| Cumulus humilis clouds | |
|---|---|
 | |
| Abbreviation | Cu hum |
| Symbol |  |
| Genus | Cumulus (heap) |
| Species | humilis (humble) |
| Variety |
|
| Altitude | 200-2000 m (656–7,000 ft) |
| Classification | Family C (Low-level) |
| Appearance | Low-altitude, flattened, wider than it is tall, fluffy heaps of clouds with cotton-like appearance. |
| Precipitation | UncommonRain,Snow orSnow pellets |
Cumulus humilis arecumuliform clouds with little vertical extent, common in the summer, that are often referred to as "fair weathercumulus". If they develop intocumulus mediocris orcumulus congestus, thunderstorms could form later in the day.[1]
They generally form at lower altitudes (500–3000 m (1,500–10,000 ft)), but in hot countries or over mountainous terrain these clouds can occur at an altitude of up to 6,000 m (20,000 ft). They show no significant vertical development, indicating that thetemperature in the atmosphere above them either drops off very slowly or not at all with altitude; that is, theenvironmental lapse rate is small ornegative. Cumulus humilis clouds often have little variance in their depths due to their constrained vertical development.[2] Cumulus humilis may be accompanied by othercloud types.
Air below the cloud base can be quite turbulent due to thethermals that formed the clouds, giving occupants of light aircraft an uncomfortable ride.[3] To avoidturbulence where such clouds are present, pilots may climb above the cloud tops. However,glider pilots actively seek out the rising air to gain altitude.
These clouds may later metamorphose intocumulus mediocris and eventuallycumulus congestus clouds whenconvection is intense enough,[4] though the presence of these types of clouds usually indicates fair weather.[1]
Morning cumulus humilis clouds are signs of an unstable atmosphere.Larger clouds or possibly thunderstorms could form throughout the day to cause bad orsevere weather in the afternoon or evening. Cumulus humilis clouds are not rain clouds but could precede a storm.
Cumulus humilis are sometimes seen beneathcirrostratus clouds, which block some of the heat from the sun and thus create aninversion, causing any cumuliform clouds to flatten and become cumulus humilis. In this case, a warm front could be approaching and rain is possible for the next 12 to 24 hours.
When cumulus humilis appear in a clear sky, they are an indicator of pleasant weather for the next several hours.
Cumulus humilis clouds are formed by rising warm air orthermals with ascendingair currents of 2–5 m/s (7–17 ft/s).[5] These clouds are usually very small convective clouds and usually form after a thermal reaches thecondensation level. They can develop into cumulus mediocris clouds but most often dissipate a few minutes after formation.[6]
