What Is Atmospheric Pressure?

Air is held to the earth by gravity. This strong invisible force pulls the air downward, giving air molecules weight. The weight of the air molecules exerts a force upon the earth and everything on it. The amount of force exerted on a unit surface area (a surface that is one unit in length and one unit in width) is called atmospheric pressure or air pressure. The air pressure at any level in the atmosphere can be expressed as the total weight of air above a unit surface area at that level in the atmosphere. Higher in the atmosphere, there are fewer air molecules pressing down from above. Consequently, air pressure always decreases with increasing height above the ground. Because air can be compressed, the density of the air (the mass of the air molecules in a given volume) normally is greatest at the ground and decreases at higher altitudes.

The air surrounding earth is called the atmosphere. Air has weight and therefore exerts pressure not only on the earth's surface but on all objects on the earth. In fact, human beings and other animals are living at the bottom of an ocean of air which exerts enormous pressure. This pressure is not felt because the blood exerts a slightly higher pressure from inside. At high altitudes where atmospheric pressure is less nose bleeding may occur due to the greater pressure of blood.

It is due to the pressure of the atmosphere that ink rises in the tube of a fountain pen, or liquid rises in a syringe when the piston is pulled.

In aircraft flying at high altitude, normal atmospheric pressure is maintained by the use of air pumps. If this were not done, the crew and passengers would experience difficulty in breathing and consequently face dangers.