A cumulus cloud is a type of cloud with noticeable vertical development and clearly defined edges. Often described as "puffy" or "cotton-like" in appearance, cumulus clouds may appear alone, in lines, or in clusters. Cumulus clouds are often precursors of other types of clouds, such as cumulonimbus, when influenced by weather factors such as instability, moisture, and temperature gradient. Cumulonimbus clouds may be associated with phenomena such as landspouts, waterspouts and tornadoes. Cumulus means "heap" or "pile" in Latin.
Cumulus clouds typically form when warm air rises and reaches a level of comparatively cool air, where the moisture in the air condenses. This usually happens through convection, where a parcel of air is warmer than the surrounding air. As it rises, the air cools at the dry adiabatic lapse rate (approximately 3°C per 1000ft or 1°C per 100m), while the dewpoint of the air falls by 0.5°C per 1000ft. When the temperature of the air reaches the dewpoint, some water 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. If the top of the cumulus cloud reaches above the altitude where the temperature is at or below the freezing level, then precipitation from the cloud is possible. The temperature of the air at ground level will determine if this falls as rain or snow.
In windy conditions, the clouds can form lines (cloud streets) parallel with the wind. In mountainous areas, they may also form lines at an angle to the wind, due to the presence of lee waves above the clouds.
Over the sea, cumulus clouds may be found in regularly spaced lines. The best examples of these lines are found in the trade winds, where they may extend for many miles. Such lines create a pattern in the vertical movement of air, causing it to roll horizontally. Between the lines of cloud are stronger, more gusty, and slightly veering winds, but beneath the lines of cloud, somewhat lighter and more backing winds prevail.
The height at which the cloud starts to form (cloud base) depends on the amount of moisture in the air parcel that forms the cloud. Humid air will generally result in a lower cloud base. In temperate areas, the base of the cumulus clouds is usually up to 8,000ft (2,400m) in altitude. In arid and mountainous areas, the cloudbase can be in excess of 20,000ft (6,000m).
Cumulus humilis clouds, appearing as small- or medium-sized puffy shapes in the sky, often occur in times of fair weather. However, cumulus clouds can grow into cumulonimbus clouds which may produce heavy rain, lightning, severe and strong winds, hail, and even tornadoes. Cumulus congestus clouds, which appear as relatively thin towers, will often grow into Cumulonimbus storm clouds.