ObserveCumulonimbus
Cb · Low · 11 September 2026 · By · Hazard
Cumulonimbus: the one cloud that can hurt you
Cumulonimbus is the only cloud genus that routinely produces hail, lightning, damaging gusts and tornadoes, and it is the only one where your observation practice has to include a decision about where to stand. It is a deep convective tower with an ice-crystal top, and it is genuinely different in kind from everything else in this atlas.
Cumulonimbus
Cumulonimbus
- Band
- Low
- Altitude
- 600 m – 12 km
- Code
- Cb
- Seasonal window
- Deep convection requires both heat and moisture. In the northern mid-latitudes the peak is late afternoon and evening from May to September over land; in the tropics it is a year-round feature of the afternoon over land and of the night over warm ocean. (May–Sep)
- Published
- 11 September 2026
- By
Weather clue
A single congestus tower that glaciates at the top, acquires a dark flat base and starts showing a rain shaft is 20-40 minutes from the mature stage. The gust front arriving before the rain is the reliable ground-level warning.
How to observe it
- Track the storm from a substantial building with a clear view in at least two directions.
- Time flash-to-thunder for every strike; write down the shortest interval in each 10-minute block.
- Note the gust front arrival separately from the rain arrival — the gap is often 2 to 10 minutes.
- Do not stand in an open field, at a viewpoint, or under an isolated tree to get a better photograph.
Hazard. This genus is associated with thunderstorms, downbursts, hail or severe gusts. Apply the 30-30 rule: a flash-to-thunder interval of 30 seconds or less puts you within about 10 km. Observe from substantial shelter, never from under an isolated tall tree, on a ridge or in the open.
The three stages, and how to tell which one you are watching
A cumulonimbus goes through a cumulus stage, a mature stage and a dissipating stage. The mature stage is the dangerous one, and it is also the easiest to identify from outside: it is the only stage with both a strong updraught and a strong downdraught running at the same time.
| Stage | Duration | What you see | Ground effects |
|---|---|---|---|
| Cumulus stage | 10-20 min | Tower growing, all updraught, no rain | None yet |
| Mature stage | 15-30 min | Glaciated anvil top, dark flat base, rain shaft | Heavy rain, hail, lightning, gust front |
| Dissipating stage | 20-30 min | Anvil spreading out, base losing definition, rain weakening | Light rain, cool outflow, clearing |
What the sky looks like just before it turns
There is a short window — often 10 to 20 minutes — in which a cumulonimbus is visibly transitioning, and it is worth knowing the signs because the safest time to leave an exposed position is during that window, not after.
The base becomes uniformly dark and suspiciously flat, and briefly may show a rotating lowering under it if a mesocyclone is present. A greenish tint in the base is a real observation and is associated with very large hail: it comes from the cloud being so optically thick that only the green-blue part of the transmitted light survives.
- Base goes uniformly dark and flat, losing the puffy cumulus look
- A sharp, low, horizontal cloud line advances ahead of the rain — the gust front or shelf cloud
- The wind suddenly shifts direction and rises sharply, with a noticeable temperature drop of 5-10 °C
- First hail, which almost always falls before or with the heaviest rain, not after
Safety: the 30-30 rule and what to do with it
The standard lightning rule is: if the time between the flash and the thunder is 30 seconds or less, the flash is within about 10 km and you should be in a substantial building or a hard-topped vehicle. Then wait 30 minutes after the last thunder before going back out.
The 30-second threshold corresponds to a distance of about 10 km, which is the typical radius within which the next strike may occur. Situations to avoid entirely: open ground, the top of a hill, a beach, a boat without a cabin, and standing under an isolated tall tree. A car with a metal roof is genuinely safe because the metal shell conducts the current around you — an open-top vehicle or a golf cart is not.
Why anvils mean the storm is dying, not growing
When the top of a cumulonimbus flattens into a wide anvil, the updraught has hit the tropopause and can no longer rise. The anvil spreads downwind and is made almost entirely of ice crystals that have been left behind.
This is why an anvil that is growing quickly downwind means the storm is strong, while a storm that is all anvil and no tower underneath is on its way out. The exception is a storm that keeps producing new towers on its upwind flank — that is a storm that is sustaining itself, and those can run for hours.
Rules of thumb
- The mature stage is the only one with simultaneous strong updraught and downdraught, and it is the dangerous one.
- A uniform dark flat base with a green tint is associated with large hail.
- 30-30 rule: 30 seconds flash-to-thunder means you are within 10 km — get inside; wait 30 minutes after last thunder.
- A growing anvil means a strong storm; an all-anvil storm with no tower is dying.
| The other cloud | Comparison — Cumulonimbus |
|---|---|
| 雨层云 · Nimbostratus | Nimbostratus is a featureless deep layer with steady, continuous rain; cumulonimbus has a defined tower, a hard anvil and rain that starts and stops abruptly. |
| 浓积云 · Cumulus congestus | Congestus is all cauliflower with no ice at the top and no anvil; the moment the top becomes fibrous and hard, you are looking at a cumulonimbus. |
The two or three genera this one is most often confused with, and the single feature that separates them.
Observation questions
Why is hail so rare in winter?
Because hail needs a very strong updraught to keep the stone aloft long enough to grow, and winter convection over land is weak because surface heating is weak. A winter thunderstorm can produce small hail, but the large stones come from the strongly buoyant summer updraughts.
Is it safe to be in a car during a thunderstorm?
Yes, if the roof is metal and the windows are up. The metal shell conducts the current around the outside of the vehicle. A convertible, a golf cart, a motorcycle or a car with a fibreglass or fabric roof does not provide that protection.