Heating of the earth near the equator leads to large amounts of upward motion and convection along the monsoon trough or Intertropical convergence zone. The divergence over the near-equatorial trough leads to air rising and moving away from the equator aloft. As it moves towards the Mid-Latitudes, the air cools and sinks, which leads to subsidence near the 30th parallel of both hemispheres. This circulation is known as the Hadley cell and leads to the formation of the subtropical ridge.
The subtropical ridge is a significant belt of high pressure situated around the latitudes of 30°N in the Northern Hemisphere and 30°S in the Southern Hemisphere. It is characterized by mostly calm winds, which acts to reduce air quality under its axis by causing fog overnight, and haze during daylight hours caused by the stable atmosphere found near its location. Air flows out from its center toward the upper and lower latitudes of each hemisphere, creating both the trade winds and the westerlies. It moves poleward during the summer, reaching its most northern latitude in early fall, before moving equatorward during the cold season.