Not a real wind, but an expression of wind shear for a given layer of atmosphere; the vector expressing the difference between the geostrophic winds at the bottom and top of the layer. It is proportional to the thickness of the layer, and is directed along the isotherms, with cold air to the left in the Northern Hemisphere, and to the right in the Southern. However, the term is used to denote a wind developing as follows: the pressure gradients which produce surface winds may be due to the presence of cold and warm air masses. The fall in pressure with height is rapid in cold air, and much less rapid in warm air. Thus, at height, air pressure in the cold air will be less than that in the warm air. This creates a high-level pressure gradient and, therefore, a wind, often described as the ‘thermal wind’. The strength of this wind is a function of its height and the temperature difference between air masses; the greater the difference, the stronger the wind.

Since there is a marked meridional temperature gradient in the troposphere, influenced at height by a powerful westerly factor, thermal winds are very strong at the point where the temperature gradient is greatest; at the polar front. The result is the polar front jet. The force of a thermal wind may be strengthened by any pressure gradient at ground level. See Chu-shih (1963) Acta Met. 33 on thermal wind in a baroclinic atmosphere, and Roth et al. (1999) Boundary-Layer Met. 92, 2.