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Weather

Fog is really a stratus cloud that forms at the ground or so close to it as too seriously impair surface visibility. Cooling is the most frequent cause of fog formation. It may be due to loss of heat by the ground because of radiation (radiation or ground fog), loss of heat of warm air following over a cooler surface (advection fog)), adiabatic cooling of air streaming over rising terrain (up-slope fog). Fog may form as a result of nonsaturated air becoming saturated through evaporation from water warmer than the air-stream fog. If the evaporation is from rain falling through colder air, the fog is referred to as frontal fog.

Thunderstorms are generally classified according to the way the initial lifting action is accomplished. They are divided into two general groups, frontal thunderstorms and air-mass thunderstorms. Each thunderstorm progreses through a cycle which consists of the three stages: 1) the cumulus stage, 2) the mature stage, and 3) the dissipating stage or anvil stage. The hazards associated with thunderstorm clouds are: lifting, hail, altimeter errors, icing, snow, turbulence, rusts. One of the weather phenomena most respected by pilots is the squall line of thunderstorms which sometimes precedes a cold front.

Icing is caused by an accumulation of super-cooled water drops which form ice on air frames, engines, propellers, rotors, windows and other exposed surfaces of an aircraft and under certain conditions inside their power plants.

Three conditions are necessary for formation of either rime or glaze ice (not frost however):

1) the presence of visible moisture

2) an outside air temperature at or below freezing

3) an aircraft whose temperature is at freezing or less

There are several forms of ice which an aviator will encounter – snow, sleet, hail, propeller ice, rotor ice, windshield and canopy ice, Pitot tube ice and structural ice.

Turbulence is caused by random fluctuations of wind flow which are instantaneous and irregular. There are four degrees of turbulence which the pilot should know: 1) light, 2) moderate, 3) severe, 4) extreme, and four causes of turbulence 1) thermal (or convective), 2) mechanical, 3) frontal, 4) large-scale wind shear. In addition to these phenomena, such as the wake turbulence.

Pilots operating at high altitudes should understand the following phenomena of high-altitude flight. Jet streams are very high-speed winds which usually flow west to east in narrow bands in the middle latitudes between 20000-40000ft. Jet streams occur near regions where there is a large horizontal temperature difference between warm and cold air masses.

Clear air turbulence (CAT) is sometimes encountered in the vicinity of jet streams, probably as a result of large wind shears.

A contrail is generated in the wake of aircraft flying in clear cold humid air. There are three types: 1) exhaust trails, 2) aerodynamic trails, 3) dissipation trails.

Canopy static is caused by solid particles (ice crystal of cirrus clouds) which brush against the canopy or other plexiglass-covered surfaces of the aircraft during flight and build up a static electric charge on them. When this static electricity is discharged, the accompanying noisy disturbance reduces radio reception.

There are ten principal sources of aviation weather information: 1) surface observation, 2) pilot balloon observation, 3) radar observation, 4) radiosonde observation, 5) weather recognizance flight, 6) meteorological rocket sounding, 7) satellite observation, 8) ships-at-sea-report, 9) automatic meteorological observation stations (AMOS), 10) pilot reports.

Answer the questions.

1. How is fog formed?

2. How are thunderstorms generally classified?

3. What hazards are associated with thunderstorm clouds?

4. What is icing caused by?

5. Could you name forms of ice which an aviator can encounter?

6. What is turbulence caused by?

7. Where do jet streams occur?

8. What does CAT stand for?

9. What condition is a contrail generated under?

10. Could you explain the formation of electric statics?

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