Thermal: Difference between revisions
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{{Short description|Column of rising air in the lower altitudes of Earth's atmosphere}} |
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{{About|the atmospheric phenomenon|other uses|Thermal (disambiguation)}} |
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{{One source|date=July 2009}} |
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[[Image:Thermal column.svg|thumb|Example of a thermal column between the ground and a cumulus]] |
[[Image:Thermal column.svg|thumb|Example of a thermal column between the ground and a cumulus]] |
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{{multiple image |
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|image1 = Bubble or vortex ring thermal model.jpg |
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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> |
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|image2 = Column or plume thermal model.jpg |
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|footer = Bubble or vortex ring thermal model (left), and column or plume thermal model (right) |
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[[File:Thermal cross section with stronger lift in darker shades of green while red is sink.jpg|thumb|Thermal cross section with stronger lift (rising air) in darker shades of green, while red is sink (descending air).]] |
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A '''thermal column''' (or '''thermal''') is a rising mass of buoyant air, a convective current in the atmosphere, that transfers heat energy vertically.<ref>{{cite web |date=2003 |title=Glider Flying Handbook, FAA-H-8083-13A |url=https://www.faa.gov/sites/faa.gov/files/regulations_policies/handbooks_manuals/aviation/glider_handbook/faa-h-8083-13a.pdf |access-date=21 January 2021 |website=FAA government handbooks |publisher=U.S. Dept. of Transportation, FAA |pages=9-6,9-7}}</ref> Thermals are created by the uneven heating of Earth's surface from [[solar radiation]], and are an example of [[convection]], specifically [[atmospheric convection]]. |
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==Thermals on Earth== |
==Thermals on Earth== |
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The |
The [[Sun]] warms the ground, which in turn warms the air directly above.<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> The warm air near 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, thus density, as the surrounding air. |
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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. |
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. |
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The size and [[ |
The size and [[Power (physics)|strength]] of thermals are influenced by the properties of the lower atmosphere (the ''[[troposphere]]''). 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, as it's suitable for forming thermals. 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, as mature thermals can't form. |
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Thermals are often indicated by the presence of visible [[ |
Thermals are often indicated by the presence of visible isolated [[Cumulus cloud|cumulus]] [[clouds]] at the top of the thermal. 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. 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. |
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The condensing water releases [[latent heat]] energy allowing the air to rise higher. Very unstable air can reach the [[level of free convection]] (LFC), rising to great heights, condensing large quantities of water and forming convective clouds causing showers or even thunderstorms. The latter are [[Cumulonimbus and aviation|dangerous]] to any aircraft flying through or nearby. |
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==Thermals beyond Earth== |
==Thermals beyond Earth== |
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Thermals are seen elsewhere in the solar system. On [[Mars]], for example, thermals are often seen in the form of [[dust devils]], |
Thermals are also seen elsewhere in the solar system. On [[Mars]], for example, thermals are often seen in the form of [[dust devils]], carrying dust instead of water vapor. Thermals are also seen on the [[sun]], typically forming hexagonal convective prisms ([[Bénard cell|Bénard cells]]). |
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Thermals are also seen on the [[sun]], typically forming hexagonal prisms ([[Bénard cell]]s). |
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==Where Thermals can be found== |
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Thermals appear around areas that have a greater resistance to temperature change than air. These areas radiate heat and warm the surrounding air. Dark earth, urban areas, and roadways are good examples of sources of thermals. [[Urban heat islands]] produce thermals due to human activity. For example, concrete and other materials used to build cities efficiently absorb heat, heating the surrounding air, this pulls cooler air from outside of the city, traveling from high to low pressure creating the thermal column. Other human influences such as automotive exhaust and waste heat generated by industries contribute to the increase in urban heat and thermal effect. |
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Water is another common source of a thermal columns when the temperature of water is higher than the air, waters [[specific heat capacity]] and high heat retention warms air and causes a rising thermal column. |
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Elevation changes also can produce a thermal effect. The tops of east facing hills and ridges are the first to be warmed by the sun pulling air up from lower elevations. |
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==In the Hunting Industry== |
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Thermals have increased in popularity with the hunting community, especially those that chase big game animals reliant on the sense of smell ([[Olfaction]]). Thermals are often small scale used to predict where human scent will blow into a particular area where hunter is intending game to occupy. |
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==See also== |
==See also== |
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* [http://bookergc.blogspot.com/2008/04/thermal-formation-and-decay.html Time-lapse video of clouds caused by thermals forming and decaying] |
* [http://bookergc.blogspot.com/2008/04/thermal-formation-and-decay.html Time-lapse video of clouds caused by thermals forming and decaying] |
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[[Category:Aviation meteorology]] |
[[Category:Aviation meteorology]] |
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[[Category:Gliding technology]] |
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[[Category:Severe weather and convection]] |
[[Category:Severe weather and convection]] |
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Latest revision as of 07:45, 18 March 2024
A thermal column (or thermal) is a rising mass of buoyant air, a convective current in the atmosphere, that transfers heat energy vertically.[1] Thermals are created by the uneven heating of Earth's surface from solar radiation, and are an example of convection, specifically atmospheric convection.
Thermals on Earth
[edit]The Sun warms the ground, which in turn warms the air directly above.[2] The warm air near the surface expands, becoming less 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, thus density, 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 strength of thermals are influenced by the properties of the lower atmosphere (the troposphere). 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, as it's suitable for forming thermals. 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, as mature thermals can't form.
Thermals are often indicated by the presence of visible isolated cumulus clouds at the top of the thermal. 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. 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 soaring and glider pilots.
The condensing water releases latent heat energy allowing the air to rise higher. Very unstable air can reach the level of free convection (LFC), rising to great heights, condensing large quantities of water and forming convective clouds causing showers or even thunderstorms. The latter are dangerous to any aircraft flying through or nearby.
Thermals are one of the many sources of lift used by soaring birds and gliders to soar.
Thermals beyond Earth
[edit]Thermals are also seen elsewhere in the solar system. On Mars, for example, thermals are often seen in the form of dust devils, carrying dust instead of water vapor. Thermals are also seen on the sun, typically forming hexagonal convective prisms (Bénard cells).
See also
[edit]References
[edit]- ^ "Glider Flying Handbook, FAA-H-8083-13A" (PDF). FAA government handbooks. U.S. Dept. of Transportation, FAA. 2003. pp. 9–6, 9–7. Retrieved 21 January 2021.
- ^ Bradbury, Tom (2000). Meteorology and Flight: Pilot's Guide to Weather (Flying & Gliding). A & C Black. ISBN 0-7136-4226-2.
External links
[edit]- What do thermals look like? - Thermal Structure and Behavior by Wayne M. Angevine
- Time-lapse video of clouds caused by thermals forming and decaying
