Weather Theory and Hazards

Standard atmosphere

The International Standard Atmosphere (ISA) defines sea-level pressure as 29.92 inHg (1013.25 hPa), temperature 15°C, and a lapse rate of approximately 2°C per 1,000 ft up to 36,000 ft. Pilots use this reference to compare actual conditions with performance charts.

Fronts and air masses

A cold front forces warm air rapidly upward, producing cumuliform clouds, showery precipitation and possible thunderstorms. A warm front has gentler slope with widespread stratiform clouds and steady precipitation ahead of the surface front. Stationary and occluded fronts combine characteristics and often bring prolonged IFR conditions.

Thunderstorms

Thunderstorms require moisture, unstable air and a lifting mechanism. The three stages are cumulus, mature and dissipating. Hazards include turbulence, hail, lightning, microbursts and wind shear. Avoid by at least 20 NM from severe cells; never penetrate a mature thunderstorm.

Icing, fog and visibility

• Structural icing occurs in visible moisture between 0°C and −20°C; clear ice is most hazardous.
• Radiation fog forms on clear nights with light wind; advection fog moves in with moist air over a cooler surface.
• MVFR is 3–5 SM and/or ceilings 1,000–3,000 ft; IFR is below 3 SM and/or ceiling below 1,000 ft.

Weather products

METARs report current conditions; TAFs forecast aerodrome weather. Area Forecasts (FA) and AIRMETs/SIGMETs describe en route hazards. Convective SIGMETs warn of severe thunderstorms and tornadoes.