UPDRAFT AND DOWNDRAFT

in meteorology, upward-moving and downward-moving air currents, respectively, that result from a number of causes. Local daytime heating of the ground sometimes causes the surface air to become much warmer than the air above, and, because the warmer air is lighter, it rises; this vertical current, called a thermal, may reach an altitude of 3 km (2 miles). Updrafts and downdrafts also occur as part of the turbulence produced when air currents or air masses collide with one another or with topographic obstacles. Strong updrafts and downdrafts occur in a thunderstorm. Updrafts characterize a storm's early development, during which warm air rises to the level where condensation and precipitation can begin. When a storm is fully developed, updrafts exist alongside downdrafts caused by the downward dragging of air by falling precipitation. These downdrafts, originating at high levels, contain cold, dense air that spreads out at the ground as a cold air wedge. Downdrafts encountered by aircraft are sometimes called air pockets because it was formerly thought that they were pockets of low pressure or vacuum.