Thermal

From Freepedia

This article is about the atmospheric phenomenon. For other uses of the term thermal, see thermal (disambiguation).

A thermal column (or thermal) is a column of rising warm air in the lower altitudes of the Earth's atmosphere. Thermals are created by the uneven heating of the Earth's surface from solar radiation, and are an example of convection. The Sun warms the ground, which in turn warms the air directly above it.

The size and strength of thermals are influenced greatly by the properties of the lower atmosphere (the troposphere). Generally, when the air is cold, bubbles of warm air formed by the ground heating the air above it, can rise like a hot air balloon. The air is then said to be unstable. If there is a warm layer of air higher up, an inversion can prevent thermals from rising high and the air is said to be stable.

Thermals are often indicated by the presence of visible cumulus clouds. The rising air in a thermal cools as it ascends, until the water vapor in the air begins to condense into visible droplets. The condensing water releases latent heat energy allowing the air to rise higher. Very unstable air can rise to great heights condensing large quantities of water and so forming showers or even thunderstorms.

Thermals are one of the three sources of lift used by glider pilots to climb.