From Wikipedia, the free encyclopedia

Example of a thermal column between the ground and a cumulus

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 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 an example of convection. The Sun warms the ground, which in turn warms the air directly above it.

When a pool of warmer air accumulates, it expands and becomes lighter (less dense) than the surrounding air mass. The mass of lighter air will then rise, but as it does so it will cool due to expansion. This process will continue until at some height the pool of air will have cooled to the same temperature as the surrounding air, at this stage the air will stop rising. Also 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 thermal top.

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. When a steady wind is present thermals and their respective cumulus clouds can align in rows oriented with wind direction. Cumulus clouds formed by the rising air in a thermal as it cools and 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 many sources of lift used by soaring birds and gliders to soar.

• Hot tower
• Vertical draft
• Atmospheric thermodynamics

• What do thermals look like? - Thermal Structure and Behavior by Wayne M. Angevine