<P> 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 . 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 include cumulonimbus 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 . There are also cumuliform clouds of limited convection that comprise stratocumulus (low - étage), altocumulus (middle - étage) and cirrocumulus (high - étage). These last three genus - types are sometimes classified separately as stratocumuliform . </P> <P> 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 vapor condenses, releasing latent heat, warming the air and spurring further convection . </P> <P> In this phase, water vapor condenses on various nuclei present in the air, forming the cumulus cloud . This creates the characteristic flat - bottomed puffy shape associated with cumulus clouds . The height of the cloud (from its bottom to its top) depends on the temperature profile of the atmosphere and the presence of any inversions . During the convection, surrounding air is entrained (mixed) with the thermal and the total mass of the ascending air increases . 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 . 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 which predicted that the droplet radius would grow unboundedly within a discrete time period . </P> <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>

Why does the height of the cumulus cloud base change
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