Reference
Cloud types illustrated guide
Every cloud type recognized by the WMO. Height, physics, and the weather it forecasts.
The three families
High clouds (16,500+ ft)
Ice crystals. Cirrus, Cirrostratus, Cirrocumulus.
Mid-level clouds (6,500-20,000 ft)
Water droplets, sometimes ice. Altocumulus, Altostratus.
Low clouds (surface-6,500 ft)
Water droplets. Stratus, Stratocumulus, Nimbostratus.
Vertical clouds
Cross multiple levels. Cumulus, Cumulonimbus.
High clouds
Cirrus (Ci)
Wispy, hair-like ice clouds. Precede warm fronts. "Mares tails" indicate storm approaches in 24-48 hrs.
Cirrostratus (Cs)
Thin, translucent ice veil. Produces 22° halo around sun/moon. Warm front approaching in 12-24 hrs.
Cirrocumulus (Cc)
Small ice ripples. Rare. "Mackerel sky." Fair weather but unstable air aloft.
Mid-level clouds
Altocumulus (Ac)
Rounded rolls or patches. "Sheep clouds." Instability aloft. Can precede storms.
Altostratus (As)
Gray/blue sheet, sun barely visible through. Warm front getting closer.
Altocumulus castellanus
Turret-topped altocumulus. Instability aloft. Convection likely by afternoon.
Altocumulus lenticularis
Lens-shaped. Mountain wave clouds. Fair but turbulent.
Low clouds
Stratus (St)
Uniform gray sheet, low. Drizzle possible. "Fog off the ground."
Stratocumulus (Sc)
Low, lumpy, gray/white. Common. Fair or drizzle.
Nimbostratus (Ns)
Thick gray rain cloud. Steady rain or snow. Not thunderstorm.
Vertical / convective clouds
Cumulus humilis
"Fair weather" cumulus. Small, flat. Everyday summer.
Cumulus mediocris
Larger cumulus with vertical growth. Building toward showers.
Cumulus congestus
Towering. May become thunderstorm within an hour.
Cumulonimbus (Cb)
The thunderstorm cloud. Anvil top, base to 60,000 ft. Rain, hail, lightning, tornadoes.
Pyrocumulus
Fire-generated cumulus. From wildfires.
Pyrocumulonimbus
Fire-generated thunderstorm. Rare, extreme.
Special / accessory clouds
Anvil (incus)
Flat spreading top of cumulonimbus.
Mammatus
Pouches hanging from anvil bottom. Follows severe storms.
Wall cloud
Lowered rotating base of supercell. Tornado precursor.
Shelf cloud (arcus)
Low wedge on gust front of thunderstorm. Marks strong winds.
Roll cloud
Detached rotating tube. Rare. Some form ahead of thunderstorm outflow.
Contrails
Aircraft ice trails. Persistent contrails indicate humid upper air.
Noctilucent clouds
Ice at 250,000 ft. Visible after sunset in polar regions.
Kelvin-Helmholtz
Breaking-wave clouds. Shear between air layers.
Fallstreaks / virga
Precipitation that evaporates before ground.
Fractus
Ragged shreds under storms.
Undulatus asperitas
Wave-like base of turbulent air. Newly recognized (2017).
Reading clouds for weather
- Fast-moving cirrus → warm front approaching.
- 22° halo → precipitation within 24 hrs 70% of time.
- Cirrocumulus mackerel sky → instability, storms possible.
- Altocumulus castellanus in AM → afternoon storms likely.
- Cumulus growing tall by 11 AM → thunderstorms by 3 PM.
- Mammatus after storm → recent severe activity.
- Wall cloud → tornado risk.
- Green tint under thunderstorm → hail-laden.