This chapter examines how unequal surface heating and the rotation of the earth generate global circulation systems in the atmosphere and oceans. The weight of air and the force of gravity pulling air towards the earth create air pressure .
Air pressure is greatest at the earth's surface and decreases with altitude.
Differences in pressure cause air to move horizontally. This air in motion is called wind . Winds move from areas of high pressure to areas of low pressure. Pressure differences between two places create pressure gradients and the resulting pressure gradient force causes air to move from high pressure areas to low pressure areas. Land and sea breezes are examples of winds caused by pressure differences that result from temperature differences over land and water surfaces.