Meteorology: Atmosphere, Clouds and Hazardous Weather
The atmosphere and pressure
Know the standard atmosphere, the lapse rate (about 2 °C per 1000 ft in the lower atmosphere), and how pressure, temperature and density change with height. Be comfortable with QNH, QFE and QNE and what each is referenced to, and with the transition altitude and transition level.
Clouds and stability
Understand stability: a stable atmosphere resists vertical displacement, giving stratiform cloud and steady precipitation; an unstable atmosphere gives cumuliform cloud, showers and turbulence. Know the cloud families and the conditions that produce each. Orographic lift, convergence and frontal lifting are the main lifting mechanisms.
Hazardous weather
Thunderstorms: know the three stages (cumulus, mature, dissipating), that the mature stage contains both strong updraughts and downdraughts, and the hazards of hail, microbursts and wind shear. Icing: clear ice from large supercooled droplets, rime ice from small droplets, and the conditions most favourable for airframe icing. Fog: radiation fog forms on clear, calm nights, advection fog forms when warm moist air moves over a cold surface.
Traps
- Assuming the most severe turbulence is at the top of a thunderstorm - it is often near the freezing level.
- Confusing the transition altitude (fixed, by QNH) with the transition level (variable, by QNH/QNE).
- Forgetting that a microburst gives a headwind then a downdraught then a tailwind - the tailwind is the dangerous phase.
Rule of thumb
For icing questions, ask: are there visible water droplets below freezing? Then icing is possible. For fog, ask: is the air cooling to its dew point, and is there a nucleus and light wind? All three must be present.