40 Role of Meteorology in Aviation

R Suresh

1. Learning Outcomes
2. Introduction
2.1. Aviation and Meteorology
2.2 Aviation meteorological information and forecast
2.3 Ever growing meteorological requirements for aviation
2.4 Weather requirements for flight planning
3. Role of surface meteorological parameters in flight operations
3.1 Principles of flight
3.2 Effect of surface temperature
3.3. Effect of station level pressure
3.4 Effect of surface Wind direction and speed
3.5 Clouds and visibility
4. Aviation significant / severe weather
4.1 Thunderstorm and precipitation
4.2 Turbulence
4.3 Mountain waves
4.4 Aircraft icing
4.5 Wake vortex / turbulence
5. Weather Forecast and Warning services to aviation
5.1 Aerodrome forecasts
5.2 Take-off forecast
5.3 Flight forecast documentation (Chart Form of Documentation)
5.4 Airfield Warnings
5.5 SIGMET warnings
6. Summary

1. Learning Outcomes

After studying this module, you shall be able to:

• Know the importance and indispensability of meteorology in aviation
• Know the impact of meteorological parameters on safe conduct of flight operations
• Know the impact of severe weather phenomena in aviation
• Appreciate the economic benefit of aviation meteorology
• Know the meteorological instrumentation relevant for aviation.

2. Introduction

Meteorology, a complex science having inputs from all basic and physical sciences, comprises of weather and climate. Weather is the instantaneous state of atmosphere which may be conceived as the sum total of the ambient air temperature, humidity, atmospheric pressure, wind, visibility, cloud and associated precipitation (drizzle, rain, shower, hail, snow etc), thunder and lightning etc. Climate is the long term average of weather. By long term we mean a minimum of 30 years. We all know that weather and climate affect the day-to-day animal and plant life. The meteorological information is quite vital in various sectors such as agriculture, transportation, energy besides housing, tourism, entertainment, mountaineering etc. In this module, let us study the meteorological input in aviation sector.

2.1 Aviation and Meteorology

It is well known that without meteorological input, an aircraft operations cannot take place. The meteorological parameters play an important role in effective and efficient conduct of aircraft operations right from the planning, take-off phase to cruising and landing phases of the operations. Meteorological data and information including forecasts and warnings have prophylactic values and a careful attention to these information ensures optimal gain to the air line agencies. When an aircraft navigates through the ocean of air, its flight is affected by the changes in the atmospheric conditions. While poor visibility, low cloud base and sudden wind shifts affects the landing and take-off phases of operations, some atmospheric phenomena like thunderstorms, line squalls, turbulence, cyclonic storms etc. are hazardous when the aircraft is in-flight. Hence, for safe aviation and at various stages of aircraft operations, a supporting and specialized meteorological service is necessary.

2.2 Aviation meteorological information and forecast

The concept of performance based navigation system (PBN) comprising of air traffic flow management (ATFM) and air traffic management (ATM) contemplated by the International Civil Aviation Organisation (ICAO) to enhance safety, capacity and efficiency in Aircraft operations, requires accurate weather information and forecast. While surface meteorological information comprising of surface temperature, wind, pressure and visibility play a crucial role in take-off operations, the surface wind and visibility are of paramount importance for landing operations.

Forecast parameters of temperature and wind (surface and upper air), surface pressure (at Runway level as well as pressure reduced to mean sea level as per international standard atmospheric conditions), weather phenomena of significance such as thunderstorm / sandstorm / dust storm, strong wind such as squall, gale force etc., fog / mist etc. that reduces surface visibility are the guiding factors for the Pilots and air traffic controllers for the safe conduct of flight operations. The benefits of improved, if not accurate, short range forecasts to the aviation industry over four airports in United States were estimated as 24.4 million dollars per annum by way of savings in operational costs, savings to passengers and reduction in fuel loads.

2.3 Ever growing meteorological requirements for aviation

The meteorological requirements of aviation are so precise and demanding that the meteorological services the world over had to grow rapidly with developments in the design and operation of different types of aircraft. Aircrafts which were flying with 200 kmph speed in early 1920s and that too at heights not beyond 3 km have been replaced by jet aircrafts with speeds exceeding 1000 kmph and flying at higher heights of about 9 – 18 km. As aviation rose higher and became faster, atmospheric sounding were extended to greater heights and data collected at forecasting offices covered larger areas. Special high level analyses and prognostic techniques were developed to cater to the aviation needs. Upper air weather observing stations increased along the air routes to measure atmospheric characteristics at increasing altitudes as the flight altitudes kept rising. While superfast and high altitude flights were on the increase, aircraft operations in the lower atmosphere continued, particularly for special purposes like spraying of pesticides on crops etc. Thus, the meteorological support to aviation extends from the ground to various altitudes upto about 12 km normally and to much higher altitudes in respect of supersonic aircraft flying with speed beyond 1200 kmph. Observed weather parameters and weather phenomena besides forecast of these for various atmospheric layers in which flight operations take place proved to be of prophylactic and economic benefits.

2.4 Weather requirements for flight planning

For economic and safe aircraft operations, flight planning is necessary in respect of long-haul flights of the jet aviation. It includes all phases of flight operations viz. take-off, climb, cruise, descent and landing. The lift and drag of the aircraft in flight depend on the air density and the cruising speed. In turn, air density varies with temperature and pressure. The flight velocity is affected by the prevailing winds at the cruising level. Hence the forecast of these parameters is used in preparing the cruising phase of the flight plan. Similarly, the total take off weight of an aircraft is related to the runway temperature and the surface winds. The take off weight has to be decreased with increasing temperature and decreasing head wind (the wind blowing against the take-off direction). Similar weather considerations are relevant for landing. Thus every phase of the flight is weather dependent and some of the weather phenomena which affect aircraft operations are briefly given as follows:

(a) Precipitation (Shower, Rain, Hail, Snow).

(b) Obscuration phenomena affecting visibility (Hydrometeors such as Fog and Mist; Lithometeors such as Haze, Sand, Dust, Smoke, Volcanic ash etc).

(c) Convective phenomena (tropical cyclone, thunderstorm/hailstorm, sand/dust storm, tornado and their bye-product such as updraft/downdraft, high windspeed, squall, severe icing).

(d) Low level and upper level turbulence in association with or without convective systems.

3. Role of surface meteorological parameters in flight operations

Let us understand the principles of flight and the role of meteorological parameters in flight operations in brief.

3.1 Principles of flight

When air flows over an aerofoil, according to Bernoulli’s theorem, it is accelerated over the upper surface and results in an excess pressure on the underneath surface. This causes a force that can be resolved into two components, viz., ‘lift’ and ‘drag’ (see Fig. 1). This is the fundamental principle used in aircraft operation / air navigation. The other forces acting on the aircraft are the ‘thrust’ which acts forward and the ‘weight’ acting towards ground. At the time of take-off, the lift has to exceed the weight of the aircraft and the thrust should overcome drag while at cruising level all the four forces should balance each other.

The ‘lift’ is proportional to air density and square of the air speed. However, air density depends on temperature and pressure of the area under operation. A few degree change in temperature alters the lift of an aircraft of several thousand kilogram weight through its influence upon density of air and buoyancy. In flight, such changes are balanced by dynamic lift, but on the ground (prior to take-off) in order to get the required lift the take-off gross weight (TOGW) consisting of passenger / cargo and fuel load need to be reduced.

3.2 Effect of surface temperature

The engine efficiency is reduced when the temperature increases. Hence to attain the optimal engine efficiency (or precisely the thrust required), the aircraft has to move for quite sometime along the runway or in other words requires longer runway. During this process excess fuel is consumed. It is quite common that few tonnes of cargo or even passenger load may have to be unloaded in these circumstances to get the required Lift. For example, a 2 oC rise cause offloading of passenger/cargo load to an extent of few hundreds of kilograms. Since sudden and abrupt changes in temperature modify the lift, it is important to know the conditions which give rise to such abnormal/abrupt change.

3.3 Effect of station level pressure

The international standard atmosphere, as defined by the International Civil Aviation Organization (ICAO) for graduation of pressure altimeters and the design and testing of aircraft, assumes 1013.25 hPa pressure and 15oC temperature at the mean sea level. The pressure altitude is defined as height corresponding to the pressure in the standard atmosphere. For flight planning purposes, the deviations from the standard atmosphere are to be reckoned with and allowances made accordingly. Every increase of 100 ft pressure altitude from the standard atmosphere warrant reduction of 500 kg to 900 kg TOGW depending on the type of aircraft.

3.4 Effect of surface Wind direction and speed

The surface wind affects the aircraft performance. The advantage gained is proportional to the wind speed and reaches a maximum if the wind direction is the same as that of runway orientation. Headwind, the wind blowing against the direction of movement of the aircraft, is favourable at take-off and landing operations. The TOGW of an aircraft has to be decreased with increasing surface temperature and decreasing headwind. If take-off has to be made when the tail wind (wind blowing along the same direction of aircraft movement) is prevailing, for technical reasons, then TOGW is to be reduced to ensure safe take-off operations. When the surface temperature is 15oC, 5200 kg additional TOGW can be plausible with a head wind of 10 kts than under near zero surface wind speed conditions.

3.5 Clouds and visibility

A cloud is a visible conglomeration of minute particles of water, ice or of both in the free air. Clouds form when air containing water vapour rises and cools in that process till some of the vapour condenses into visible aggregates of minute particles called nuclei. The shape and structure of clouds depend on the availability of condensation nuclei, water vapour and the type of ascending motion (instability). Clouds obscure the vision and are often associated with precipitation, turbulence and electrical phenomena. Fog, mist, dust storms, heavy precipitation etc. are responsible for poor visibility conditions. Fog, which can be considered as cloud at or near the surface, reduces the horizontal visibility to less than 1000 metre. The impairment of visibility due to suspensoids (like polluted particulate matters, dust, smoke, water droplets etc.) in the atmosphere often affects terminal operations (take-of / landing). Accurate assessment of horizontal visibility and forecast of cloud (amount and the height of the base of the cloud above ground level) are highly desirable for terminal operations

4. Aviation significant /severe weather

Let us discuss about some of the aviation significant/severe weather phenomena.

4.1 Thunderstorm and precipitation

A thunderstorm is a well grown vertically developed convective cloud producing thunder, lightning and occasionally hail under favourable atmospheric conditions (Fig. 2). While thunderstorm (often known as Cumulonimbus (Cb) cloud) cloud of vertical height just exceeding 6 km produces thunder and lightning, Cb cloud height reaching well beyond 18-20 km in tropical atmosphere is not at all uncommon. Thunderstorms may give rise to few spells of intense rain and shower and associated reduction in visibility which is detrimental for aviation flight operations. Cb cloud cells have a typical dimension of a few km in length and breadth and atleast 6 km in height. These individual Cb cells often align as a continuous line with little or no break between the cells stretching a few hundreds of km in length and a few tens of km in breadth. This type of line formation is often known as line squall or squall line thunderstorm.

Thunderstorms are often associated with intense vertical updrafts and downdrafts during their to aircraft. Thunderstorms, with or without precipitation, affect not only the aircraft performance but also cause psychological impact on the air crews and inconvenience to the passengers in view of jarring /deafening sound, turbulence and bumpiness, blinding flash of light and induced electro magnetic field from the lightning. Intense convective phenomena like thunder storms, squalls and hailstorms are dangerous to aviation in all its phases of operation. Other convective weather phenomena such as dust storm and sandstorm also affect aviation as the genesis mechanism of these phenomena are similar to thunderstorm but for the fact that due to lack of moisture precipitation and lightning are scarce in these storms.

4.2 Turbulence

Turbulence associated with convective systems can be predicted with considerable accuracy. Turbulence results in bumpiness of the aircraft. But the clear air turbulence (CAT) is often difficult to detect and predict. CAT generally occurs in patches of varying sizes extending from 50 to 500 km across. It is the manifestation of micro scale eddies embedded in the general upper air synoptic scale flow patterns. It is, therefore, possible only to forecast the synoptic scale area within which turbulent patches are likely to occur. Aircrafts encountering CAT may lose altitude of a few thousand feet altitude in a matter of few seconds, as per available records.

Clear air turbulence is associated with a variety of synoptic scale flow patterns and meteorological phenomena. Generally, CAT is more frequent in the vicinity of a jet stream, tropopause and the high level lows and troughs. Whenever wind speed changes more than 6 km per hour in a layer 300 m thick, moderate turbulence is commonly observed. However, such wind variation in upper atmosphere are restricted to limited upper air radio-sonde/GPS sonde and Pilot Balloon observations, location specific and also not normally recorded or routinely reported throughout the world. Hence one is forced to fall back on the Numerical weather Prediction model outputs and/or synoptic systems for predicting turbulence.

4.3 Mountain waves

When the wind passes over the lee side of a mountain ranges almost close to right angles, the air initially tends to slide down the slope but it regains its normal altitude a few kilometers downstream. During this process, a wave motion in the vertical with undulating rise and fall builds up. The updrafts in the rising sector of the wind flow are similar to thunderstorm updrafts and often are as high as 80 kmph. Similar magnitude ofdowndrafts are also not uncommon. Mountain wave may extend to great heights several times (usually twice the height of mountain) higher than the mountain range responsible for their origin. The high vertical velocities may disturb the flight altitude of the aircraft in flight leading to disastrous consequences. In addition to the wave type disturbance, strong eddy motion is possible on the lee side of the mountain (Fig. 3). Thus flying at various levels lee wards is risky. Under favourable conditions, a typical cloud identified as lenticularis (lenticular cloud) forms which are indicative of existence of mountain wave.

4.4 Aircraft icing

The super cooled droplets in cold clouds instantaneously freeze when their stability in disturbed by the mechanical impact they suffer when an aircraft with its body temperature less than 0° C strikes them. Under these conditions icing on the aircraft may result (Fig. 4). It decreases lift and increases weight and drag of an aircraft. Further, accumulation of ice affects the control of the aircraft.

There are different types of ice formation affecting the flights. Hoarfrost is a form of ice that gets deposited on the body of the aircraft colder than the dew point of the air. When an aircraft passes from a colder to a warmer region, it may experience hoarfrost. Rime ice, consisting of white and opaque ice pellets, is formed by freezing of small super cooled water droplets struck by an aircraft. This is generally encountered when flying is through stratiform clouds of small vertical extent. Because of its granular structure, this form of ice is easily broken away and is not dangerous. Smooth and glossy form of clear ice is the most dangerous form of icing, affecting the stability of the aircraft. The super cooled water droplets flow over the surface of the flying aircraft and slowly freeze. During this process a thin layer after layer of ice forms and firmly sticks to the body of the airframe. Such icing normally occurs when the flight is through cumulonimbus clouds.

4.5 Wake vortex / turbulence

An aircraft at the time of landing leaves behind it a vortex of air which poses maximum danger to another incoming aircraft since turbulence is often induced in such cases. The wake vortex (Fig. 5) from a large, heavy aircraft can easily flip a small aircraft upside down. As a bigger aircraft needs lots of circulation to support its weight in flight, it is generally considered dangerous to follow a bigger aircraft flying at lower speed. Well defined procedures are followed by the air traffic controllers for the temporal and spatial clearance of incoming aircrafts to avert wake turbulence. Moreover, the air crews are trained to be alert and avert the wake turbulence.

5. Weather Forecast and Warning services to aviation

The procedures of providing documentation and briefing to the aircrews of aircraft before their take-off and the other forecast services rendered to aviation are internationally standardized. A brief description of the aviation forecasts is given below.

5.1 Aerodrome forecasts

Terminal Aerodrome Forecasts (TAF) required for landing and take-off operations, with 9 hrs validity for domestic operations and 30 hrs validity for international operations, are issued by India Meteorological Department (IMD) every day with fixed overlapping periods covering the entire period of aircraft operation at an aerodrome. TAF consists of mean surface wind speed and direction, surface visibility, weather conditions such as rain, shower, thunderstorm, dust storm, sand storm, fog, mist, haze and clouds of different categories. Relevant extracts of these are included in the meteorological documentation to the pilots. These forecasts are made available to the air traffic control units (ATC) as well. In addition to these forecasts, twenty four hours current weather observations are made and based on these observations, routine Meteorological Aerodrome Report (METAR) with the expected changes / outlook for the next two hours called TREND forecast are normally issued at half-hourly intervals.

5.2 Take-off forecast

Take-off forecast is normally issued three hours prior to the scheduled take-off time for pre-flight planning purpose. It consists of surface wind, temperature and pressure. This forecast is used for cost effective pre-flight planning, estimating optimum TOGW etc.

5.3 Flight forecast documentation (Chart Form of Documentation)

Forecast information of winds and temperatures, known as Wind/Temp charts, at different flight levels upto and little above the cruising level along with significant weather information, known as SIG (i.e. significant) Weather charts, that may be encountered enroute are provided in chart form. The significant weather may include zones of thunderstorms, line squalls, turbulence, tropical cyclones, CAT etc. As icing is a severe aviation hazard, information about freezing level is also included in the significant weather (SIGWX) chart. The above chart along with composite TAF covering the flight route and alternate route(s) is often called as chart form of documentation (CFD). For low level flights (usually upto 10000 ft above ground level (a.g.l), wind/temperature information at various levels and forecast information about the cloud, visibility, significant weather are provided in another chart form called Met T3 chart.

Local weather forecast covering a radial distance upto 100 nautical mile (n.m) from the airport are issued three times daily. This forecast is made available in Area control for the purpose of vectoring the aircraft. This local forecast is also used by the air crews of low level and short haul flights.

5.4 Airfield Warnings

Airfield warnings are meant for parked and moored aircraft in an airfield. The weather elements for which the warnings issued are strong winds, low level wind shear, convective phenomena accompanied by hail, squall etc. Usual forecasting techniques are followed for these phenomena except for low level wind shear for which the shear experienced by the incoming Pilot, when passed through ATC, shall be used for the next two hours or till such time another incoming Pilot reports no such phenomenon is experienced by him whichever is earlier.

5.5 SIGMET warnings

Designated Meteorological Watch Offices (MWO), covering Flight Information Regions (FIR) for which they provide service, issue SIGMET warnings for the following expected or observed aviation severe/significant weather with a validity of generally 4 hrs duration (maximum 6 hrs in respect of Volcanic Ash and Tropical cyclone).

• Ø Active thunderstorm area
• Ø Tropical revolving storm
• Ø Severe line squall
• Ø Heavy hail
• Ø Severe icing
• Ø Severe turbulence
• Ø Marked mountain wave
• Ø Widespread sand storm / Dust storm
• Ø Volcanic Ash
• Ø Radioactive cloud

The primary purpose of SIGMET warning is for in-flight service on the enroute severe weather since long haul flights might have departed from its origin airport when the severe weather phenomena was not prevailing in the enroute/destination airport at the time of its departure from the originating airport. As such, that Pilot needs to be warned on these enroute/destination severe and significant weather before reaching the destination. Hence, SIGMET warning is disseminated to ATS units of the neighbouring FIRs and all airports in the SIGMET warning originating FIR at highest priority. The issued warning is immediately broadcast to the Pilots by the ATS units by means of HF communication when they pass over their FIR.

6. Summary

We have learnt the following in this lession:

• ü Meteorology plays a vital role in aviation flight operations.
• ü The importance and indispensability of meteorology in aviation
• ü The current weather parameters and forecast/warning information of severe/aviation significant weather are used by the airport and airline operators for the effective and efficient conduct of flights and to conduct safe and cost effective flights
• ü Socio-economic benefit of Meteorology.
• ü Meteorological instrumentation relevant for aviation.