Cumulus cloud over the River Tay today 15th March. Overnight the wind shifted to the South West bringing warmer moist air. Cumulus clouds are a type of low-level cloud that can have noticeable vertical development and clearly defined edges. Cumulo- means "heap" or "pile" in Latin. 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 6,500 feet (2,000 m) in altitude unless they are the more vertical cumulus congestus form. Cumulus clouds may appear by themselves, in lines, or in clusters and 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 cumuliform clouds, which include stratocumulus clouds, cumulonimbus clouds, cirrocumulus clouds, and altocumulus clouds.
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 vapour 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.