Meteorology - Products, Wind, Stability, Icing and Visibility
Products to know
METARs are routine hourly observations, SPECI reports special changes, TAFs are aerodrome forecasts with validity periods and TEMPO/BECMG/PROB30 change groups, GFAs give graphical cloud and weather plus icing and turbulence, AIRMETs and SIGMETs flag hazards, and PIREPs are pilot reports. All times are UTC. Ceiling is the lowest broken or overcast layer; SKC means sky clear; vertical visibility appears when the sky is obscured.
Wind
In METARs and TAFs, wind direction is true; winds passed by ATIS and ATC are magnetic. That single difference catches many candidates. Read dddffGggKT correctly: the first two digits are direction, then mean speed, then gusts. A gust spread means mechanical turbulence and a real limit on light aircraft and RPAS.
Stability, icing and visibility
Steep lapse rates and unstable air bring gusty surface winds, showers and turbulence; stable air brings smooth conditions, haze, low stratus and fog. Icing is most serious with freezing rain or freezing drizzle - supercooled droplets that accrete on props, intakes and airframe, costing lift and adding weight. A small temperature/dewpoint spread signals fog risk; snow, smoke and haze cut visibility and, more importantly, cut your visual line of sight.
Performance
Density altitude - high elevation, high temperature, high humidity - degrades both aerodynamic and battery performance, so plan shorter flights with more reserve.
Traps
- Reading a gust as the steady wind.
- Treating a TAF as current weather.
- Forgetting that reported winds are true while ATC winds are magnetic.
Worked example
METAR wind 26018G28KT means 260° true at 18 kt gusting 28. Plan and set limits for 28 kt, and expect the wind ATC gives you to be magnetic and therefore different by the local variation.