Cumulonimbus is the thunderstorm cloud: a massive tower, sometimes more than ten kilometres high, whose dark base lies close to the ground and whose top spreads into an anvil. It is the only cloud genus able to produce lightning, hail, violent showers and destructive gusts all at once.
- Level: Low level (0 to 2,000 m)
- Base height: 0 to 2,000 m (top up to the tropopause)
- Appearance: massive tower, dark base, anvil-shaped top
- Associated weather: thunderstorm, violent showers, hail, lightning, gusts
How cumulonimbus forms
Three ingredients are needed: moist air near the ground, an unstable atmosphere (rising air stays warmer than its surroundings) and a trigger, such as ground heating, high ground or a front. A cumulus then grows into a congestus, and its tops, now cold enough to freeze, take on a fibrous look: that is the stage at which we speak of cumulonimbus. The cloud climbs to the tropopause, about 11 km in the standard atmosphere, where the air becomes stable and halts it; the top then spreads into an anvil.
Inside, strong updraughts (tens of metres per second in the most violent storms) carry droplets up to freezing heights, where they freeze, grow and become electrically charged through friction and collisions. The falling precipitation drags cold air downward: this downdraught spreads out as a gust front when it hits the ground and can produce very strong gusts. A thunderstorm cell generally lasts less than an hour at any one spot.
How to recognise cumulonimbus
You recognise it by its size and its vertical build: an enormous tower, cauliflower-topped and then smooth and fibrous, sometimes spread into a horizontal anvil. The base is very dark, almost black, and flat, often with ragged clouds streaming beneath. Under it you can see curtains of rain or hail, and sometimes hear thunder.
When the anvil carries pouch-like bulges called mammatus, the cloud is mature. The roll of cloud that leads the storm, the arcus or shelf cloud, announces the gust front. Thunder can be used to judge distance: sound travels about 340 m per second, so each three seconds between flash and rumble means roughly one kilometre.
What weather cumulonimbus brings
It does not announce the weather; it brings it: a violent shower, sometimes hail, lightning, a wind that strengthens suddenly and rapid changes of temperature. In a few minutes the temperature can drop by several degrees. Hail is made of ice at least 5 mm across, by the WMO definition.
After it passes, the sky clears quickly and the air is cooler and washed clean. Summer storms often start in the afternoon, at the peak of the heat, and die out in the evening; those on fronts can cross at night.
Risks and precautions
Cumulonimbus is the most dangerous cloud. Lightning can strike several kilometres from the rain, and you are exposed as long as you can hear thunder. Take shelter in a building or a closed car, away from isolated trees, water and ridges, and wait a good while after the last rumble before going out again. Gusts can bring down branches; hail damages crops, glass and vehicles. Sudden downpours cause run-off and flash floods: never drive through a flooded road.
Check the official warnings for your country: they give the warning level and the areas concerned.
In the sky over Toronto today
Here is what the Open-Meteo forecast of 2026-11-02 08:13 gives for Toronto, averaged over the next 24 hours: 77% of the sky covered at low level, 58% at middle level and 76% at high level.
For this kind of cloud the level that matters is this one: at low level the sky is mostly covered (77% on average).
No thunderstorm appears in the next 48 hours of the model: no cumulonimbus is expected.
These percentages are model values on a grid: they say how much of the sky is occupied at each level, not which kind of cloud is there. They change at every update.
Frequently asked questions
How tall is a cumulonimbus?
Its base is low, in the low level, but its top climbs to the tropopause, about 11 km in the standard atmosphere, and sometimes higher in the most intense storms.
How can I tell how far away a storm is?
Count the seconds between the flash and the thunder. Sound travels about 340 m per second: three seconds correspond to one kilometre. If the gap shortens, the storm is approaching.
What should I do in a thunderstorm outdoors?
Get into a building or a closed car, and move away from isolated trees, open water, ridges and metal objects. Do not shelter under a tree. Wait a good while after the last thunder before setting off again.
Sources
- World Meteorological Organization (WMO), International Cloud Atlas, WMO-No. 407: definitions of the genera, levels and base heights for temperate regions. https://cloudatlas.wmo.int (copyright WMO; no text or image from the Atlas is reproduced here).
- Drawing and text on this page: original to this site, schematic.