Air near the equator is heated more than air near the poles. Heated air expands. Thus warm air at the equator is less dense than cold air at the poles. The density of the air determines the force with which it presses down on the earth's surface. This force is measured as air pressure.
Cold air presses down on the earth with a greater pressure than warm air.
Cold air is said to have a high pressure. Warm air is said to have a low pressure. Key Questions 1. Describe what happens to the temperature at different times of the day. Vocabulary atmosphere, troposphere, stratosphere, mesosphere, ionosphere, temperature, molecule, ozone Procedure 1.