Meteorology and aviation

Aviation and meteorology

From the earliest days of aviation it has been essential to understand the role of meteorology.  Even with current high technology aircraft, it is impossible to ignore the enormous power and potential hazards of the weather. Personnel in Air Traffic Services must be able to appreciate different types of weather conditions and the way they affect the operation of aircraft.  There are many different weather factors which each have important influences on flying conditions.

Relation between aircraft operations and meteorology

Pressure and altimetry

The relationship between pressure and altimetry is perhaps the most important, since altitude measurements depend on the variation of pressure with height in the atmosphere.  An aircraft altimeter measures the pressure at the level of the aircraft and converts this to an estimated altitude.  This can be done because atmospheric pressure is greatest at the Earth’s surface, and always decreases with height, and the rate at which it does so is known.  A suitable reference level such as mean sea level, aerodrome elevation or standard setting is input by the pilot and the altimeter will indicate the vertical distance of the aircraft above this surface.

Air temperature

Temperature, together with pressure, affects the air density and therefore the efficiency of engines and the lifting force created by the wings.  A temperature below the freezing point is of course critical for icing and an aircraft flying through clouds or in precipitation at low temperatures may be subject to icing.

Humidity

Condensation depends on humidity and dew point and is responsible for much of the weather that we experience.  Fog and clouds reduce visibility.  Precipitation, in the form of rain, snow or hail, can affect runway conditions.

Wind and Jet streams

Wind affects flying in many ways.  Changes in wind speed can affect the lift created by the airflow over the wings of the aircraft.  Rapid changes of wind speed and/or direction with height, known as wind shear, can be especially dangerous during take-off and landing.

Turbulence in the wind can also produce rapid variations in updraughts and downdraughts.  Jet streams are fast moving rivers of air which can strongly influence the ground speed of aircraft.

Meteorological Hazards to Aviation

  • Fog- fog can have a very significant effect on aircraft operations.  In some cases landing in fog will be possible using instrument landing systems (ILS) while in other cases diversion to other airfields will be required.  Fog conditions can change quite rapidly and an awareness of the possible formation or clearing of fog is vital. There is fog if the visibility is less than 1000m and the obscuring agent is water droplets. Relative Humidity (RH) will be near 100%.
  • Icing- icing on aircraft is a major hazard to aviation as it can disturb the aerodynamic flow of air over an aircraft resulting in higher stall speeds and loss of lift.  It can also cause damage to or flame-out in turbine engines.
  • Wind shear -wind shear may be defined as variations in wind speed and/or direction along an aircraft’s flight path which may displace an aircraft abruptly from its flight path and which may require substantial control inputs to counter it.

Low Altitude Wind shear is defined as wind shear on the final approach path or along the runway, and on the take-off and initial climb-out flight paths.

Low altitude wind shear is a major hazard to light aircraft. The effects of wind shear in the take-off, approach and landing phases of flight have been responsible for numerous accidents to light aircraft.

During take-off and landing, when flying at low speeds, aircraft are more susceptible to changes in the wind.  As an aircraft moves through different layers it may encounter winds of varying speeds and/or directions. This phenomenon, known as wind shear, can produce significant changes in lift causing an aircraft to suddenly gain or lose height.

  • Turbulence turbulence  when aircraft moves through air currents it undergoes changing accelerations (which jostle it from its smooth flight path).  This jostling is called turbulence.

Turbulence is associated with jet streams and also common in clouds which have strong up draughts and downdraughts, such as thunderstorms, and may cause an aircraft to lose height rapidly and unexpectedly.  The hazards in thunderstorms are not limited to turbulence but also include hail, lightning and micro bursts.

DC8 after flight in extreme CAT ( clear air turbulence) over Rocky mountains lost a engine and  a part of wing – 09.12.1992

  • Volcanic ash - jet engines may be suffocated by volcanic ash, leading to engine failure.  In addition, its abrasive action may reduce the transparency of the windscreen making a subsequent landing hazardous.  Aircraft weather radar is normally not able to detect the presence of volcanic ash.
  • Thunderstorms -the thunderstorm is complex  extreme phenomena including:
  • Strong winds and downdraughts;
  • Extreme turbulence;
  • Lighting;
  • Microburst;
  • Icing;
  • Hail;

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