Clouds are the sky's weather forecast, written in water vapor. Long before radar and satellites, sailors and farmers read the shapes overhead to know whether to bring in the harvest or batten down the ship. You can still do the same today: the thin wisps of a fair morning, the puffy cotton balls of a summer afternoon, and the dark anvil of an approaching storm each tell a different story.
There are dozens of named cloud varieties, but nearly all of them are combinations of just a few basic types, organized by their shape and the altitude where they form. Learn those building blocks and you can glance up and make a surprisingly good short-term forecast.
Cloud names come from a handful of Latin words describing shape and height. Master these roots and most cloud names decode themselves:
Cirro– means high and wispy. Alto– means mid-level. Strato– means a flat, layered sheet. Cumulo– means a heaped, puffy pile. Nimbo– (or the suffix –nimbus) means it's producing rain or snow. So a "cumulonimbus" is a heaped raincloud, and a "stratocumulus" is a lumpy layer. Once you know the parts, the names stop being intimidating.
Meteorologists sort clouds into three altitude families plus the towering clouds that span all levels. Here are the ones you'll actually see and what each typically means.
| Cloud | Height | Looks like | Weather clue |
|---|---|---|---|
| Cirrus | High (above 20,000 ft) | Thin, wispy streaks | Fair now; change may be 24h away |
| Cirrostratus | High | Thin veil, halo around sun | Rain or snow often within a day |
| Altocumulus | Mid (6,500–20,000 ft) | Gray-white "cotton ball" rows | Possible afternoon storms if seen on a warm morning |
| Altostratus | Mid | Gray sheet, sun looks frosted | Steady rain may be approaching |
| Stratus | Low (below 6,500 ft) | Flat gray overcast | Drizzle, mist, gloomy but calm |
| Stratocumulus | Low | Lumpy gray layer with gaps | Usually dry; rarely heavy rain |
| Cumulus | Low base, builds up | Puffy white "cauliflower" | Fair weather — unless they keep growing |
| Cumulonimbus | Low to very high | Towering anvil-topped giant | Thunderstorms, heavy rain, hail |
| Nimbostratus | Low to mid | Thick, dark, featureless | Continuous rain or snow |
High clouds form above about 20,000 feet (6,000 m), where it's so cold they're made entirely of ice crystals. Cirrus are the delicate, feathery white streaks of a fair day, sometimes called "mares' tails." On their own they don't bring rain, but if they thicken into a milky cirrostratus veil, weather is often changing within 24 hours.
Mid-level clouds sit roughly 6,500 to 20,000 feet up. Altocumulus appears as patches or rows of gray-and-white globs — a "mackerel sky." Seeing them on a humid summer morning is a classic hint that the air is unstable and thunderstorms could fire by afternoon. Altostratus is a featureless gray sheet that turns the sun into a dim, frosted disk and often thickens ahead of steady rain.
Low clouds form below 6,500 feet. Stratus is the flat, gray overcast of a dreary day — essentially fog that's lifted off the ground — and may produce light drizzle. Cumulus clouds are the friendly puffballs of a nice afternoon, with flat bottoms and bright, rounded tops. Fair-weather cumulus are harmless. But watch them: if they keep building taller through the day, they can grow into the most dramatic cloud of all.
When a cumulus cloud explodes upward — sometimes 10 miles (16 km) high, all the way to the top of the lower atmosphere — it becomes a cumulonimbus, the thunderstorm cloud. Its flat, spreading top forms the classic anvil shape, a sign the cloud has reached as high as it can go. Cumulonimbus clouds produce the full menu of severe weather: torrential rain, lightning, damaging wind, and hail. If you see one bearing down with a dark, low base, it's time to head indoors and check for a severe thunderstorm warning.
You don't need instruments to make a useful guess about the next few hours. High wispy cirrus thickening into a veil? Change is coming. Puffy cumulus staying small and scattered? Enjoy the nice day. Towers shooting up fast on a muggy afternoon? Storms are likely. A flat gray lid all morning? Expect a calm but gloomy day. Clouds are the visible result of the same temperature, moisture, and pressure that computer forecast models crunch — you're just reading the output with your own eyes.
Almost every cloud is a mix of a few basic types named for their shape and height: wispy cirrus high up, layered stratus low down, heaped cumulus in between, and the towering cumulonimbus that brings storms. Learn those few words and the sky turns into a forecast you can read anywhere, anytime, without a single gadget.
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