A column of rising air. Buildings and automobile traffic heat the air above cities and this heated air rises in a column, called a thermal.

Sailplanes gain altitude by circling in a thermal and being carried upward by it. Thermals are easily found, because clouds normally form at their top. Air containing water vapor is carried upward until it reaches a level at which the vapor condenses and becomes visible droplets of liquid water. These droplets form the cloud.