Weather and Weather Services
Stability and moisture
The standard atmosphere at sea level is 15 C and 29.92 in Hg, with a lapse rate near 2 C per 1,000 ft. Air that cools faster than standard with height is unstable and supports cumuliform clouds, turbulence, and gusty surface winds. Air that cools slowly or warms with height (an inversion) is stable and produces stratiform clouds, haze, and smooth air. The dew point spread controls cloud base and fog.
Reading the products
METARs report observed conditions, TAFs forecast them, and PIREPs fill in conditions aloft. AIRMETs cover moderate hazards such as turbulence, icing, IFR conditions, and mountain obscuration. SIGMETs cover severe or extreme turbulence, severe icing, volcanic ash, and dust storms. Convective SIGMETs imply thunderstorms. A ceiling is the lowest broken or overcast layer, not the lowest scattered layer.
Traps
- Fog forms when temperature and dew point converge: radiation fog on clear calm nights, advection fog when warm moist air moves over a cold surface.
- Structural icing requires visible moisture and freezing temperatures, and is most likely between 0 C and about minus 20 C.
- A microburst gives a headwind-then-tailwind shear; go around at the first unexplained airspeed loss and sink.
Rule of thumb
Estimate cumulus bases with (temperature minus dew point in C) times 400 ft AGL. A 5 C spread suggests bases near 2,000 ft; a 1 C spread suggests fog or very low ceilings.