Visual reference
The cloud atlas
All 10 primary cloud types plus the rare and beautiful ones — organized by altitude, with formation mechanisms and what each one tells you about tomorrow's weather.
High clouds (above 20,000 ft)
Cirrus
Wispy ice-crystal clouds. "Mare's tails." Fair weather now but often precede an incoming warm front within 24 hours.
Cirrostratus
Thin white veil covering the entire sky. Produces halos around the sun/moon. Warm front usually within 12-24 hours.
Cirrocumulus
Small round patches in a herringbone pattern. 'Mackerel sky.' Weather changing but rarely severe.
Mid clouds (6,500-20,000 ft)
Altostratus
Grey or blue-grey sheet covering the whole sky. Sun visible as through frosted glass. Rain within hours.
Altocumulus
Grey-white patches with rolls or waves. Common on warm humid mornings — signals afternoon thunderstorm potential.
Low clouds (below 6,500 ft)
Stratus
Uniform grey layer. Can produce mist or drizzle. Fog is stratus at ground level.
Stratocumulus
Lumpy layer of low clouds. Often persists all day. Rarely produces significant rain.
Nimbostratus
Thick dark grey layer producing continuous precipitation. Warm front territory.
Clouds with vertical development
Cumulus
Puffy fair-weather clouds. If they grow tall, they can become cumulonimbus.
Cumulonimbus
The thunderstorm cloud. Anvil top, dark base, can produce hail, lightning, and tornadoes.
Notable cloud features
Wall cloud
Localized lowering of a rain-free cumulonimbus base. Rotating = tornado possible.
Shelf cloud
Long horizontal wedge attached to a thunderstorm's leading edge. Signals damaging winds.
Roll cloud
Rare detached horizontal tube of cloud. Not a shelf cloud.
Mammatus
Pouch-like clouds hanging under an anvil. Follow severe storms.
Lenticular
Lens-shaped stationary cloud over mountains. Often mistaken for UFOs.
Kelvin-Helmholtz
Wave-shaped cloud like ocean waves. Rare and beautiful.
Undulatus
Wavy horizontal ripples across a cloud layer.
Asperitas
Chaotic wavy underside. Newest official cloud type (2017). Photogenic.
The high-altitude oddities
Noctilucent
Ice crystals at 47 mi altitude (mesosphere). Glow at high-latitude summer twilight.
Nacreous
Rainbow-colored polar stratospheric clouds. Winter only, high latitudes.
Pyrocumulus
Cumulus formed over a wildfire or volcano.
Pyrocumulonimbus
A fire thunderstorm. Can produce lightning and even tornadoes (see fire tornado explainer).