Ordinary cumulus clouds are puffy heaps, often with flat bases. Cumulonimbus clouds are enormous vertically developed storm clouds that can produce lightning, heavy precipitation, hail and damaging winds.
Look at vertical growth
Fair-weather cumulus have relatively limited height compared with width. When rising air remains strong, a cumulus tower can grow taller and taller, eventually developing the high, icy anvil characteristic of cumulonimbus.
How to recognize the anvil
At the upper reaches of a thunderstorm, air spreads outward, forming a flattened, fibrous top. Its anvil can extend far from the strongest rain beneath the storm.
Not every cumulus becomes a storm
Cloud growth depends on atmospheric instability, moisture and lifting. Small cumulus clouds can flatten or dissipate with no precipitation at all. A rapidly growing tower deserves more attention than an isolated small puff.
What to do when thunder occurs
Do not wait for rain to reach you. Move inside a substantial building or enclosed vehicle at the first thunder, and follow local National Weather Service alerts for severe-weather warnings.
Frequently asked question
Can a cumulonimbus cloud have a white top?
Yes. The sunlit ice-rich anvil may be bright white while the deep base below appears very dark.
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