Wind is air in motion. The Sun heats the ground, the ground heats the air, and warm air becomes less dense and rises while cooler, denser air sinks to take its place. Air moves from high pressure toward low pressure, and the bigger the pressure difference, the faster the wind. Because the Earth rotates, moving air turns to the right in the Northern Hemisphere and to the left in the Southern Hemisphere, which splits each hemisphere into three circulation cells and gives regions their prevailing winds.