Adopt-a-City in the USA What is Air Pressure?

How do we know air exists? We can feel air on a breezy day. We can see the affect air has on a flag. Think of all ways we can use air. People use air to sail, breathe, operate power tools, wind surf, or generate electrical power.

Air is matter. It has mass (weight) and takes up space. Here is a demonstration you can do at home or school to show that air has weight.

Air is composed of different gases: nitrogen, oxygen, carbon dioxide, water vapor, and other gases. Air molecules are in constant motion. As they move, they come in contact with surfaces. Air molecules push and press on the surfaces they contact.

Have you ever bounced a basketball? The air molecules push against the surfaces they contact, in this case the inside walls of the ball. When the air molecules bounce around inside the basketball they push hard enough on the walls of the round ball to keep it's shape, even after it hits the floor when you dribble. It bounces right back up to its spherical shape. The leather or rubber that the ball is made of is hard enough to contain the air. We say there is air pressure inside the ball.

The amount of force per unit area that air molecules exert on a surface is called air pressure. Air pressure is caused by all of the air molecules in the Earth's atmosphere pressing down on the Earth's surfaces.

Air pressure can change with elevation and temperature. Changes in air pressure influence and allow us to predict weather conditions. At sea level, air pressure is an average 14.7 lbs per square inch. The same pressure in metric units is 1013 millibars (1013 dyne/cm2), or 1033 grams/cm2. Weather maps often show pressure as the reading of a mercury barometer, in units of mercury column height. The average reading is 29.92 inches of mercury equivalent to 760 millimeters.

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