Subtropical High

subtropical high, one of several regions of semipermanent high atmospheric pressure located over the oceans between 20° and 40° of latitude in both the Northern and Southern hemispheres of the Earth. These highs are associated with the subsidence of the Hadley cell and move several degrees of latitude toward the poles in the summer. The circulation around the highs is clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere. In both hemispheres, subsidence is greater on the eastern sides of the highs. The subsiding air warms by compression and, coupled with cooling of the lowest layers overlying the cold ocean currents normally found off the west coasts of the continents, forms a pronounced temperature inversion (warm air over cold), called the trade-wind inversion. The inversion acts as a barrier to vertical convection and is largely responsible for the aridity and high frequency of fog found along the west coasts of the subtropical continents, especially in summer.