Clouds
Cloud Types for Sailors — What the Sky Is Telling You
Published · Last checked October 2026
Clouds are the sailor's free forecast. Cirrus spreading and thickening from the west often means a depression 12 to 24 hours away; a halo in cirrostratus is a classic warning; altostratus means rain within hours; nimbostratus brings steady rain; small cumulus mean fair weather but towering cumulonimbus bring squalls, sudden wind shifts and thunder.
| Cloud | Height | Looks like | Means for sailors |
|---|---|---|---|
| Cirrus | High | Wispy mares' tails | Front may be on the way |
| Cirrocumulus | High | Tiny ripples | Usually harmless |
| Cirrostratus | High | Milky veil, halo | Warm front within about 12–24 h |
| Altocumulus | Middle | Grey-white heaps | Fair; towers mean storms later |
| Altostratus | Middle | Grey sheet, watery sun | Rain within hours |
| Nimbostratus | Low | Thick, dark, rain | At the warm front |
| Stratocumulus | Low | Lumpy grey rolls | Mostly dry |
| Stratus | Low | Flat grey, drizzle | Warm sector, poor visibility |
| Cumulus | Low to middle | Puffs with flat bases | Fair; growing ones bring showers |
| Cumulonimbus | Towering | Huge, with an anvil | Squalls, hail, thunder |
Reading a sequence
One cloud on its own tells you little. A sequence tells a story: cirrus thickening into cirrostratus, then altostratus, with a falling barometer and a backing wind, is the textbook approach of a warm front. See weather fronts explained.
Shower clouds and squalls
Under a cumulonimbus, cold air pours down and spreads out ahead as a gust front. Expect the wind to increase sharply and shift, often by 30 degrees or more, as the cloud arrives. Reef before it reaches you.
Learn to recognise them all in Weather School.