The rotation of the Earth on its axis causes day and night. As the Earth rotates, only one-half of the Earth faces the sun at any given time. The half facing the sun is light (day) and the half facing away from the sun is dark (night). Uneven heating by the Sun creates pressure differences. As the air gets heated up more, the pressure and density decrease. And wind flows between areas of high and low pressure because the Earth is warmer at the equator than at the poles.