Formation
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. If atmospheric conditions are suitable, this can lead to deep 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. The size of the cloud 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 micrometers, 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.
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