SUBTROPICAL HIGH

one of several regions of semipermanent high atmospheric pressure located over the oceans near 35 latitude in both the northern and southern hemispheres of the Earth. The highs, which are apparently of dynamic rather than thermal origin, are most intense and closest to the poles in summer. The circulation around the highs is clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere. There is also, especially on the eastern and equatorward sides of the highs in the lower portions of the troposphere, a component of motion directed across the isobars (lines connecting points of equal atmospheric pressure) toward lower pressure. This low-level divergent motion is compensated by motion toward the centre at high levels and a broad zone of descending air, most noticeable over the eastern half 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.