Cloud Atlas
The sky is a forecast. What you see now tells you about the next 6–36 hours.
High clouds (> 20,000 ft) — the advance warning
- Cirrus (Ci) — thin wispy "mare's tails". If spreading and thickening from a consistent direction → warm front in 24–36 h.
- Cirrostratus (Cs) — milky veil. Halo around sun or moon. System 18–24 h out.
- Cirrocumulus (Cc) — ripples / "mackerel sky". Unstable air aloft.
Mid clouds (6,500–20,000 ft)
- Altostratus (As) — gray sheet, sun appears as ground glass disk. Rain within 6–12 h from a warm front.
- Altocumulus (Ac) — puffy rolls/patches. Altocumulus castellanus (turrets) in the morning → thunderstorm risk by afternoon.
Low clouds (< 6,500 ft)
- Stratus (St) — flat gray ceiling. Drizzle possible. Often with fog.
- Stratocumulus (Sc) — lumpy layer. Trade-wind skies; generally benign.
- Nimbostratus (Ns) — dark rain cloud; the front has arrived.
Convective — the ones that matter in the moment
- Cumulus humilis — fair-weather. Flat tops, cotton-ball shape.
- Cumulus congestus — tall, cauliflower tops building vertically. Shower risk.
- Cumulonimbus (Cb) — anvil top, dark base. Squalls, lightning, possible waterspouts. Reef early and give them space.
A fast-moving Cb can produce a gust front of 30–50 kt ahead of the visible rain. Watch the water surface for the dark line ("cat's paws" accelerating to whitecaps) — that's your warning.