Cumulonimbus
A cumulus that kept growing until it became a thunderstorm several kilometres tall.
Latinheap of rainLevelLow base, top reaches the stratosphereBase600–2,000 m
What it looks like
A tower of cloud, dark and ragged at the base and brilliant white higher up. When the top reaches the stable air of the tropopause it spreads sideways into a flat anvil pointing downwind. Underneath: heavy showers, hail, gusts and lightning.
What it means
A storm. At twenty kilometres it is a photograph, overhead it is a reason to be indoors. Count the seconds between flash and thunder and divide by three for the distance in kilometres.
Not to be confused with
Cumulus congestus, which is tall but still has a sharp cauliflower top and no anvil, and nimbostratus, which rains steadily for hours without any drama.
Field note. If you can hear thunder, lightning is close enough to matter. Get off hilltops and away from isolated trees and open water.
Seen one? Add it to your log or check with the identifier.