A term applied to sections of the atmosphere where trends in pressure (pressure surfaces) are at an angle to trends in temperature. (The precise definition refers to the intersection of isobars and isopycnals—levels, or surfaces, of equal density—but, for most purposes, isotherms can replace isopycnals). The number of intersecting isobars and isopycnals is a measure of baroclinicity (Nakamura (1992) J. Atmos. Sci. 17). Moving polewards, temperatures fall very rapidly, while pressure remains constant in that direction, but falls with height (Garreaud (1999) Monthly Weather Rev. 127). At some point, the slope for isotherms intersects with the slope for isobars, so that the two intersect in a baroclinic zone; most often in mid-latitudes in winter. Barry et al. (2002) Nature 415 see the baroclinic zone as a transport mechanism.

In the mid-latitude baroclinic zones spontaneous generation of weather systems such as depressions and thunderstorms is common. (Sanders and Hoffman (2002) Weather & Forecast. 17, 4). These are baroclinic disturbances, characteristically, on synoptic charts, with strong meridional pressure gradients in the constant-pressure surfaces and vertical wind shear. When the temperature gradient along the meridians is very steep, atmospheric cells break down into cyclonic and anticyclonic eddies. This failure is known as baroclinic instability (Lesiur et al. (2000) J. Turbulence 1, 1), and is characterized by the ascent of warmer, and the descent of colder, air.