Air Masses and Cloud Types

An air mass is a large body of air with broadly similar temperature and humidity. Around the British Isles, common air masses include polar maritime (cool, moist, unstable with showers), tropical maritime (warm, moist, often cloudy with poor visibility), polar continental (cold and dry in winter), tropical continental (hot and dry in summer), and returning polar maritime (air that has travelled south then returned north, often unstable).

Cloud type helps identify what the air mass and fronts are doing. High clouds such as cirrus and cirrostratus often signal approaching frontal weather. Altostratus and nimbostratus bring widespread grey cloud and steady rain. Cumulus suggests convection and fair-weather instability; towering cumulus and cumulonimbus warn of heavy showers, squalls, thunder, and gusts. Stratus and low cloud often accompany drizzle, hill fog, or sea fog.

For the skipper, this is not academic classification. Cloud and air-mass clues help you decide whether the forecast is arriving early, whether visibility is likely to deteriorate, and whether squalls or gusty conditions are developing ahead of the next front.