Cumulus is the cloud everyone draws without thinking: a white, domed heap shaped like a cauliflower, with a bright top and a flat, slightly grey base. It forms on sunny days when warm air rises from the ground, and it follows the rhythm of the day: it appears in late morning and fades by late afternoon.
- Level: Low level (0 to 2,000 m)
- Base height: 0 to 2,000 m
- Appearance: white heaps with domed tops, flat greyish base
- Associated weather: fair weather; if it grows, showers and storms possible
How cumulus forms
The sun warms the ground, the ground warms the air touching it, and this lighter air rises in bubbles called thermals. As it climbs it expands and cools by about 1 °C per 100 m while it is unsaturated. It finally reaches its dew point: the vapour condenses into droplets and the cloud appears. The height of that condensation sets the flat base of the cumulus.
You can estimate it with a simple rule: the base lies about 125 m above the ground for every degree of difference between the air temperature and its dew point. With 25 °C and a dew point of 15 °C you get a base near 1,250 m. Above the base, condensation releases heat, which keeps the air rising and makes the top grow into domed towers.
How to recognise cumulus
Cumulus has sharp outlines, a domed top and a flat, darker base. It stands alone, with blue sky between clouds. Meteorologists distinguish species by height: cumulus humilis, flat and small, is the fair-weather cloud; mediocris, of medium depth, has a budding top; congestus, tall and turbulent, is a big cumulus that heralds showers.
When its tops fray into fibrous ice, it has become cumulonimbus: at that point the storm is not far away. Compared with stratocumulus, its elements are separated by gaps of blue and have domed tops; compared with altocumulus, it is bigger, lower and sharper in outline.
What weather cumulus signals
Small cumulus humilis that appear in the morning and fade in the evening signal settled, fine weather. It is the signature of an ordinary summer's day. The further the day goes, the bigger they grow, and they vanish as the sun drops and the thermals die.
On the other hand, cumulus that develop early and upwards, becoming congestus, signal afternoon showers. When their tops reach a great height they turn into cumulonimbus. On a morning with a clear sky and strong humidity, big cumulus can develop before noon: the risk of thunderstorms rises.
Risks and precautions
A cumulus humilis is harmless, and glider pilots use it to climb. The risk lies in what it can turn into. Watch the congestus: if they grow fast and their tops turn fuzzy, take shelter before the shower breaks. In the mountains, set off early and plan to come down before mid-afternoon.
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).
With 16.9 °C and a dew point of 9.0 °C around 14:00, the base of a fair-weather cumulus would sit near 990 m above Toronto, at roughly 125 m per degree of difference.
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
Why is the base of a cumulus flat?
Because it marks the height at which rising air reaches its dew point. Every bubble of air from the ground reaches saturation at about the same height, hence a flat base.
How can I calculate the height of a cumulus?
Estimate the base as about 125 m per degree of difference between the temperature and the dew point at the ground. With 20 °C and a dew point of 10 °C it is near 1,250 m. The top is measured differently: it depends on the stability of the atmosphere.
Does cumulus mean rain?
A small, isolated fair-weather cumulus gives none. If it grows into a big, tall cauliflower it can give showers, and even a thunderstorm if it becomes cumulonimbus.
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.