Meteorology
Decode before you interpret
A METAR gives wind (direction true, speed, gusts), visibility (statute miles in US reports, metres in ICAO), present weather, sky cover in oktas (FEW 1-2, SCT 3-4, BKN 5-7, OVC 8), temperature and dew point, altimeter, and remarks. The ceiling is the lowest BKN or OVC layer; a scattered layer is not a ceiling. TAFs use the same codes with FM, TEMPO and BECMG change groups, and TEMPO means temporary fluctuation, not a prevailing condition.
Altimetry: below the transition altitude fly the local altimeter setting; above it, standard 29.92 inHg or 1013 hPa. Getting the switch wrong produces a real altitude error of roughly 30 ft per 0.01 inHg.
Hazards: clear-air turbulence near jet streams, airframe icing in the 0 to minus 20 Celsius band in visible moisture, low-level wind shear and microbursts (headwind gain then sudden loss on final), and convective activity flagged by SIGMET or AIRMET. Fog forms as temperature and dew point converge, typically within 2-3 Celsius.
Worked example. Field elevation 2,000 ft, altimeter 29.62, OAT plus 30 Celsius. Pressure altitude = 2,000 + (29.92 minus 29.62) x 1,000 = 2,300 ft. ISA temperature there is about 15 minus (2 x 2.3), roughly 10 Celsius, so the ISA deviation is plus 20. Density altitude is approximately 2,300 + (120 x 20) = 4,700 ft: a high, hot, heavy takeoff.