World Meteorological Organisation
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. 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 India is a member country of the WMO. Meteorological requirements in India are looked after by the Indian 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
The function of IMD in regard to aviation can be broadly classified as follows:
- Maintenance of IMD observatories for taking observations of meteorological elements.
- Maintenance of IMD communications network to exchange meteorological data
- Maintenance of forecasting offices at civil airfields.
- Collection and statistical analysis of meteorological data and their periodical publication in the form of climatic summaries, tables and atlases.
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:
- 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.
- 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”.
- 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.
- RAWIN Observatories. 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”.
- 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.
- 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
- Apart from the static observatories mentioned above, the IMD has the benefit of 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.
Another important sources of meteorological observations are aircraft on routine or nonroutine flights. Reports from aircraft are known as “AIREP”. They are of immense use in forecasting for aviation purposes.
Rockets and Satellites
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.
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. 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.
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
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 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.
Area Met Watch Offices
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.
|Active thunderstorm area.
Severe line squall.
Marked mountain waves.
Widespread dust storm.
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 CHARTS, FORECASTS AND BRIEFING
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
The following types of charts are used in plotting meteorological observations:
|Surface Weather Charts.
Upper wind Charts.
Constant Pressure Charts.
a. 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
b. 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.
c. 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 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. 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.
d. Auxiliary 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
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
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.
(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
(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
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:
|Surface Weather Charts.
Upper Wind Charts.
Constant Pressure Charts.
Other Special Charts (Auxillary charts).
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. The chart is prepared on a base map covering India and immediate neighbourhood.
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.
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.
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.
Thickness Charts & Other Auxiliary Charts. These charts contain data, the details of which are beyond the scope of this book.
Interpretation and Plotting of Various Symbols
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.
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.