "Mass Met Briefing". In a met briefing

the Met Officer gives the following information:

(a) Salient features observed on latest Surface & Upper air charts.

(b) Present state of weather at base and diversionary airfields with emphasis on

elements adverse for flying.

(c) Local forecast for base and diversionary airfields for the next 6-12 hours with specific

mention of any weather warnings in force and comments on the likelihood of their further

extension.


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23. At the end of the briefing, aircrew may ask any relevant question either to clarify doubts or to

elicit supplementary information required.

24. For cross-country flights, the met briefing is conducted in the Met Section. The briefing is

invariably supplemented by written forecasts on standard forms for the route as well as for terminal

and diversionary airfields. Flight planning is done only after obtaining proper met briefing.

25. In this connection, it is well to remember the following points:

(a) The Met Officer is authorised to provide current weather and forecast information for

flying. He is not authorised to comment on the feasibility or otherwise of undertaking specific

flight. Decisions on flights are taken by officers who are duly delegated this responsibility.

These decisions are no doubt, taken after due consideration of the met briefing. However,

many other considerations such as the limitations of the aircraft, capability of the pilot,

availability of navigational aids etc., play important role.

(b) Weather forecasting is not an exact service and despite the best efforts, Met

forecasts issued for flight are likely to have deviations and inaccuracies. To derive maximum

benefit of Met section, an aviator must take Met briefing prior to departure in person and

should make an endeavour to understand the Met charts. In cases of delays etc. an update

must be obtained. It is his duty to report to ATC of any encounter with significant weather or

its existence in the reportable vicinity. He should inculcate the healthy habit of debriefing Met

Officer and flight commander for taking remedial actions.

26. From the foregoing it will be clear that an elaborate organisation exists for feeding

meteorological information to a Met Section. The input and output of a normal Met Section are shown

schematically in Table 2-1. A pilot should make the best use of the organisation and the facilities

provided by it. In turn, he should contribute to its usefulness by rendering in flight and post flight

weather reports and by free and frank discussion with the Met Officer regarding weather experienced

by him.

Table 2-1: Input and Output f rom a Normal Met Section


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SAMPLE QUESTIONS

1. Choose correct answer / answers.

(a) SYNOP is recorded every:

(i) Hour. (ii) Half an hour.

(ii) Three hours.

(b) Metar is recorded every:

(i) Half an hour. (ii) Hour.

(iii) Half an hour during adverse weather period.

(c) SPECI / SPECIAL are recorded every:


(iii) Any time.

(d) CMR / Weather Warning are generally issued with a minimum notice of:


(iii) Two hours.

(e) Satellite pictures are received every:

(i) 1 hour. (ii) 3 hours.

(iii) 6 hours.

(f) RAREPs are taken at every:


(iii) Any time. (iv) 3 hrs in case of no echo and 1 hr in

case of echo.

(g) TAFs are valid for usually:(i) 6 hours. (ii) 9 hours.

(iii) 12 hours.

(h) Local/ Area forecasts are usually valid for:

(i) 6 hours. (ii) 9 hours.

(iii) 12 hours.

(j) If CMR is issued, it will be followed by weather warning:

(i) Surely. (ii) Not necessary.

(iii) May be.


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(k) Tephigram is used to asses:

(i) Temperature. (ii) ALTICOR.

(iii) Instability.


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17

CHAPTER 3

MET OBSERVATIONS AND ELEMENTS

Introduction

1. Meteorological observations of many elements are made with the help of suitable

instruments. However, there are certain elements for the observation of which either no instruments

have been developed or the instruments are too complicated and expensive to be put into general

use. Such elements are observed visually by trained and experienced personnel. There are eight

elements of meteorological observations that are recorded viz. pressure, temperature, humidity, wind,

air density, clouds, precipitation & visibility.

Atmospheric Pressure

2. The weight of a column of air standing on unit square area and extending vertically to the

uppermost levels of the atmosphere is known as 'atmospheric pressure’. Pressure can be expressed

in many ways e.g. hPa, Millibars, pounds per square inch, grams per square centimetre etc.

3. The most accurate method of measuring pressure is by balancing the weight of the column of

air against a column of mercury in a glass tube, which has vacuum at the top. The instrument, which

utilises this principle, is known as a "mercury barometer". The readings of the length of the column of

the mercury, corrected for some factors are used to express the pressure. At mean sea level the

pressure is of the order of 760 mm or 29.92" of mercury. The unit of pressure in common use in

meteorology is the 'hectopascal'. The mean sea level pressure is of the order of 1000 hPa.

4. The pressure in the atmosphere decreases with height. In order that the pressure at two

places at different elevations can be compared, it is necessary that the readings are ‘reduced’ to a

common level. Reduction is generally done to mean sea level. For this some corrections are needed.

5. The corrections referred to above are due to the fact that the weight of the column of mercury

is dependent on the temperature and on the acceleration due to gravity. To make two readings at

different times or different places comparable, the readings are "reduced" to a common value of

temperature and gravity also.

6. Mercury barometers are delicate instruments and are unsuitable for mobile units. An

instrument, which is sturdy, compact and suitable for carrying from one place to another, is known as

an Aneroid barometer and is illustrated in Fig 3-1. It consists of a chamber made of two corrugated

lids; hermetically sealed after removing the air inside. As the atmospheric pressure changes the lids

are pressed closer together or are displaced away from each other. The movements is magnified and


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transmitted to a needle which moves along a

graduated dial; giving readings of pressure. The

aneroid barometer is initially adjusted by comparing it

with a mercury barometer.

7. Aircraft altimeters are actually aneroid

barometers in which the dial graduations are in terms

of height in the atmosphere (feet or meters) instead of

units of pressure. Fig 3-1: Aneroid Barometer

8. Measurements of pressure in the upper air are made by releasing aneroid type barometers

along with hydrogen-filled balloons. The readings are transmitted by radio signalling arrangement,

which works automatically and has been previously calibrated in the laboratory.

9. An aneroid barometer which gives a continuous and permanent record of pressure on a chart

is known as a "Barograph".

Temperature

10. Temperature is a measure of the degree of warmth of a substance. It is generally determined

by means of a thermometer, which works on the principle of expansion of a liquid with increase of

temperature. For measuring air temperature, the thermometer is kept in a well ventilated louvered

wooden screen known as "Stevenson Screen" which shields the thermometer from the Sun's rays.

11. Temperature can be expressed in terms of different scales. The common scales are

Fahrenheit, Centigrade (or Celsius) and Absolute (or Kelvin). The relationship between these scales

is as follows:

F = 32 + 9 X C

5

C = 5 X (F - 32)9

A = 273 + C

12. Another method of measuring temperature is by utilising the principle of the expansion of a

strip of metal with increase of temperature. A bimetallic coil is generally used. The coil winds or

unwinds with changes in temperature. This movement is magnified and transmitted to a needle,

which moves across a graduated dial or chart. Measurement of temperatures in the upper air is done

by this method. The "thermograph" which gives a continuous and permanent record of temperature is

also based on this principle.


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Met Observations and Elements 19

Ai r Densi ty

13. Density is defined as mass per unit volume. Density of air in the atmosphere is not measured

directly. It is calculated from the well known gas equation.

P = RT

ρ

Where P is pressure, ρ is density, T is temperature and R is a constant known as "gas constant"

whose value is 287.0 Joules per kg K. This formula gives a density of 1225 grams per cubic metre for

dry air at ISA sea level pressure of 1013.2 mb and a temperature of 15º C (288.15º A).

Humidity

14. Humidity is a general term used in regard to the water vapour content in the atmosphere. It

can be expressed in several ways. The more common ones used in aviation meteorology are given

below:

(a) Relative Humidity. Air at a given temperature can hold only a certain quantity of

water vapour and no more. The ratio of the actual amount of water vapour held to the

maximum it can hold at that temperature is known as "relative humidity". It is usually

expressed as a percentage. Air is said to have reached "saturation" when it holds the

maximum water vapour for that temperature, i.e. when the relative humidity is 100%.

(b) Dew Point Temperature. The amount of water vapour required to saturate air is

more at higher temperatures. Thus if air having water vapour is cooled, a stage will be

reached when the water vapour present is sufficient to saturate the air. Any further cooling

results in excess water condensing into water droplets (dew) on near by solid objects. The

temperature to which air has to be cooled to reach saturation is defined as dew point

temperature. A low dew-point temperature indicates low water vapour content.

(c) Wet-Bulb Temperature. This is the temperature attained by a thermometer bulb

from which free evaporation of water is taking place. The wet bulb temperature is intermediate

between the air temperature and dew point temperature, but all three are equal when the air

has reached saturation.

(d) Humidity Mixing Ratio. This is the mass of water vapour present in unit mass of

air. It is generally expressed as grams of water vapour in kilograms of (dry) air.

(e) Vapour Pressure. It is that part of atmospheric pressure, which is exerted by water

vapour. It is expressed in hPa.


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15. Humidity is generally measured by covering the bulb of a thermometer with wet muslin cloth

and measuring the wet bulb temperature. The difference between the air temperature and the wet

bulb temperature is noted and from this a calculation of relative humidity, dew-point temperature etc.

can be made with the aid of formula or tables.

16. Humidity in the upper air is measured by winding muslin on a bimetallic coil and allowing the

ends of the muslin to dip in a can of water.

17. The length of the human hair, which is completely de-greased, is sensitive to changes in

humidity. This principle is used in the "Hygrograph" which is an instrument giving continuous and

permanent record of relative humidity.

Wind

18. Wind is air in a horizontal motion. Its direction is represented by the point from which it blows

and is specified either in one of the 16 points of compass or in degrees from true North reckoned in a

clockwise sense. At some airfields the ATC adopts a convention of expressing the wind direction as a

deviation from the orientation of the runway in use, the deviation being given in terms of the hour hand

of clock. Thus when runway '36' is in use, a wind blowing from East would be specified as "wind 3 O

clock" one from West as "wind 9 O clock" etc. This method of specifying wind direction enables a pilot

to make a quick assessment of the crosswind component.

19. The direction of the wind is measured by a vane, which rotates freely on a spindle and

continuously aligns itself with the wind. Wind direction near a runway can be estimated by means of

windsocks. At landing grounds where ATC facilities do not exist smoke candles are used to give a

visual indication to the pilot.

20. The speed of the wind is measured by an

instrument known as an "Anemometer". It consists of

an assembly of cups, which is mounted on a spindle

(Fig 3-2). The cups rotate with the wind. The speed of

rotation is proportional to the speed of the wind. The

speed is measured either by means of a counter or by

electrical methods in which case the reading can be

taken on a panel fixed in the Met Section or the ATC.

Wind speed is expressed in knot or kilometres per

hour.

Fig 3-2: Anemometer and Wind Vane

21. Wind speed is rarely steady. The irregular oscillation in speed is known as "gustiness".

When there is unusual gustiness the mean speed as well as the peak speed in gusts are reported.


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22. Winds at higher levels are measured by letting off hydrogen filled balloons and tracking their

movement by means of optical or radio theodolites.

23. An instrument which gives a continuous and permanent record of wind is known as an

"Anemograph".

24. In the absence of an anemometer, wind speed can be estimated quite reliably by using what

is known as the "Beaufort Scale". The lower stages of this scale are given in table 3-1.

Effect of Wind Speed Range (kts)

Smoke rises vertically Less than 1

Slight deviation in rising smoke 1 - 3

Leaves rustle 4 - 6

Leaves and twigs in constant motion 7 - 10

Small branches in motion 11 - 16

Small trees begin to sway 17 - 21

Large branches in motion 22 - 27

Whole trees in motion More than 27

Table 3-1: Beaufort scale

Clouds

25. Observations of clouds consist of three different aspects:

(a) Type of Cloud. Clouds have been classified into different types each bearing a

name, identification of the type is done through experience.

(b) Amount of Cloud. The coverage of a particular kind of cloud or the total coverage

of all kinds of clouds is determined as a fraction of the sky. It is expressed in Okta i.e. in eight

parts of the sky. Thus when half the sky is covered, the amount is given as 4 Okta.

(c) Height o f Base of Cloud. This is determined from estimation by trained observers

with reference to nearby hills or other high features. The height of base of clouds can be

measured also. During daytime a hydrogen filled balloon with a known rate of ascent is

released. The time it takes to enter into the lower portions of the clouds is noted and from this

the height of base calculated. At night a "cloud searchlight" may be used. The searchlight

throws a powerful vertical beam of light, which is intercepted, by the cloud. The vertical angle

of the patch of light on the cloud is measured from a point at a known distance from the

searchlight. By simple trigonometry, the height of base of cloud can be calculated. A

"Ceilometer" is a cloud searchlight in which the patch of light on the cloud is located by


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photoelectric method. Laser Range Finder (LRF) is used to estimate cloud base by focussing

a laser beam on clouds overhead.

Height of base of cloud in all SYNOP, METAR and SPECI reports refers to height above ground level(AGL).

Visibility

26. Visibility is a measure of the degree of transparency of the atmosphere. It is expressed in

terms of the distance in meters upto which objects are visible to the naked eye and can be recognised

as such.

27. During daytime, visibility is estimated by using landmarks at known distance. At night the

method adopted is to estimate the equivalent daytime visibility by means of lights of standard candle

power at specified distances. If such lights are not available, existing lights are used for estimation.

Runway Visual Range

28. Runway visual range (RVR) is reported on those airfields where Scopograph or

Transmissometer is installed. RVR is reported only when visibility is less than 2000 m.

Rainfall

29. Rainfall is measured in terms of the depth of accumulation over level ground if run off is not

permitted. It is expressed in inches or millimetres. A rainguage is merely a container, which catches

the rain and keeps it stored until the next observation. The depth of water is measured by pouring the

water in a measuring glass, which is suitably graduated. Snowfall is measured by melting the snow

and measuring the rainfall equivalent.

Weather Phenomena

30. Weather phenomena are identified and classified through experience by trained observers.

Apart from the kind of phenomenon, its character and intensity are also judged.

31. The following main phenomena occur in different season:

(a) Haze. Atmospheric obscurity due to moisture, dust or smoke wherein visibility is

reduced to 2 to 5 km. It is called haze if humidity is > 75%. In case < 75% it is called Dust

haze or Smoke haze.

(b) Mist. Moist haze is said to exist when visibility ranges from 1km to less than 2km.


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or visibility reported by Met section is different from his observation in the air. On many occasions he

is in a better position to judge these elements than an observer on the ground, especially in case of

cloud height and location of Cb cells etc.

SAMPLE QUESTIONS

1. Choose correct answer/answers.

(a) Visibility in Haze is:

(i) 2 to 4 km. (ii) 3 to 5km.

(iii) 4 to 5 km. (iv) 2 to 5 km.

(b) Visibility in Mist is:

(i) 1 to < 2 km. (ii) 1 to 2 km.

(iii) 2 to 3 km.

(c) Fog means visibility is:

(i) 1 km or less. (ii) Less than 1km.

(iii) 1to 2 km.

(d) Squall can be reported if it lasts for minimum of:

(i) 1 minute. (ii) 2 minutes.

(iii) 5 minutes.

(e) Thunderstorm can be reported when:

(i) Thunder is heard. (ii) Lightening is seen.

(iii) Precipitation commences.

(f) RVR is reported only when visibility is less than:

(i) 1 km. (ii) 2 km.

(iii) 3 km.

(g) RVR is reported with the help of:

(i) Scopograph. (ii) Barograph.

(iii) Anemograph.

(h) In Dust raising winds (DRW) visibility is less than:(i) 1 km. (ii) 3 km.

(iii) 5 km.


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(j) Showers would be experienced from:

(i) ST. (ii) SC.

(iii) CU.

(k) An AS cloud patch is likely to cause generally:

(i) Drizzle. (ii) Rain.

(iii) Shower.


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27

CHAPTER 4

STRUCTURE OF THE ATMOSPHERE

Composition of Air

1. The atmosphere, in its dry state is a mixture of many gases of which nitrogen and oxygen are

by far the most abundant, accounting for almost 99% of its content. By weight there are nearly three

parts of nitrogen to one part of oxygen. This composition shows very little variation upto high levels.

The atmosphere also contains, in widely varying quantities, minute particles of dust, smoke and other

impurities, which cause obscurity of the atmosphere. This phenomenon is important from the point of

view of aviation. Certain layers of the atmosphere contain ozone. Ozone is responsible for absorptionof ultra-violet radiation from the sun. The atmosphere also contains traces of other gases like carbon

dioxide, hydrogen, helium etc.

2. Actually the atmosphere is never dry. Water vapour in varying quantities is present, chiefly in

the lower layers. Water vapour also behaves as a gas; frequently, however, it condenses into liquid

or solid precipitation like rain or snow. From the point of meteorology, water vapour is thus a very

important constituent of air.

Layers in the Atmosphere

3. The atmosphere can be conveniently divided into layers, which have different characteristics

in regard to variation of temperature with height. Although the vertical extent of these layers is not

constant, the variations are not too large and have been averaged by means of a large number of

observations. The main atmospheric layers are schematically represented in Fig 4-1.

Troposphere

4. This is the layer closest to the surface of the earth. It is characterised, on an average, by a

fairly uniform fall of temperature with height. The fall continues regularly until it ceases more or less

abruptly at a height of several miles above the earth's surface. Practically all the water vapour

content of the atmosphere is contained in the troposphere and all weather phenomena are confined to

it.

Tropopause

5. The upper boundary of the troposphere is known as the tropopause. Its mean height at the

equator is 16.5 km, above sea level, at the poles it is about half this value.


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Stratosphere

Fig 4-1: The Atmosphere

6. This extends from the tropopause upto a height of about 50 kms above mean sea level. In

the stratosphere the temperature is nearly constant with height or increases slowly. Clouding and

weather phenomena are practically absent.

Ozone

7. Ozone is a form of oxygen with three atoms in a molecule instead of the usual two. If all the

ozone in the atmosphere were concentrated into a layer near sea level, its vertical depth would not

exceed 0.3 cm. Most of the ozone content is concentrated in the "ozone layer" which is partly in the


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31

CHAPTER 5

ATMOSPHERIC PRESSURE

Variation of Pressure

1. Pressure is never steady, even at one location it changes continuously. At any given instant

the sea level pressure is different in different parts of the globe. This gives rise to high pressure areas

and low pressure areas which are important in weather processes. The movement of these systems

give rise to pressure changes at any station.

2. Superimposed on these changes there are fairly regular oscillations of pressure on a daily aswell as annual scale. The daily oscillations are of a tidal nature and have two maxima (10 am and 10

pm, and two minima (4 am, and 4 pm). The range of oscillation is high in the tropics, being about 3-4

hPa in India. The seasonal oscillation gives a maximum of pressure in winter and a minimum in

summer.

3. The change of pressure at a station during a given period is known as "pressure tendency". It

is useful in judging the movement of pressure systems. In the middle latitudes pressure tendency

refers to 3 hours preceding the time of observation. In India this is of little use because of the large

tidal oscillations which make the variation of pressure due to movements of pressure systems

insignificant. Pressure tendencies are, therefore, worked out for 24 hours preceding the time of

observation.

4. It is common knowledge that pressure decreases progressively with height in the atmosphere.

The rate at which pressure decreases with height depends to some extent on the temperature and to

a smaller extent on the humidity. Knowledge of this rate provides us with a convenient tool for

determining heights in the atmosphere by means of pressure measurements. The altimeter used on

an aircraft employs this principle.

Al timeter

5. The aircraft altimeter is an aneroid barometer in which the dial graduations are in units of

height instead of units of pressure. We saw earlier that pressure varies with time as well as space.

Further, the variation along the vertical depends on the temperature and humidity. Thus the readings

of an altimeter carried on one aircraft are not strictly comparable with those on another aircraft. Since

aircraft in flight should maintain proper height separation amongst themselves to avoid collision, it is

essential that the readings of altimeters carried on different aircraft are comparable. This can be

achieved in one of two ways:


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(a) Readings of the altimeter are corrected at every stage for the local value of pressure

and for air temperature and humidity to obtain true altitudes. This is a laborious procedure

and is impracticable in flight.

(b) The scale of an altimeter is graduated according to some uniformly accepted

standard conditions of temperature and humidity in the atmosphere, and all altimeters on

aircraft flying in a given air space initially set the altimeter to a single value of pressure.

Although this procedure gives somewhat fictitious values of altitude, it ensures proper vertical

separation amongst aircraft, and is followed universally.

International Standard Atmosphere

6. For the purpose of graduation of altimeter scales, a fictitious atmosphere known as the

International Standard Atmosphere (ISA) has been adopted. This is practically the same as the ICAN

atmosphere. This atmosphere has the following characteristics:

(a) Sea level air density is 1225 gms/cubic meter.

(b) Sea level pressure is 1013.25 hPa.

(c) Sea level temperature is +15° C

(d) Temperature decreases with height at a rate of 6.5°C/km up to 11 km above which it

is constant at -56.5°C.

(e) The atmosphere is completely dry.

7. Heights corresponding to various pressures in this atmosphere are used in graduating the

dials of altimeters. Thus when the sub-scale of an altimeter is set to the sea level pressure in the ISA

i.e. 1013.25 hPa, the altimeter will read 10,000 ft at a pressure of 696.8 hPa, 25,000 ft when the

pressure is 376.0 hPa etc. Since the actual atmosphere rarely corresponds exactly with the ISA, the

readings are not equal to true heights and may sometimes differ considerably from true heights.

Al timeter Settings

8. To make altimeter readings comparable, all aircraft flying in a given area and height band at

any time should set the sub-scale of altimeter to a single pressure value. In the vicinity of an airfield

the most convenient value is what is known as the "altimeter setting" or QNH of the airfield. This is

the current value of pressure over the airfield reduced to mean sea level according to the standard

rate of variation of pressure with height in the ISA. An altimeter set to this value will read the elevation

of the airfield while on ground. While in flight in the vicinity of the airfield, altimeter set to this value will

indicate reasonably true altitudes. QNH setting thus enables aircraft to keep clear of obstructions of

known elevation in the vicinity of the airfield. Current QNH values are invariably supplied with

METARs and are transmitted on R/T by ATC. Other types of settings in the vicinity of an airfield are:


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Atmospheric Pressure 33

(a) QFE. It is defined as the station level pressure reduced to Aerodrome reference

point (ARP) as per ISA conditions. If QFE is set while on ground the altimeter will read zero.

(b) QNH. It is defined as the station level pressure reduced to MSL as per ISA

condition. If QNH is set while on ground the altimeter will read station elevation.

(c) QFF. It is defined as the station level pressure reduced to MSL as per actual

conditions. It is used for plotting on surface charts.

9. QNH or QFE settings are advantageous in the vicinity of an airfield because apart from

ensuring vertical separation amongst aircraft, they enable aircraft to keep clear of obstructions since

the altimeter readings give reasonably true altitudes or elevations above airfield level. These settings

are however, unsuitable for cross-country flight for the simple reason that the settings vary from point

to point and it is impracticable to obtain local values at every stage of the flight. Aircraft on cross-

country flight thus use what is known as "Standard Altimeter Setting", which is the mean sea-level

pressure in the ISA, viz. 1013.25 hPa or 29.92 inch. QNE is the altitude indicated when 1013.25 hPa

is set on subscale.

10. Altitudes indicated with this setting are known as "Pressure altitudes" or merely as "Indicated

altitudes" "Flight levels" allotted by ATC to aircraft on cross-country flight refer to pressure altitudes.

11. Since the pressure at mean sea level as well as the temperature and humidity at various

levels rarely correspond to conditions in the ISA, indicated altitudes differ from true altitudes on most

occasions.

12. Pressure Al titude. Pressure altitude is the altitude in ISA, where the prevailing pressure at

station exists.

13. Density Altitude. Density altitude is the altitude in ISA at which air density is same as the

observed density. A higher density altitude means lower air density.

Al ti tude Correction (ALTICOR)

14. The correction to be applied to the indicated altitude to obtain the true altitude is known as

‘altitude correction’ or in short as alticor. This is frequently required by a pilot while flying in

mountainous terrain or in high-level bombing operations. The forecast alticor can be obtained from

Met Section for the required levels at any point on the route.

ALTICOR = Indicated Altitude - True Altitude. (5.1)


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15. It is useful for a pilot to memorise the following thumb rules in connection with indicated

altitudes:

(a) When an aircraft flies from an area of high pressure to one of low pressure, thealtimeter over-reads; caution is therefore, to be exercised.

(b) Similarly pressure remaining unchanged, an altimeter over-reads when the aircraft

flies from warmer to colder air.

16. While in flight a rough calculation of the alticor can be made if the value of mean sea level

pressure at the point, the air temperature at the flight level and tables of temperatures at different

heights in the ISA are available. MSL pressures are generally given in route forecasts for the different

sectors of the route. Most aircraft have OAT (outside air temp) gauges fitted. If there is no OATgauge the temperature given in the forecasts can be used. A pilot should carry with him an abridged

table of conditions in the ISA.

17. The calculation is made in two stages and then the final result obtained. The stages are:

(a) For obtaining the correction due to pressure differences, add 30 ft for every millibar

difference when the actual MSL pressure is higher than 1013.2 hPa and subtract 30 ft for

every millibar difference when the actual MSL pressure is lower than 1013.2 hPa.

(b) For obtaining the correction due to temperature difference, add 1% of indicated

altitude for every difference of 3°C when the actual temperature is higher than ISA

temperature at flight level and subtract 1% of indicated altitude for every difference of 3°C

when the actual temperature is lower than ISA temperature at flight level.

Combine both these corrections algebraically to get the final result.

D Value

18. D Value is defined as the difference between true altitude and Indicated altitude. It can be

represented as

D = T - I (5.2)

Where D = D Value

T = True Altitude

I = Indicated Altitude.

D value is used by pilots flying very low level missions, when true altitude becomes critical due to low

terrain clearance and is obtained by:


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T = D + I (5.3)

On the other hand Alticor is used by aircrew for bombing ops, since they would like to know Indicated

altitude at which precisely the bomb is to be released.

Thus, if A is Alticor

A = I - T

∴ I = A + T (5.4)

Equation (5.3) & (5.4) clearly indicate that Alticor and D-Value are correction which differ only in -ve or

+ve sign, so that correction is additive in both the cases.

Sea Level Pressure Patterns

19. It has been mentioned before that the distribution of pressure over a large area is not uniform.

When values of mean sea level pressure are plotted on a chart the pattern of distribution of pressure

can be best perceived by drawing lines of equal pressure. These lines are called, "isobars" and are

generally drawn at intervals of 2 hPa.

20. Experience shows that isobars exhibit certain characteristic configuration whose locations

change on successive charts with some amount of continuity. You would already have noticed this is

the daily met briefing.

21. The main types of

configurations are given below

and are illustrated in Fig 5-1.

(a) Low. It is a

region of relatively low

pressure with nearly

circular and concentric

isobars around the centre

where pressure is lowest. When it is intense it is known as a depression or a cyclone.

Fig 5-1: Types of Isobar Systems

(b) Secondary Low. It is a shallow low within the area covered by a deeper one.

(c) Trough of Low. In this region the isobars extend outwards from an area of low

pressure so that pressure is lower in the trough than on both sides.


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(d) High or Anticyclone. It is a region with relatively high pressure and more or less

circular isobars with highest pressure at the centre.

(e) Ridge. Isobars extending outwards from an area of high-pressure so that pressure

is higher in the ridge than on both sides.

(f) Col. The region of relatively flat distribution of pressure located between two highs

and two lows.

Static and Moving Pressure Systems

22. The study of the development, movement and dissipation of pressure systems is a very

important branch of meteorology and is known as ‘synoptic meteorology’. This subject will be

introduced in a later chapter. The reader should however be aware that, broadly speaking, there are

two categories of pressure systems:

(a) Static of semi-permanent pressure systems which show little movement in any one

season.

(b) Moving pressure systems whose horizontal dimensions are of a somewhat smaller

scale and appear as moving disturbances in the large-scale systems.

23. As examples, the great Siberian anticyclone and the equatorial trough of low pressure belong

to the first category. Western disturbances and monsoon depression belong to the second category.

24. If we make a broad generalisation, high-pressure areas are associated with settled and fair

weather, while low-pressure areas are associated with adverse weather phenomena like rain and

thunderstorm.

SAMPLE QUESTIONS


(a) If QNH is set on subscale on ground the Altimeter will indicate:

(i) Height. (ii) Altitude of Station.

(iii) Elevation of Station.

(b) If QFE is set on ground, the altimeter will indicate:

(i) Elevation. (ii) Zero.

(iii) ARP.


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(c) QNE in flight means:

(i) True Altitude. (ii) Indicated Altitude.

(iii) Stn Pressure reduced to MSL as per actual condition.

(d) Alticor is used for:

(i) Recee Missions. (ii) Bombing ops.

(iii) Interception.

(e) D-Value is used for:

(i) Bombing. (ii) Interdiction.

(iii) Ground attack ops.

(f) When an aircraft flies from High pressure area to area of low pressure, altimeter will

(i) Over read. (ii) Under read.

(iii) Be same.

(g) An aircraft flying from A to B expenses drift to starboard, the ac is going towards an

area of:

(i) Low. (ii) High.

(iii) Col region.

(h) Pressure Altitude means nothing but:

(i) True Altitude. (ii) Indicated Altitude.

(iii) Actual value of Pressure corresponding to the altitude you are flying.

(j) On a rainy day the length of Runway required in comparison to sunny day would be:

(i) More. (ii) Less.

(iii) Same.

(k) Higher density altitude indicates the performance of the aircraft would be

(i) Less efficient. (ii) More efficient.

(iii) Does not indicate performance of aircraft.


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39

CHAPTER 6

TEMPERATURE

1. Temperature is the degree of warmth of a substance. There are various processes by which

heat is transferred from one body to another.

2. Heat is a form of energy. As heat is extracted from a body, the molecules composing the

body lose energy and their random motions and vibrations decrease. The molecules get rearranged in

a more orderly fashion. As the heat is removed from the body, the molecular motions decrease

further. A stage is reached when the molecules are at complete rest and no further heat can be

removed from the body. The temperature at which this happens is the same for all matter. It is knownas the absolute zero as this is the lowest temperature that can possibly be attained. The zero of the

absolute scale of temperature is based on this principle. In terms of the Celsius scale, absolute zero is

reached at –273.15O C.

Methods of Heat Transfer

3. Heat can be transferred from one body to another by four ways:

(a) Conduction. The energy of molecular motions of a hotter body is physically

transferred to the molecules of an adjacent body. Conduction is an important process of heat

transfer very close to the ground.

(b) Convection. This process of heat transfer applies only to liquids and gasses. In

this process parcels of hot fluid from one part are bodily transferred to a colder part of the

fluid. This is an important process of heat transfer within the atmosphere.

(c) Advection. This is a process by which heat is transferred from one area to

another through horizontal wind motion i.e. bodily movement of air masses.

(d) Radiation. Unlike the above processes, radiation does not depend on any

medium for the transfer. Every body, whatever its temperature, emits energy in the form of

electromagnetic waves which travel through space at the same speed as radio waves, but are

of much shorter wave length. The amount of energy radiated depends on the temperature,

being proportional to the fourth power of the temperature. The radiant energy is absorbed by

a body, the amount of absorption depending on the nature of the substance.


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Sources of Heat in the Atmosphere

4. Although the interior of the earth is very hot and is in a molten state, the solid crust does not

permit appreciable penetration of heat to the surface. The main source of heat for the surface of theearth and the atmosphere surrounding it is the radiation emitted by the Sun.

5. The sun’s temperature is estimated to be about 6000O

C at its surface. At this temperature it

emits an enormous amount of energy in the form of radiation. The radiation is mainly in the form of

visible light, although a part of it is in the form of invisible energy in the ultra violet and infra-red

regions. The Sun is able to keep up this constant supply of energy, because within its interior, energy

is being continuously produced by a process similar in principle to the thermonuclear fusion as in the

hydrogen bomb.

6. The ability of any body to absorb radiation from another source depends upon the nature of

the body as well as the wavelength of the radiation. Except for ozone, the other constituents of air

cannot appreciably absorb the radiation received from the Sun. During daytime this radiation passes

through the atmosphere without heating it and reaches the surface of the earth where it is absorbed.

The earth re-radiates this energy in longer wavelengths. The atmosphere is capable of absorbing this

radiation in longer wavelengths. Thus, although the Sun is the primary source of heat, the source from

which the atmosphere gets heat is the earth. The earth being the secondary source of heat,

temperature in the lower atmosphere decreases with distance from earth, i.e. with height.

7. The above is only a simplified explanation. In actual fact dry air is transparent even to the

longwave radiation from the earth. On the other hand, water vapour absorbs a part of this longwave

radiation. However, it is found that the actual rate of decrease of temperature with height in the

atmosphere is smaller than the one which would result from a process of heat transfer from the earth

by longwave radiation only. Other important methods of heat transfer from the earth to the

atmosphere are:

(a) Convection.

(b) Advection.

(c) Latent heat of condensation of water vapour, which has been transported upwards.

Surface Temperature

8. The primary influences controlling the temperature of the ground and the air layers very close

to it are the incoming solar radiation and the outgoing longwave radiation from the earth, the nature of

the ground surface and the horizontal transference of heat by wind.

9. Maximum heating due to insolation takes place when the Sun is highest in the sky due to two

reasons:


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Temperature 41

(a) The amount of radiation received on unit area is highest when the rays are incident

vertically.

(b) Rays travel through the shortest path and hence suffer least loss by absorption or

reflection or scattering.

The broad features of the distribution of average temperature over the earth’s surface can be

explained on the basis of the varying elevations of the Sun - greatest in equatorial regions

and decreasing towards the poles. The seasonal variations, warm in summer and cold in

winter, may be explained similarly.

Diurnal Variation

10. The temperature attained by the ground depends on the nature of the surface. The rise in

temperature by absorption is inversely proportional to the specific heat of the substance. Water having

the highest specific heat experiences relatively smaller temperature changes, while the solid materials

of the earth’s surface have a smaller specific heat and temperature changes are therefore greater on

land.

11. During day, ground temperatures may be much higher than the air temperature, sometimesby as much as 10

O C. Maximum temperature is reached about 2 hours after midday.

12. At night the ground cools because the earth emits longwave radiation. At the time of the

minimum temperature, the ground is colder than the air close to it sometimes by about 5O

C when the

sky is clear and radiation effect is at its maximum. Minimum temperature is reached near about

sunrise time.

13. The diurnal variation of air temperature near the ground is least over the oceans and highest

over the interior of continents. Other things being equal, diurnal variation is greatest when the sky is

clear and the wind is calm.

Effects of the Source of Air

14. The temperature of air does not depend on these factors only: it varies widely according to

the source of supply of the air. Air coming from higher latitudes is usually colder than air from lower

latitudes.

Upper Air Temperature

15. The Tropopause is highest over equatorial belt. The lowest temperatures are found in upper

troposphere over the equator. The tropopause is not continuous from the equator to poles. It has a


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break at about Lat. 30O.The tropical tropopause is about 8 kilometers higher than the polar

tropopause.

Temperature and Aviation

16. Apart from its importance in the generation of weather phenomena, temperature has a direct

bearing on aviation. Engine performance and cooling systems depend on the variation of temperature

in the atmosphere. The performance of an aircraft (both piston and jet type) is affected by the density

of air, which in turn is inversely proportional to the temperature at constant pressure. High

temperature implies lower density and so has an adverse effect on engine performance. This effect is

usually greatest during take off, but it should also be considered at other stages of flight, especially for

jet aircrafts.

Ai r f ield Reference Temperature

17. In sighting a runway, its length should be planned not only in regard to the type of aircraft that

are likely to be operated, but in regard to the temperature and pressure that prevail in the locality. For

purposes of planning, a temperature referred as airfield reference temperature (ART).

SAMPLE QUESTIONS


(a) The source from which the atmosphere gets heat is:

(i) Directly from Sun. (ii) Earth.

(iii) Sun’s heat reflected from Earth.

(b) During day, maximum temperature is reached at about:

(i) Midday (ii) 2 hours after midday

(iii) 2 hour before midday.

(d) Minimum temperature is reached at near about:

(i) Sunset (ii) Sunrise.

(iii) At midnight.


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43

CHAPTER 7

ATMOSPHERIC OBSCURITY

Importance of Visibility in Aviation

1. Visibility is a measure of the degree of transparency of the atmosphere. In Meteorology it is

defined as the distance upto which prominent objects can be seen by naked eye and recognised as

such. An aviator's interest in visibility arises because he wants to know how far off he will be able to

see various things like landmarks, targets, obstructions, beacon lights, other aircraft, runways etc.,

while he is in flight or when he is about to make an approach and landing. Met reports of visibilityfrom ground stations therefore provide an aviator with vital information, which may prepare him

mentally for a difficult landing or a diversion.

2. At some airfields, blind landing systems like ILS, GCA etc. are provided for guiding aircraft for

a take-off or landing under conditions of poor visibility. Even these systems have their limitations and

a large majority of airfields in India do not have even these aids. Thus, even for experienced aviators

poor visibility still remains a serious hazard.

Shortcoming of Visibility Reports

3. It must be realised that visibility reported by a Met section is not fully representative. It has

the following shortcomings:

(a) The report gives horizontal visibility at ground level. This may differ considerably form

vertical and slant visibility.

(b) The report gives horizontal lowest visibility after scanning all directions. It is well

known that visibility can be substantially different in different directions.

(c) Estimate of visibility depends on the illumination of the landmarks selected for

observation.

(d) Visibility reported at night is actually an estimate of the equivalent daytime visibility.

This can be quite misleading at airfields, which have a system of electric lighting of variable

intensity on the runway. For example when the reported visibility is one km, a light of 20

candle power will be just visible at one km, but a light of 1,000,000 candle power (such as a

high intensity approach light) will be visible at over three km.


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Runway Visual Range (RVR)

4. To overcome these shortcomings and to determine as closely as possible the conditions that

would be experienced on landing or on take-off, at some airfields, observations are taken of the

runway visual range (RVR) which is the visual range of those objects or lights provided on the runway

for the guidance of the aircrew. These observations are made by the runway controller (or other

observer) positioned at one end of the runway. RVR is determined only when the visibility is marginal

for flying. When the visibility is more than about 2 km, the difference between conventional visibility

and RVR is not significant for aircraft operations. RVR is reported with the help of an instrument

called transmissometer or Scopograph. Since IAF airfields do not have these instruments, at most of

airfields, Runway visibility instead of RVR is reported by following the procedure outlined above.

Vertical and Slant Visibili ty

5. When a shallow layer of haze or

fog is covering an airfield the horizontal

ground visibility is poor. However, an

aviator flying over the airfield above the

haze layer may be able to see the airfield

clearly. This is deceptive since, on

approach for landing, he would suddenly

encounter poor visibility. This is illustrated

in Fig 7-1.

6. From the above it is evident that

horizontal visibility may differ considerably

form vertical visibility and slant visibility

(also known as oblique visibility). No fixed

relationship exists between these three

and each situation gives a different

relationship depending on the thickness of

the haze layer and vertical density

distribution of particles constituting the

haze (Fig 7-2).

Fig 7-1: Obscuring of an Aerodrome by Fog /Haze as an Aircraft Descends to Land

Fig 7-2: Oblique Visibility through Haze LayersIn-Flight Visibility

7. Haze and fog layers are usually confined to a few thousand feet above ground level. Furtheraloft the transparency of the atmosphere increases considerably except in cloud and precipitation.

Nevertheless the ability to locate distant aircraft while in flight depends on many factors:


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Atmospheric Obscuri ty 45

(a) Size, colour, illumination, background and speed of the remote aircraft.

(b) Sight of the observer.

(c) Bearing of line of sight relative to the sun or moon. Visibility is lower looking towards

the sun or moon than away from it.

(d) Transparency of the cockpit windows.

8. Theoretically visibility in the stratosphere should be excellent because of the absence of cloud

and haze particles. In practice, however, this is not the case due to two reasons:

(a) There is greater dazzle in the stratosphere due to the fact that the apparent

brightness of the sun increases and that of the sky decreases.

(b) Due to the absence of objects for sighting (viz. clouds), the eye adjusts itself to focus

midway between distant and near vision, with the result that an aircraft cannot be sighted

unless it comes fairly close.

9. From the above it is evident that for in-flight conditions the transparency of the atmosphere is

not the same thing as visibility.

Cause of Poor Ground Visibili ty

10. Poor visibility of layers near the ground may be caused due to the following phenomena:

(a) Dust Haze. Widespread dust may be held in suspension for a number of days due

to persistent strong winds caused by a steep pressure gradient, especially over desert areas

or large rivers like Brahmaputra in summers. Winds of this type are usually known as dust-

raising winds. The dust may be carried to great distances in the prevailing low-level wind

circulation. At times it is carried vertically to 10-15,000 ft. In extreme conditions ground

visibility may reduce to less than 1500 m. Vertical visibility is usually very poor. At night the

wind speed decreases and some of the dust settles down leading to slight improvement in

visibility. This type of dust haze is common over Northwest India in summer (May - June).

The haze extends even upto Northeast India but visibility progressively improves as one goes

further east from UP. Very infrequently this type of haze prevails in Northern India in the late

winter months. In this case the haze originates from the arid stretch of Iraq to Rajasthan.

Due to the widespread nature of the dust haze, diversions are difficult. Airfields to the south

of the dust-fetching areas are most suitable for diversions.

(b) Moist Haze. Due to the condensation of water vapour consequent to cooling of air

layers near the ground, moist haze, mist or fog may prevail. Moist haze usually occurs in the

early morning hours and dissipates due to heating of the sun. It is principally a winter hazard


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because of clear skies and consequent greater cooling at night. Mist and fog will be

discussed in greater detail in subsequent paragraphs.

(c) Smoke Haze. Smoke emitted from industrial or domestic source spreads as a haze

layer when the wind is calm or very light and a strong ground inversion exists. The wind

speed is then insufficient to cause undue horizontal diffusion, and the inversion (and

consequent stability of air) confines the smoke within the inversion layer, which may extend at

most to 3000 ft. Under extreme conditions ground visibility may reduce to 1500 m, but usually

it is 2-4 kms. Smoke haze is principally a winter hazard because of stronger ground

inversions resulting from clear skies at night. The haze sets in before sunrise and usually

dissipates by 0930 hrs, because of the breakdown of the inversion and the freshening of the

wind. The prevalence of smoke and the prevailing wind direction as governed either by the

pressure pattern or minor local circulations.

(d) Dust Storms. The visibility in a duststorm may range from less than 10 m to 1000

m. Very poor visibility lasts for short duration of about half an hour, but after the duststorm

passes away, dust may remain suspended for many hours giving only partial improvement in

visibility. If the duststorm is followed by a light shower the improvement in visibility is rapid. It

must be remembered that duststorms occur in a season when the general visibility itself is not

good due to suspended dust. Improvement of visibility referred to here should, therefore, be

interpreted in this context.

(e) Precipitation. The visibility in rain depends both on the size of the drops as well as

their number in a given volume. Light rain has little effect, moderate

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