baroclinic

(physics) Of, pertaining to, or characterized by baroclinity.

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. Strong baroclinicity indicates a steep horizontal temperature gradient with associated thermal winds. The situation occurs thus: with distance from the equator, patterns of isobars and isotherms are broadly alike, each sloping from a maximum, to a minimum at the poles. However, the slope for isotherms is much steeper than for isobars, so that at some point the two intersect, and this happens most often in mid-latitudes in winter.

In such areas, known as baroclinic zones, the isotherms at height show a sudden polewards fall (i.e. there is a sudden increase in the meridional temperature gradient), cutting across the vertically zoned isobars. (Imagine temperatures falling very rapidly polewards while pressure remains constant in that direction, but falls with height.) In the baroclinic zones of the mid-latitudes, spontaneous generation of weather systems such as depressions and thunderstorms is common. 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, and is characterized by the ascent of warmer, and the descent of colder, air.