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Encyclopedia > Thermal wind

The thermal wind is not actually a wind, but a wind difference between two pressure levels p1 and p0, with p1 < p0. It is only present in an atmosphere with horizontal gradients of temperature, i.e. baroclinicity. In a barotropic atmosphere the geostrophic wind is independent of height. The name stems from the fact that this wind flows around areas of low (and high) temperature in the same manner as the geostrophic wind flows around areas of low (and high) pressure. Wind is the roughly horizontal movement of air (as opposed to an air current) caused by uneven heating of the Earths surface. ... Geopotential height is a vertical coordinate referenced to Earths mean sea level — an adjustment to geometric height (elevation above mean sea level) using the variation of gravity with latitude and elevation. ... In thermodynamics, temperature is the physical property of a system that underlies the common notions of hot and cold —something that is hotter has the greater temperature. ... In fluid dynamics, the baroclinity (sometimes called baroclinicity) is a measure of the stratification in a fluid. ... A barotropic atmosphere is one in which the density depends only on the pressure, so that isobaric surfaces are also surfaces of constant density. ... The geostrophic wind is defined as the wind resulting from the balance between the Coriolis force and the pressure gradient force. ... The use of water pressure - the Captain Cook Memorial Jet in Lake Burley Griffin, Canberra. ...

The thermal wind equation is

f mathbf{v}_T = mathbf{k} times nabla ( phi_1 - phi_0 ),

where the φx are geopotential height fields with φ1 > φ0, f is the Coriolis parameter, and is the upward-pointing unit vector in the vertical direction. The thermal wind equation does not determine the wind in the tropics. Since f is small or zero there, the equation reduces to stating that nabla ( phi_1 - phi_0 ) is small. Geopotential height is a vertical coordinate referenced to Earths mean sea level - an adjustment to geometric height (elevation above mean sea level) using the variation of gravity with latitude and elevation. ... The Coriolis frequency, f, is equal to twice the rotation rate of the Earth multiplied by the sine of the latitude. ... In mathematics, a unit vector in a normed vector space is a vector (most commonly a spatial vector) whose length is 1. ... In astronomy, geography, geometry and related sciences and contexts, a direction passing by a given point is said to be vertical if it is locally aligned with the gradient of the gravity field, i. ... The tropics are the geographic region of the Earth centered on the equator and limited in latitude by the two tropics: the Tropic of Cancer in the northern hemisphere and the Tropic of Capricorn in the southern hemisphere. ...


Examples of thermal wind

Temperature contrast between equator and pole: Increasing westerlies with height

(This applies similarly to the Southern hemisphere, and because f is negative there, it yields the same result.)

On the Northern hemisphere, the polar region is cold (low temperature) while the equatorial region is warm (high temperature). Because the thermal wind circles the area of low temperature in the same manner as the geostrophic wind flows around a cyclone, namely counter-clockwise in the Northern hemisphere, the thermal wind in the northern hemisphere mid latitudes is westerly (i.e. is directed eastward). This can be seen by looking at a globe from above the North Pole — a westerly current flows counter-clockwise around the globe. Insert non-formatted text here The Northern Hemisphere is the half of a planets surface (or celestial sphere) that is north of the equator (the word hemisphere literally means half ball). On the Earth, the Northern Hemisphere contains most of the land and population. ... The CYCLONE, an early computer built in 1959 by Iowa State University, was based on the Institute for Advanced Study (IAS) architecture developed by John von Neumann. ... A clockwise motion is one that proceeds like the clocks hands: from the top to the right, then down and then to the left, and back to the top. ... Westerly, founded in 1669 by John Babcock, is a beachfront community on the south shore of Washington County, Rhode Island. ... The North Pole is the northernmost point on the Earth. ...

What does this mean for the vertical wind profile in the mid-latitude NH troposphere? If the thermal wind is westerly, the atmospheric flow will become more westerly with height, as the thermal wind describes the wind change with height. Therefore, if at a certain level, say, at the top of the boundary layer, the wind speed is close to zero, the wind speed will have a strong eastward component at higher levels.

This simple argument basically describes the jet stream, a westerly current of air with maximum wind speeds close to the tropopause which is basically (even though other factors are also important) a result of the temperature contrast between equator and pole. Jet streams are fast flowing, relatively narrow air currents found in the atmosphere at around 12 km above the surface of the Earth, just under the tropopause. ... The tropopause is a boundary region in the atmosphere between the troposphere and the stratosphere. ...

Advection of warm or cold air: Turning of the geostrophic wind with height

If the geostrophic wind at a level advects (i.e. transports) warm or cold air, the thermal wind causes a turning of wind direction with height. A similar argument as in the other example with regard to how the thermal wind is related to the temperature distribution can be made. Wind direction is the direction from which the wind is blowing. ...

The outcome is that a geostrophic wind that advects warm air into a region of colder air causes the wind to turn right (clockwise, veering) with height, while cold air advection into a region of warmer air results in the wind turning left (counter-clockwise, backing).

Further reading

  • Holton, James R.: An Introduction to Dynamic Meteorology, 2004. ISBN 0-12-354015-1
  • Vasquez, Tim: Weather Forecasting Handbook, 2002. ISBN 0-9706840-2-9

  Results from FactBites:
Thermal wind applied (729 words)
Strong westerly winds blow in the upper troposphere and lower stratosphere of the midlatitudes.
The strength of the wind is proportional to the pressure gradient between A and B, i.e.
As an example we test whether the observed zonal wind in a cross section of the troposphere through western Australia and eastern Siberia, during the southern summer, is in thermal wind balance with the observed meridional temperature field (Fig 2).
geostrophic wind: Definition and Much More from Answers.com (840 words)
A hypothetical wind based upon the assumption that a perfect balance exists between the horizontal components of the Coriolis force and the horizontal pressure gradient force per unit mass, with the implication that viscous forces and accelerations are negligible.
The thermal wind is directed approximately parallel to the isotherms of air temperature with cold air to the left and warm air to the right in the Northern Hemisphere, and vice versa in the Southern Hemisphere.
The geostrophic wind is defined as the wind resulting from what is called the geostrophic balance between the Coriolis force and the pressure gradient force acting on a parcel of air, causing the wind to blow parallel to isobars of pressure in the earth's atmosphere.
  More results at FactBites »



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