CONVERGENCE AND DIVERGENCE

in meteorology, the enlarging and spreading out of air, as well as the rate at which this takes place. The twin term refers specifically to the horizontal inflow or outflow of air. The convergence of horizontal winds, as in one of the convergence zones, causes air to rise and the velocity of the rising air to increase aloft, whereas the divergence of horizontal winds causes downward motion of the air (subsidence). Ground-level atmospheric pressure is not affected by convergence, because divergence occurs simultaneously at higher levels. In the region of the subtropical high-pressure centres, such as those over the oceans of the Northern Hemisphere, the clockwise flow of the large anticyclones causes the ocean's surface layer to converge because the water flow is deflected to the right of the wind direction. As the surface water converges, it tends to pile up in the centre of the vortex and must sink; this subsidence is aided by the increased density (salinity) caused by rapid evaporation. Conversely, in the Aleutian or Icelandic low-pressure areas, divergence of the surface water occurs because of the counterclockwise flow of the large cyclones.