The Subtropical High-Pressure Belt and Rain Shadows

Most of the world's great hot deserts owe their existence to a single planetary-scale circulation feature: the Hadley cell, first described in physical terms by the English lawyer and amateur meteorologist George Hadley in a 1735 paper explaining the trade winds. Warm, moisture-laden air rises near the equator along the Intertropical Convergence Zone, cools as it ascends, and releases its moisture as the heavy tropical rainfall that sustains equatorial rainforests. That same air, now largely wrung dry, spreads poleward at high altitude and sinks back toward the surface at roughly 30° North and South latitude, warming adiabatically as it descends under increasing atmospheric pressure. Adiabatic warming lowers the relative humidity of already-dry air still further and actively suppresses cloud formation, producing broad belts of persistent high atmospheric pressure — the subtropical highs, exemplified by semi-permanent features such as the Azores High in the North Atlantic and the South Pacific High — beneath which skies remain clear and rainfall stays minimal for most of the year.