"A thermal column (or thermal) is a column of rising air. Thermals are created by the uneven heating of the Earth's surface from solar radiation. The Sun warms the ground, which in turn warms the air directly above it. The sun warms different surfaces more than others - asphalt, concrete, grass, and water - all heat differently. The warmer air expands, becoming less dense (lighter) than the surrounding air mass. The mass of lighter air rises, and as it does, it cools due to its expansion at lower, high-altitude pressures. It stops rising when it has cooled to the same temperature as the surrounding air. Associated with a thermal is a downward flow surrounding the thermal column. The downward moving exterior is caused by colder air being displaced at the top of the thermal."