In 1735, George Hadley, a British physicist (1685–1768), formulated a model to explain the general pattern of global atmospheric circulation. The Hadley model describes, in the simplest form, a large-scale circulation in Earth's atmosphere, with a rising motion of air over the equatorial regions and a descending motion northward and southward toward the midlatitudes.
Massive convection in the equatorial Pacific lofts air from the surface to altitudes above the tropopause. The tropopause is coldest in equatorial regions, and water precipitates in the form of rain and ice from the convected air masses. This injects very dry air into the region of the tropopause and above. The air moves generally toward the poles and descends in the process, accounting for our observations of exceedingly dry air layers at midlatitudes. Frequently, moist layers occur above the dry regions. Wind shear causes this by transporting thin layers from nearby moist regions.