Cloud Atlas: Types and Classification
Clouds are the open book of the sky. Knowing how to "read" them allows you to predict the coming weather, just as sailors and shepherds have done for centuries. In this guide you'll learn to identify the 10 main cloud types, what they indicate about future weather, and some spectacular phenomena few have seen.
What Are Clouds?
Clouds are water or ice suspended in the air. They form when humid air rises, cools, and water vapor condenses into millions of microscopic droplets or ice crystals.
Cloud Classification by Height
The Three Height Levels
- HIGH CLOUDS (6-12 km): Prefix "CIRRO" - Made of ice crystals
- MIDDLE CLOUDS (2-6 km): Prefix "ALTO" - Mix of water and ice
- LOW CLOUDS (0-2 km): No special prefix - Made of water droplets
- VERTICAL DEVELOPMENT CLOUDS: From ground to 12+ km
The 10 Main Cloud Types
HIGH CLOUDS (6-12 km) ❄️
1. CIRRUS (Ci) - "Feathers in the sky"
White, thin, fibrous clouds. Look like feathers or mare's tails. Made of ice crystals.
What they indicate: If cirrus appear from the WEST and increase, a warm front is approaching. Rain in 12-36 hours.
2. CIRROCUMULUS (Cc) - "Mackerel sky"
Small white balls in regular pattern. Look like fish scales.
What they indicate: Weather change coming. May rain in 12-24 hours.
3. CIRROSTRATUS (Cs) - "The veil"
Thin, uniform veil covering entire sky. Produce HALOS around sun/moon.
What they indicate: Clear sign of coming rain! Warm front 6-24 hours away.
MIDDLE CLOUDS (2-6 km) ⛅
4. ALTOCUMULUS (Ac) - "Sheep clouds"
Medium-sized clouds in groups or layers. White or gray with visible shadows.
What they indicate: Variable weather. On warm summer mornings = possible afternoon storms.
5. ALTOSTRATUS (As) - "Gray sky"
Uniform gray or bluish layer covering entire sky. Sun appears diffuse.
What they indicate: Warm front very near. Persistent rain in coming hours.
6. NIMBOSTRATUS (Ns) - "Rain cloud"
Dark gray, thick, uniform layer. Produce CONTINUOUS rain or snow
What they indicate: It's ALREADY RAINING! Or about to. Persistent rain for hours.
LOW CLOUDS (0-2 km) ☁️
7. STRATOCUMULUS (Sc) - "Gray rolls"
Rounded masses or rolls. Gray or whitish. Rarely produce rain.
What they indicate: Cloudy but generally DRY weather.
8. STRATUS (St) - "Fog that doesn't touch ground"
Uniform gray layer very low. Like fog but not touching ground.
What they indicate: Gray, cold, unpleasant weather. Can persist for days.
9. CUMULUS (Cu) - "Fair weather clouds"
White fluffy clouds. Look like "cotton balls" or "cauliflower". Flat base, rounded top.
What they indicate: If SMALL = guaranteed fair weather. If growing large = possible afternoon storm.
VERTICAL DEVELOPMENT CLOUDS (0-12+ km) ⚡
10. CUMULONIMBUS (Cb) - "Storm cloud"
The most powerful cloud! Giant tower reaching 15-18 km height. Very dark base. Anvil-shaped top.
Produces: Lightning, torrential rain, hail, violent winds (100+ km/h), tornadoes
What they indicate: ⚠️ IMMEDIATE DANGER - SEEK SHELTER NOW
What Weather Do Clouds Predict?
Deterioration Sequence (Warm Front Approaching)
- Day 1 - Morning: Clear blue sky
- Day 1 - Noon: CIRRUS appear from west
- Day 1 - Evening: CIRROSTRATUS (veil, halo)
- Day 2 - Dawn: ALTOSTRATUS (gray sky)
- Day 2 - Morning: NIMBOSTRATUS
- Day 2 - Noon: CONTINUOUS RAIN!
Special and Rare Clouds
MAMMATUS - "Celestial Udders"
Hanging pouches like udders below cloud base. Indicate very strong downdrafts. Appear AFTER very severe storms.
Danger: High. Severe storm nearby or just passed.
LENTICULAR CLOUDS - "Flying Saucers"
Lens or flying saucer shaped clouds. Appear near mountains. Indicate SEVERE TURBULENCE for aircraft.
Fun fact: Many "UFO" sightings are actually lenticular clouds.
ARCUS (Shelf Cloud) - "Apocalypse Roll"
Horizontal roll or arc advancing menacingly. Leading edge of severe storm.
Danger: EXTREME. Violent winds coming (80-150 km/h possible). Seek shelter IMMEDIATELY.
VIRGA - "Ghost Rain"
Rain falling from cloud but EVAPORATING before reaching ground. Air below is very dry and hot.
Conclusion
Clouds are much more than sky decoration. They are nature's early warning system, functioning for millions of years before we invented satellites and radars.
See also our guides on Isobar Maps, DANAs and Fronts and Low Pressure.