Energy from the sun heats the area. Let's say the soil region warms more rapidly than the adjacent water and grass surfaces. The soil in return radiates heat to the air directly above it. The warm air develops into a thermal, or warm parcel of rising air. The thermal then rises and moves off as another thermal develops. This process will continue as long as there is energy from the sun to heat the soil and the air.
The Shodor
Education Foundation, Inc.