thermal wind balance

The thermal wind balance occurs between the horizontal temperature gradient and the vertical shear of the geostrophic wind (i.e., thermal wind). During frontogenesis or frontolysis, there is not a thermal wind balance because ageostrophic motions are occurring to cause the wind parallel to the isobars to depart from geostrophic. During frontogenesis, for example, the horizontal pressure gradient increases with height due to the increase in the low-level horizontal temperature gradient. This causes the total wind at a particular level to accelerate in an across-isobar direction toward lower pressure; the acceleration being more dramatic with height. The Coriolis force responds to this new wind speed and eventually adjusts its direction back parallel to the isobars. The result is a larger thermal wind (or geostrophic wind shear) in order to balance the larger low-level temperature gradient.