Module 13 - Meteorology
Air Masses and Cloud Types
Polar maritime air reaching the UK is commonly cool, moist and unstable, supporting showers and good visibility between them. Tropical maritime air is warm and moist and can bring extensive low cloud, drizzle, fog and reduced visibility, especially on windward coasts and hills.
Continental and returning air masses are modified as they move. A label describes broad origin and history, not an immutable parcel. Sea track, season, stability, land crossing and fronts can change the weather experienced at the coast.
A progression from cirrus through thicker layered cloud can support the forecast of an approaching warm-frontal system. Cumulonimbus supports a risk of heavy showers, squalls, thunder and gusts. Use the sequence with pressure, wind, visibility and official information; one cloud is not a clock.
| Evidence | Possible implication | Check before acting |
|---|---|---|
| Polar maritime | Cool, moist, showery and unstable | Season, sea track and current forecast |
| Tropical maritime | Warm, moist, low cloud, drizzle or fog | Windward coast, sea temperature and visibility reports |
| Layered cloud thickening | Possible frontal approach | Pressure tendency and forecast timing |
| Cumulonimbus | Heavy shower, squall, thunder or gust risk | Movement, warnings and safe sea room |
Optional quick check
Section 6 of 15
What is the safest conclusion from thickening high cloud and a falling barometer?
Sources and factual review
Reviewed 20 August 2026 - Compass Revision editorial review.
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