
Aeronautical Meteorology
The branch of meteorology that deals with atmospheric effects on the operation of
vehicles in the atmosphere, including winged aircraft, lighter-than-air devices
such as dirigibles, rockets, missiles, and projectiles. The air which supports flight
or is traversed on the way to outer space contains many potential hazards.
Poor visibility caused by fog, snow, dust, and rain is a major cause of aircraft
accidents and the principal cause of flight cancellations or delays.
The weather conditions of ceiling and visibility required by regulations for crewed
aircraft during landing or takeoff are determined by electronic and visual aids
operated by the airport and installed in the aircraft. The accurate forecasting of terminal conditions is critical to flight economy,
and to safety where sophisticated landing aids are not available. Improved prediction
methods are under continuing investigation and development, and are based on mesoscale
and microscale meteorological analyses, electronic computer calculations, radar
observations of precipitation
areas, and observations of fog trends. See also Mesometeorology; Micrometeorology.
Atmospheric turbulence is principally represented in vertical currents and their
departures from steady, horizontal airflow. When encountered by an aircraft, turbulence
produces abrupt
excursions in aircraft position, sometimes resulting in discomfort or injury to passengers, and sometimes even structural
damage or failure. Major origins of turbulence are (1) mechanical, caused by irregular
terrain below the flow of air; (2) thermal, associated with vertical currents produced
by heating of air in contact with the Earth's surface; (3) thunderstorms and other
convective clouds (Fig. 1); (4) mountain wave, a regime of disturbed airflow leeward of mountains or hills, often comprising both smooth
and breaking waves formed when stable air is forced to ascend over the mountains;
and (5) wind shear,
usually variations of horizontal wind in the vertical direction, occurring along
air-mass boundaries, temperature inversions (including the tropopause), and in and near the jet stream.
While encounters with strong turbulence anywhere in the atmosphere represent substantial
inconvenience,
encounters with rapid changes in wind speed and direction at low altitude can be
catastrophic.
Generally, wind shear
is most dangerous when encountered below 1000 ft (300 m) above the ground, where
it is identified as low-altitude wind shear. Intense convective microbursts, downdrafts
usually associated with thunderstorms, have caused many aircraft accidents often
resulting in a great loss of life. The downdraft emanating from convective clouds,
when nearing the Earth's surface, spreads horizontally as outrushing rain-cooled
air. When entering a
microburst outflow, an aircraft first meets a headwind that produces increased performance by way of
increased airspeed
over the wings. Then within about 5 s, the aircraft encounters a downdraft and then a tailwind with decreased performance. A large proportion
of microburst accidents, both after takeoff and on approach to landing, are caused
by this performance decrease, which can result in rapid descent.
Turbulence and low-altitude wind shear can readily be detected by a special type
of weather radar, termed
Doppler radar. By measuring the phase shift of radiation backscattered by
hydrometeors and other targets in the atmosphere, both turbulence and wind shear
can be clearly identified. It is anticipated that Doppler radars located at airports,
combined with more thorough pilot training regarding the need to avoid microburst
wind shear, will provide desired protection from this dangerous aviation weather
phenomenon. See also Doppler radar.
Since an aircraft's speed is given by a propulsive component plus the speed of the
air current bearing the aircraft, there are aiding or retarding effects depending
on wind direction in relation to the track flown. Wind direction and speed vary
only moderately from day to day and from winter to summer in certain parts of the
world, but fluctuations of the vector wind at middle and high latitudes in the troposphere
and lower stratosphere
can exceed 200 knots
(100 mph).
The role of the aeronautical meteorologist is to provide accurate forecasts of the
wind and temperature field, in space and time, through the operational ranges of
each aircraft involved. For civil jet-powered aircraft, the optimum flight plan must always represent a compromise among
wind, temperature, and turbulence conditions. See also Upper-atmosphere dynamics; Wind.
The jet stream is a
meandering, shifting current of relatively swift wind flow which is embedded
in the general westerly
circulation at upper levels. Sometimes girdling the globe at middle and subtropical latitudes, where the strongest jets are found,
this band of strong winds, generally 180–300 mi (300–500 km) in width, has great
operational significance for aircraft flying at cruising levels of 4–9 mi (6–15
km). The jet stream challenges the forecaster and the flight planner to utilize tailwinds to the greatest extent possible
on downwind flights
and to avoid retarding headwinds as much as practicable on upwind flights. As with considerations of wind and temperature,
altitude and horizontal coordinates are considered in flight planning for jet-stream
conditions. Turbulence in the vicinity of the jet stream is also a forecasting problem.
See also Jet
stream.
An electrical discharge or lightning strike to or from an aircraft is experienced
as a blinding flash and a
muffled explosive sound. Atmospheric conditions favorable for lightning
strikes follow a consistent pattern, characterized by solid clouds or enough clouds
for the aircraft to be flying intermittently on instruments; active precipitation
of an icy character;
and ambient air temperature near or below 32°F (0°C). Saint Elmo's fire, radio static,
and choppy air often
precede the strike. However, the charge separation processes necessary for the production
of strong electrical fields is destroyed by strong turbulence. Thus turbulence and
lightning usually do not
coexist in the same space. See also Atmospheric electricity; Lightning.
Modern aircraft operation finds icing to be a major factor in the safe flight. Icing
usually occurs when the air temperature is near or below freezing (32°F or 0°C)
and the relative
humidity is 80% or more. Clear ice is most likely to form when the air temperature
is between 32 and −4°F (0 and −20°C) and the liquid water content of the air is
high (large drops or many small drops). As these drops impinge on the skin of an aircraft, the surface temperature
of which is 32°F (0°C) or less, the water freezes into a hard, high-density solid.
When the liquid water content is small and when snow or ice pellets may also be present, the resulting rime ice formation is composed of drops and encapsulated air, producing an ice that is less dense
and
opaque in appearance. Accurate forecasts
and accurate delineation of freezing conditions are essential for safe aircraft
operations.