Guide
How to identify clouds (without guessing)
This site uses the WMO (World Meteorological Organization) cloud classification as the backbone. The goal is practical: figure out what you’re seeing, and what it usually implies — without pretending clouds are perfect weather predictors.
The 10 official “genera” (WMO)
Every cloud you see belongs to one (and only one) genus.
- Cirrus (Ci)
- Cirrocumulus (Cc)
- Cirrostratus (Cs)
- Altocumulus (Ac)
- Altostratus (As)
- Nimbostratus (Ns)
- Stratocumulus (Sc)
- Stratus (St)
- Cumulus (Cu)
- Cumulonimbus (Cb)
Tip: “nimbo-” implies precipitation; “strato-” implies layers; “cumulo-” implies heaps; “cirro-” implies high/ice.
The finger test (for ‘mackerel sky’ clouds)
If the sky is full of little bumps/ripples, hold up your hand at arm’s length and compare the size of the cloud elements.
- Cirrocumulus: tiny grains < ~1° (about your little finger width).
- Altocumulus: 1–5° (bigger than little finger, smaller than ~3 fingers).
- Stratocumulus: > ~5° (≈ three fingers or more).
Halos vs coronas
A quick optical clue that narrows the search.
- Halo around the Sun/Moon → commonly cirrostratus (ice crystals).
- Corona (colored ring close to the Sun/Moon) → can happen with thin altocumulus.
A classic pattern: warm-front approach
One common progression (not guaranteed) is clouds thickening and lowering:
Cirrus → Cirrostratus → Altostratus → (lower) Stratus/Nimbostratus
This is why thin high veils can be the opening act for later steady rain.