ObserveCumulus
Cu · Low · 18 August 2026 · By
Cumulus: reading the fair-weather heaps
Cumulus is the first cloud most people learn to name, and the one most often named wrongly. It is a convective cloud: it exists because a parcel of air near the ground got warmer than its surroundings and rose until it hit its condensation level. Everything you can read from a cumulus — base height, edge sharpness, vertical growth rate — comes directly from that single fact.
Cumulus
Cumulus
- Band
- Low
- Altitude
- 500 m – 3 km
- Code
- Cu
- Seasonal window
- In the northern mid-latitudes cumulus fields are most reliable from late April to early September over land, peaking two to four hours after local solar noon. In winter the same airmass produces far fewer, flatter cumulus because surface heating is weak. (Apr–Sep)
- Published
- 18 August 2026
- By
Weather clue
Scattered humilis and mediocris over land in the afternoon means a stable, dry-ish boundary layer and no rain today. A single congestus tower among flat cumulus means one locally favoured spot is being fed harder than the rest.
How to observe it
- Measure the surface temperature and dew point first, then predict the base height before you look up.
- Pick one tower and time it for two minutes; note whether the top is rising, flat, or dissolving.
- Hold your arm out at full length to size the elements — a useful, portable scale reference.
- Photograph the same tower twice, 15 minutes apart, from the same spot so the growth is measurable.
The base height is a thermometer you can see
The flat bottom of a cumulus sits at the lifting condensation level (LCL), the altitude where a rising parcel finally cools to its dew point. A useful field approximation for the lowest 2 km is base (m) ≈ 125 × (temperature − dew point), both in °C.
That gives you a genuine measurement from the ground. If the air is 24 °C and the dew point is 16 °C, the spread is 8 °C and the bases will sit near 1000 m. If the spread is 3 °C, bases sit near 375 m and the sky looks low and crowded. Pilots use exactly this number — low bases plus high terrain is where visual flight rules stop being safe.
Four species, in the order they appear during the day
On a settled summer day a cumulus field usually walks through its species in sequence. Recognising which one you are looking at tells you how far into the day's convection you are.
| Species | Shape | Typical top | What it is telling you |
|---|---|---|---|
| Cumulus fractus | Ragged shreds, no defined base | below 1000 m | Morning, or the decaying remains of yesterday's cloud |
| Cumulus humilis | Wider than tall, flat pancake | 1000-1500 m | Convection has just started; the day is stable so far |
| Cumulus mediocris | Roughly as tall as it is wide, slightly ragged top | 1500-2500 m | Convection is deepening; watch it |
| Cumulus congestus | Clearly taller than wide, cauliflower towers | 3000-5000 m | Strong updraughts; a shower or a cumulonimbus is possible within the hour |
Sharp edges mean a healthy updraught
A cumulus with crisp, hard outlines is being fed by a steady updraught that keeps replacing the droplets as they evaporate at the margin. A cumulus whose edges are going soft and fuzzy is dying: the updraught has been cut off and the cloud is a leftover.
Watch one tower for two minutes. If the top is visibly rising and the sides stay defined, you are watching a cloud that will get bigger. If the top flattens into a wide anvil-ish smear, the tower has hit a stable layer and the convection is capped.
How cumulus turns into something worth respecting
Congestus is the doorway. Two signs say the doorway is being walked through: the top stops looking like cauliflower and starts looking like a hard, glaciated cap, and the base goes dark and flat with a suspiciously smooth underside.
Once you see that transition, stop and note the time. From first glaciation to a mature cumulonimbus with rain at the ground is often 20 to 40 minutes in a humid, unstable summer airmass — fast enough that a walk you started in clear sunshine can end in a downpour.
- Base darkening and turning uniform: the cloud is now optically thick
- A hard, fibrous top instead of cauliflower: ice has taken over above the freezing level
- First visible rain shaft below the base, usually ragged at the edges
- Sudden cool gust at ground level: downdraught has reached you before the rain has
Rules of thumb
- Cumulus base height ≈ 125 × (temperature − dew point) in metres for the lowest 2 km.
- Species order fractus → humilis → mediocris → congestus tracks the day's convective energy.
- Crisp edges mean active updraught; fuzzy edges mean the cloud is dissolving.
- Congestus with a glaciated top is a cumulonimbus in progress — expect 20-40 minutes to maturity.
| The other cloud | Comparison — Cumulus |
|---|---|
| 层积云 · Stratocumulus | Stratocumulus comes in sheets with dark, lumpy bases joined together; the elements are wider than they are tall and do not grow vertically through the day. |
| 高积云 · Altocumulus | Altocumulus elements are smaller and sit higher; hold your arm out — an altocumulus element is about the size of your thumbnail, a cumulus element is much larger. |
The two or three genera this one is most often confused with, and the single feature that separates them.
Observation questions
Why do cumulus clouds have flat bottoms?
Because the bottom marks a single altitude: the lifting condensation level. Every parcel rising from the well-mixed surface layer reaches saturation at the same height, so the cloud starts at that height and nowhere below it. The top has no such constraint, which is why it is ragged and constantly changing.
How fast can a cumulus grow?
In a humid, unstable summer airmass a congestus tower can add several hundred metres of depth in ten minutes. From first glaciation of the top to a mature cumulonimbus with rain at the surface is often 20 to 40 minutes, which is faster than most people expect.