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'{{about|the atmospheric phenomenon|other uses|thermal (disambiguation)}} {{One source|date=July 2009}} [[Image:Thermal column.svg|thumb|Example of a thermal column between the ground and a cumulus]] 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 are an example of [[convection]], specifically [[atmospheric convection]]. The [[Sun]] warms the ground, which in turn warms the air directly above it.<ref>{{cite book |last= Bradbury |first= Tom |title= Meteorology and Flight: Pilot's Guide to Weather (Flying & Gliding) |publisher= A & C Black |year= 2000 |id = ISBN 0-7136-4226-2}}</ref> Dark earth, urban areas and roadways are good sources of thermals. The warmer air near the surface expands, becoming less [[density|dense]] than the surrounding air mass. The mass of lighter air rises, and as it does, it cools due to its expansion in the lower pressure of the higher altitude. 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. The size and [[power (physics)|strength]] of thermals are influenced by the properties of the lower atmosphere (the ''[[troposphere]]''). Generally, when the air is cold, bubbles of warm air are formed by the ground heating the air above it and 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 [[temperature inversion|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 cloud|cumulus]] [[clouds]] at the apex of the thermal. When a steady wind is present, thermals and their respective cumulus clouds can align in rows oriented with wind direction, sometimes referred to as "cloud streets" by [[Lift (soaring)|soaring]] and [[Glider (sailplane)|glider]] pilots. Cumulus clouds are formed by the rising air in a thermal as it ascends and cools, 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 reach the [[level of free convection]] (LFC) and thus 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 (soaring)|lift]] used by [[soaring birds]] and [[Unpowered aircraft|gliders]] to [[Gliding flight|soar]]. Thermals on the sun typically form hexagonal prisms ([[Bénard cell]]s). == See also == * [[Atmospheric thermodynamics]] == References == {{Reflist}} == External links == * [http://www.rcsoaring.com/docs/thermals_2006.pdf What do thermals look like?] - ''Thermal Structure and Behavior'' by Wayne M. Angevine * [http://bookergc.blogspot.com/2008/04/thermal-formation-and-decay.html Time-lapse video of clouds caused by thermals forming and decaying] [[Category:Weather]] [[Category:Convection]] [[Category:Atmospheric thermodynamics]]'