<P> 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 . </P> <P> 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 . They generally form during high - pressure systems, such as after a cold front . </P> <P> 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). </P> <P> 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 are no water droplets . These can occur when winds tear the cloud and incorporate the environmental air or when strong downdrafts evaporate the water . </P>

Where do cumulus clouds form in the atmosphere
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