Flexi Says: Because air is a gas, its molecules have a lot of energy. Air molecules move a lot and bump into things. For this reason, they exert pressure. Air pressure is defined as the weight of the air pressing against a given area. At sea level, the atmosphere presses down with a force of about 1 kilogram per square centimeter (14.76 pounds per square inch). The terms "high pressure" and "low pressure" describe relative differences in air pressure. These differences cause air to move from areas of high pressure to areas of low pressure.