Why Clouds Have Flat Bottoms

On a clear summer afternoon, the clouds above a landscape often share one striking feature. Their bases all sit at the same height, forming a level line across the sky, while their tops rise to different heights and take irregular shapes. The explanation lies in what happens to air as it rises. Air warmed by the ground becomes lighter than the air around it and begins to rise. As it climbs, the pressure around it falls, so the parcel of air expands, and expanding air cools. The amount of water vapor the parcel carries does not change on the way up, since no water is added or taken away. What does change is how much vapor the air is able to hold, because cool air can hold less than warm air. Sooner or later the rising parcel cools to the point where it can no longer hold all the vapor it began with. The surplus condenses into tiny droplets, and those droplets are what we see as cloud. That point is reached at a particular temperature, and therefore at a particular height. Parcels rising from the same stretch of ground begin with roughly the same temperature and the same humidity, so all of them reach that temperature at the same altitude. The flat base of a cloud is that altitude made visible. The tops differ because they depend on how much energy each parcel carried upward.