Cumulus clouds look like white fluffy balls of cotton wool and mark the vertical extent of convection or thermal uplift of air taking place in the in the atmosphere. The level at which condensation and cloud formation begins is defined by the flat cloud base, and its height will depend upon the humidity of the rising air. The more humid the air, the lower the cloud base.
Small cumulus clouds are evidence of relatively stable air during fair weather. Once condensation begins, uplift continues only for a short distance before the temperature of rising air falls below that of the surrounding atmosphere. Since cumulus clouds have no existence apart from thermal convection, they are diurnal clouds when over land, forming mid morning and disappearing again after sunset. They may also form over sea when the sea surface temperature is warmer than the air passing over it. Then similar convection currents can develop.
Young fair weather cumulus have sharply defined edges and bases while the edges of older clouds appear more ragged, an artifact of cloud erosion. Evaporation along the cloud edges cools the surrounding air, making it heavier and producing sinking motion or subsidence outside the cloud. This downward motion inhibits further convection and the growth of additional thermals from below, which is why fair weather cumulus typically have expanses of clear sky between them. Without a continued supply of rising air, the cloud begins to erode and eventually disappears. Fair weather cumulus typically have a lifetime of 5 minutes to about half an hour.