Why Clouds Matter
Clouds are more than scenery — they're visible evidence of atmospheric processes happening in real time. A skilled weather observer can look at the sky and make educated guesses about incoming weather just by identifying cloud types and noting how they're changing. The World Meteorological Organization (WMO) classifies clouds into 10 principal genera, organized by altitude and form.
The Three Altitude Levels
Cloud classification begins with altitude:
- High clouds (above ~20,000 ft): Cirrus, Cirrocumulus, Cirrostratus
- Mid-level clouds (~6,500–20,000 ft): Altocumulus, Altostratus, Nimbostratus
- Low clouds (below ~6,500 ft): Stratus, Stratocumulus, Cumulus, Cumulonimbus
Note: Cumulus and Cumulonimbus are called vertical development clouds because they can span all three levels.
High Clouds
Cirrus
Thin, wispy streaks or curls of white at high altitude. Composed entirely of ice crystals. Often called "mare's tails," they may signal an approaching warm front 24–48 hours away when appearing in increasing numbers.
Cirrostratus
A thin, sheet-like veil that covers large portions of the sky. Often produces a halo around the sun or moon — a classic sign that rain may arrive within 12–24 hours.
Cirrocumulus
Small, rounded white puffs arranged in rows at high altitude. Nicknamed a "mackerel sky." Less common than other cirrus types and usually short-lived.
Mid-Level Clouds
Altocumulus
Gray or white patches, layers, or waves of cloud in the mid-levels. When they appear on a warm, humid morning, afternoon thunderstorms are possible later in the day.
Altostratus
A gray or blue-gray sheet covering the whole sky. The sun may appear as if seen through frosted glass. Typically precedes steady, widespread precipitation.
Nimbostratus
A thick, dark gray layer that completely blocks the sun and is actively producing rain or snow. Associated with frontal systems and prolonged precipitation.
Low Clouds
Stratus
A uniform gray layer, like fog that's lifted off the ground. Usually brings drizzle rather than heavy rain. Associated with stable, stagnant air masses.
Stratocumulus
Low, lumpy gray or whitish clouds in patches or rolls — the most common cloud type globally. Often form in the subsiding air around high pressure systems and typically don't produce significant rain.
Vertical Development Clouds
Cumulus
The classic "fair weather" cloud — puffy white with flat bases and rounded tops. When they remain small and scattered, conditions are pleasant. If they grow tall ("towering cumulus"), storm development may be underway.
Cumulonimbus
The king of clouds. Towering thunderstorm cells that can reach the tropopause and produce lightning, heavy rain, hail, strong winds, and even tornadoes. Often identifiable by their characteristic anvil top, which forms when the cloud spreads out at the tropopause.
Quick Reference: Clouds and Likely Weather
| Cloud Type | Likely Weather |
|---|---|
| Cirrus (increasing) | Frontal rain possible in 24–48 hrs |
| Cirrostratus with halo | Rain likely within 12–24 hrs |
| Nimbostratus | Steady, prolonged rain or snow now |
| Cumulonimbus | Thunderstorm imminent or ongoing |
| Stratocumulus | Overcast but mostly dry |
Practice cloud identification daily — it's one of the most rewarding and practical skills in amateur meteorology.