Cumulus Clouds

Cumulus means “heap” or “pile” in Latin, and true to its etymology, these clouds resemble puffs of cotton scattered over the skies. They start forming from an altitude of 1000 meters (330 feet) above sea level and develop vertically, appearing as rising mounds, domes, or towers. The sunlit parts of these clouds are mostly brilliant white while their bases are relatively dark.


The base of the cumulus cloud, which is usually a kilometer broad, is often flat. The top of the cloud has rounded towers, giving this cloud its characteristic cauliflower or popcorn like appearance. These clouds may appear alone, in lines, or in clusters.

Like all clouds, cumulus (abbreviated “Cu”) also take many species and varieties, depending on the the size, shape and form of the elements. Such types are:

Cu humilis: Cumulus Humilus are the type of cumulus clouds that have small vertical development, or which does not precipitate or disturb the atmosphere. They are, as such called fair-weather cumulus. All the cloudlets within this group are strikingly similar, with uniform flat base and height.

Cu mediocris: Cumulus mediocris is the cumulus clouds characterized by moderate vertical development, or which disrupts the atmosphere in a moderate way. The upper sprouting of this species is not very marked, and it has the shape of a small cauliflower.

Cu congestus: Culumus congestus is a strongly sprouted cumulus species with sharp outlines and characterized by its resemblance to a cauliflower. They occur in the form of very high towers, the tops of which have puffs that may detach from the main portion of the cloud on account of the wind.

Cu fractus: Cumulus fractus is the cloud species where the elements are irregular and small. They present a ragged, shredded appearance, as if torn.

Cu radiatus is a variety of cumulus where the elements arrange in straight parallel bands that, to the naked human eye, seem to converge towards a point of the horizon. Between the lines of these clouds are strong and gusty winds, but beneath the lines of cloud, somewhat lighter and more backing winds prevail.


Cumulus clouds usually form during sunny days, when the warm air rises and reaches a level of comparatively cool air, where the moisture in the air condenses and undergoes convention.

When the hot air rises and encounters the cool air, the temperature of the air falls. When this temperature reaches the dew point, some water vapor condenses out of the air to form the cloud. The size of the cloud depends on the temperature profile of the atmosphere and the presence of any inversion.

The height at which the cloud starts to form depends on the amount of moisture in the air parcel. When the air is humid, the cloud forms at a lower altitude. In temperate areas, the base of the cumulus clouds is usually up to 8,000ft (2,400m) in altitude, whereas in arid and mountainous areas, the cloud base is usually in excess of 20,000ft (6,000m).

Cumulus congestus forms when cumulus mediocris type of cloud undergoes a transformation and grow in size, and sometimes, when the altocumulus castellanus or stratocumulus castellanus types of clouds undergo a similar transformation.


Cumulus clouds contain millions of tiny water droplets. The clouds that form at higher levels might contain some ice crystals also.


Cumulus clouds, especially Cu Humilis are relatively thin and associated with fair weather. They do not have sufficient quantities of water vapor to precipitate.

The exception is the Cu Congestus, which is associated with bad weather. When the atmosphere becomes unstable and strong, upward air currents form, and this moisture laden air enriches the Cu Congestus cloud. The result is heavy rains, flash floods, and thunderstorms. If the atmosphere turns very unstable and the air currents further intensify, Cu Congestus clouds transform into cumulonimbus clouds that cause thunderstorms in an even higher intensity.

Glider pilots often pay close attention to cumulus clouds, as they can be indicators of rising air drafts or thermals underneath.


Most types of cumulus clouds dissipate on account of the winds.

When the top of the cumulus cloud reaches above the altitude where the temperature is at or below the freezing level, precipitation takes place in the form of rain or snow, depending on the temperature of the ground.

When the atmosphere is unstable, the Cu Congestus type of Cumulus clouds grow into cumulonimbus clouds.