Weather Theory, Reports, and Forecasts
Why air moves and what the numbers say
Stability drives most weather questions. A stable atmosphere resists vertical motion: smooth air, stratus clouds, steady precipitation, and haze. An unstable atmosphere supports vertical development: cumulus, turbulence, showers, and thunderstorms. Compare the environmental lapse rate to the standard 2°C per 1,000 ft; air that stays warmer than its surroundings as it rises is unstable.
Moisture rules of thumb: the spread between temperature and dew point narrows toward saturation and fog. Estimate cumulus bases with (temperature − dew point) ÷ 2.5 × 1,000 ft AGL. A cold front brings rapid lifting, squall lines, and a narrow band of heavy precipitation; a warm front brings a broad shield of stratiform cloud and steady rain.
Decode reports systematically. In a METAR, the ceiling is the lowest broken or overcast layer - never scattered. Winds are true on a TAF and magnetic on a METAR/ATIS? No: winds aloft forecasts are true, surface winds are reported in degrees magnetic. AIRMETs cover moderate conditions (Sierra = IFR/mountain obscuration, Tango = turbulence, Zulu = icing); SIGMETs cover severe or hazardous conditions.
Trap: treating a scattered layer as a ceiling, or misreading the forecast valid period.
Worked example: METAR KABC 121755Z 27015G25KT 3SM -RA BKN012 OVC030 18/16 A2992. Surface wind 270° at 15 gusting 25; visibility 3 statute miles, light rain; ceiling 1,200 ft AGL; temperature 18°C, dew point 16°C (spread 2) - bases near 800 ft.