Cumulus Cloud - Description

Description

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.

Cumulus clouds that have formed over the ocean may be found in regularly spaced lines or patterns. These "cloud streets" form when convective currents lying parallel to the ground and each other create updrafts and downdrafts. One current rotates one way, while the two on either side of it rotate in the opposite direction. The distance between the centers of the clouds is three times the height (diameter) of each of the convective currents. The inversion created by the downdraft causes the clear space between the lines of cloud.

The height at which the cloud forms (the cloud base) 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 6,000 feet (1,800 m) above ground level, but it can range up to 8,000 feet (2,400 m) in altitude. In arid and mountainous areas, the cloud base can be in excess of 20,000 feet (6,100 m).

Cumulus clouds can be composed of ice crystals, water droplets, supercooled water droplets, or a mixture of them. 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. 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. 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. Furthermore, large droplet size is roughly inversely proportional to the droplet concentration per unit volume of air. In places, cumulus clouds can have "holes" where there 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.