4. Atmospheric Circulation

Varying amount of insolation received by the earth causes differential heating of the earth and its atmosphere. Temperature difference thus produced account for the density differences in the air. Air expands when heated and gets compressed when cooled. This results in variations in the atmospheric pressure. The result is that it causes the movement of air from high pressure to low pressure, setting the air in three-dimensional motion on global scale. Air in horizontal motion is wind. Atmospheric pressure also determines when the air will rise or sink. The wind redistributes the heat and moisture across the planet, thereby, maintaining a constant temperature for the planet as a whole. The vertical rising of moist air cools it down to form the clouds and bring precipitation. There is, in fact, an intimate relationship between winds and pressure, and knowledge of pressure variations is a prerequisite to understanding air motion.