The ten basic cloud types
Every cloud in the sky belongs to one of ten genera defined by the World Meteorological Organization, organised by the altitude of the cloud base and by shape. This reference lists each genus with its abbreviation, altitude étage, appearance and the weather it tends to bring, plus filters by level and keyword so you can identify what you see overhead.
How it works
Cloud names combine a height prefix and a shape root:
cirro- = high alto- = middle strato- = layered (flat)
cumulo- = heaped nimbo- / -nimbus = rain-bearingHigh clouds (cirrus, cirrocumulus, cirrostratus) are made of ice crystals; low and middle clouds are mostly water droplets. Cumulus and cumulonimbus grow vertically across étages, with cumulonimbus reaching the tropopause and forming the classic anvil top of a thunderstorm. The étage filter restricts the list to high, middle, low or vertical clouds, and the keyword filter matches name, appearance or weather text.
The ten genera at a glance
| Genus | Abbrev | Étage | Appearance | Weather signal |
|---|---|---|---|---|
| Cirrus | Ci | High | Thin white streaks or filaments | Fair, but may precede a front |
| Cirrocumulus | Cc | High | Small white tufts in rows (“mackerel sky”) | Changing weather |
| Cirrostratus | Cs | High | Thin sheet, halo around sun/moon | Rain within 12–24 hours |
| Altocumulus | Ac | Middle | Grey/white patches in groups | Changeable; “mackerel sky” at mid-level |
| Altostratus | As | Middle | Uniform grey veil, sun appears “watery” | Continuous rain or snow approaching |
| Nimbostratus | Ns | Middle/Low | Thick dark featureless layer | Steady prolonged rain or snow now |
| Stratocumulus | Sc | Low | Low grey rolls or patches | Light rain possible; often overcast |
| Stratus | St | Low | Uniform grey sheet, like elevated fog | Drizzle; can reduce visibility |
| Cumulus | Cu | Vertical | Puffy white heaps, flat base | Fair weather (small); showers (large) |
| Cumulonimbus | Cb | Vertical | Towering with anvil-shaped top | Heavy showers, hail, thunderstorms |
Reading the sky for weather
Cloud identification is most useful when you read a sequence rather than a single cloud type. Classic sequences to know:
Approaching warm front — the standard sequence hours before rain arrives: cirrus streaks appear first (ice-crystal wisps at altitude), thickening to a uniform cirrostratus veil that produces a sun or moon halo, then lowering to altostratus (the watery sun effect), then nimbostratus as steady rain begins. This progression can span 12 to 24 hours from the first cirrus to the rain.
Afternoon convective storms — on warm humid days, small fair-weather cumulus (Cu humilis) in the morning can build through the afternoon into towering cumulus (Cu congestus) and then cumulonimbus if instability is sufficient. The appearance of an anvil-shaped top on a cumulus tower is the signal that the storm has reached the tropopause and heavy showers with lightning are likely.
Stable overcast — a large area of stratus or stratocumulus with no vertical development typically produces only drizzle or light rain. The sky looks uniformly grey with no texture; the base sits low and flat. This is the “classic British winter sky” pattern associated with stagnant high pressure and fog-prone conditions at ground level.
Species and varieties
Each genus is further divided into species (describing cloud shape and structure) and varieties (describing arrangement and transparency). Common species include:
- Cumulus humilis — flat-based, low vertical development, fair weather
- Cumulus congestus — vertically developed, cauliflower top, showers possible
- Cumulonimbus capillatus — cumulonimbus with the characteristic fibrous anvil top, indicating a mature thunderstorm
- Stratus nebulosus — uniform featureless stratus, the lowest-visibility variety
- Cirrus uncinus — hook-shaped cirrus (“mares’ tails”), often a warm front signature
This reference covers the ten base genera. The WMO’s International Cloud Atlas is the definitive source for the full species and variety classification.