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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 [[atmosphere of Earth|Earth's atmosphere]], a form of atmospheric [[Vertical draft|updraft]]. Thermals are created by the uneven heating of 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 |isbn = 0-7136-4226-2}}</ref> ==Thermals on Earth== The warmer air nearer to the surface expands, becoming less [[density|dense]] than the surrounding air. The lighter air rises and cools due to its expansion in the lower pressure at higher altitudes. It stops rising when it has cooled to the same temperature as the surrounding air. Dark earth, urban areas, and roadways are good examples of sources of thermals. 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 referred to as 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 "[[Horizontal convective rolls|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. The latter are [[Cumulonimbus and aviation|dangerous]] to any aircraft. Thermals are one of the many sources of [[lift (soaring)|lift]] used by [[soaring birds]] and [[Unpowered aircraft|gliders]] to [[Gliding flight|soar]]. ==Thermals beyond Earth== Thermals are seen elsewhere in the solar system. On [[Mars]], for example, thermals are often seen in the form of [[dust devils]], viewed from the [[Mars rover|Martian rovers]] and orbiters. Thermals are also seen on the [[sun|solar]], typically forming hexagonal prisms ([[Bénard cell]]s). ==See also== * [[Air current]] * [[Atmospheric thermodynamics]] * [[Cumulus cloud]] * [[Gliding]] ** [[Hang gliding]] * [[Thermal energy]] == 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:Aviation meteorology]] [[Category:Severe weather and convection]] [[Category:Atmospheric thermodynamics]]'