Thermal

A thermal is a column of rising air created by uneven heating of the earth's surface by the sun. Dark surfaces like asphalt parking lots, plowed fields, and rocky terrain absorb more solar radiation and heat the air above them, which then rises because it is less dense than the surrounding air.

Thermals are essential for soaring flight — glider pilots seek out thermals to gain altitude, circling within the rising air column to climb hundreds or thousands of feet. Experienced glider pilots can identify thermals by observing cumulus clouds (which form at the top of thermals where rising air cools to its dew point), watching birds soaring in circles, and noting terrain features likely to produce thermals.

For powered aircraft, thermals manifest as turbulence — the familiar bumpy ride on a sunny afternoon. The strength of thermals varies from gentle (1-2 knots of lift) to powerful (10+ knots in desert or mountain environments). Thermals are most active in the afternoon, weaken toward evening, and are generally absent at night.