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Cumulus clouds

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Cumulus clouds are a genus-type of low-level cloud that can have noticeable vertical development and clearly defined edges. Cumulo- means "heap" or "pile" in Latin.[1] They are often described as "puffy" or "cotton-like" in appearance, and generally have flat bases. Cumulus clouds, being low-stage clouds, are generally less than 2,000 metres (6,600 ft) in altitude unless they are the more vertical cumulus congestus form. Cumulus clouds may appear by themselves, in lines, or in clusters.

Cumulus Clouds of MayCumulus Clouds of May. Photo copyright by CGC. 

Cumulus clouds are often precursors of other types of cloud, such as cumulonimbus, when influenced by weather factors such as instability, moisture, and temperature gradient. Normally, cumulus clouds produce little or no precipitation, but they can grow into the precipitation-bearing congestus or cumulonimbus clouds. Cumulus clouds can be formed from water vapor, supercooled water droplets, or ice crystals, depending upon the ambient temperature. They come in many distinct subforms, and generally cool the earth by reflecting the incoming solar radiation. Cumulus clouds are part of the larger category of free-convective cumuliform clouds, which includecumulonimbus clouds. The latter genus-type is sometimes categorized separately as cumulonimbiform due to its more complex structure that often includes a cirriform or anvil top.[2] There are also cumuliform clouds of limited convection that comprise stratocumulus clouds (low-étage), altocumulus clouds (middle-étage) and cirrocumulus clouds (high-étage).[3]


 Cumulus clouds form via atmospheric convection as air warmed by the surface begins to rise. As the air rises, the temperature drops (following the lapse rate), causing the relative humidity (RH) to rise. If convection reaches a certain level the RH reaches one hundred percent, and the "wet-adiabatic" phase begins. At this point a positive feedback ensues: since the RH is above 100%, water vapour condenses, releasing latent heat, warming the air and spurring further convection.

In this phase, water vapor condenses on various nuclei present in the air, forming the cloud. This creates the characteristic flat-bottomed puffy shape associated with cumulus clouds.[4][5] The size of the cloud depends on the temperature profile of the atmosphere and the presence of any inversions.[6] During the convection, surrounding air is entrained (mixed) with the thermal and the total mass of the ascending air increases.[7]

Rain forms in a cumulus cloud via a process involving two non-discrete stages. The first stage occurs after the droplets coalesce onto the various nuclei. Langmuir writes that surface tension in the water droplets provides a slightly higher pressure on the droplet, raising the vapor pressure by a small amount. The increased pressure results in those droplets evaporating and the resulting water vapor condensing on the larger droplets. Due to the extremely small size of the evaporating water droplets, this process becomes largely meaningless after the larger droplets have grown to around 20 to 30 micrometres, and the second stage takes over.[7] In the accretion phase, the raindrop begins to fall, and other droplets collide and combine with it to increase the size of the raindrop. Langmuir was able to develop a formula[note 1] which predicted that the droplet radius would grow unboundedly within a discrete time period.[8]


 The liquid water density within a cumulus cloud has been found to change with height above the cloud base rather than being approximately constant throughout the cloud. At the cloud base, the concentration was 0 grams of liquid water per kilogram of air. As altitude increased, the concentration rapidly increased to the maximum concentration near the middle of the cloud. The maximum concentration was found to be anything up to 1.25 grams of water per kilogram of air. The concentration slowly dropped off as altitude increased to the height of the top of the cloud, where it immediately dropped to zero again.[9]

Cumulus clouds can form in lines stretching over 480 kilometres (300 mi) long called cloud streets. These cloud streets cover vast areas and may be broken or continuous. They form when wind shear causes horizontal circulation in the atmosphere, producing the long, tubular cloud streets.[10] They generally form during high-pressure systems, such as after a cold front.[11]

 The height at which the cloud forms depends on the amount of moisture in the thermal 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 below 550 metres (1,800 ft) above ground level, but it can range up to 2,400 metres (7,900 ft) in altitude. In arid and mountainous areas, the cloud base can be in excess of 6,100 metres (20,000 ft).[12]

Cumulus clouds can be composed of ice crystals, water droplets, supercooled water droplets, or a mixture of them.[1] The water droplets form when water vapor condenses on the nuclei, and they may then coalesce into larger and larger droplets. In temperate regions, the cloud bases studied ranged from 500 to 1,500 metres (1,600 to 4,900 ft) above ground level. These clouds were normally above 25 °C (77 °F), and the concentration of droplets ranged from 23 to 1300 droplets per cubic centimeter (380 to 21,300 droplets per cubic inch). This data was taken from growing isolated cumulus clouds that were not precipitating.[13] The droplets were very small, ranging down to around 5 micrometers in diameter. Although smaller droplets may have been present, the measurements were not sensitive enough to detect them.[14] The smallest droplets were found in the lower portions of the clouds, with the percentage of large droplets (around 20 to 30 micrometers) rising dramatically in the upper regions of the cloud. The droplet size distribution was slightly bimodal in nature, with peaks at the small and large droplet sizes and a slight trough in the intermediate size range. The skew was roughly neutral.[15] Furthermore, large droplet size is roughly inversely proportional to the droplet concentration per unit volume of air.[16] In places, cumulus clouds can have "holes" where there are no water droplets. These can occur when winds tear the cloud and incorporate the environmental air or when strong downdrafts evaporate the water.[17][18]