Weather and Meteorology
Weather decides whether a glider flies at all. The exam asks you to identify lift sources, estimate thermal tops from the temperature-dewpoint spread, recognize mountain wave and its hazards, and read a TAF or winds-aloft forecast.
Where lift comes from
- Thermals: sun heats the surface, air rises along the dry adiabatic lapse rate (about 5.4 degrees F per 1,000 ft) until it reaches the convective condensation level and forms cumulus.
- Ridge lift: wind within roughly 30 degrees of perpendicular to the ridge, strongest on the windward side; the lee side holds rotor turbulence.
- Mountain wave: stable air crossing a ridge produces lenticular clouds, smooth lift upwind of the wave crest, and violent rotor below and downwind.
Stability and cloud base
- A steep lapse rate and a large surface-to-500-mb spread mean instability, good thermals, and cumulus; a stable layer caps convection and leaves blue, hazy air.
- An inversion traps haze and stops thermals at its base.
- Estimate cumulus bases with the spread: temperature minus dewpoint in degrees C, multiplied by about 400 ft, gives cloud base AGL.
- A small spread means low bases, showers, and a thunderstorm risk; a very large spread means high bases and possibly gusty, dusty surface winds.
Worked example
Surface temperature 24 C and dewpoint 12 C give a spread of 12. Multiply by 400: about 4,800 ft AGL for cumulus bases. With ridge tops at 3,000 ft, working height is roughly 1,800 ft above the ridge -- enough for a local flight, but the high bases suggest dry, possibly strong thermals and gusty surface winds.
Traps that cost marks
Using the Fahrenheit spread with the 400-ft multiplier; assuming clear blue sky means no lift; treating lenticular clouds as a sign of smooth air; and ignoring the wind gradient that makes the surface wind differ from the wind at 3,000 ft.