What Is Air Pressure?
You can think of our atmosphere as a large ocean of air surrounding the Earth. The air that composes the atmosphere is made of many different gases. Nitrogen accounts for as much as 78 percent of the volume, while oxygen accounts for 21 percent. The remaining one percent is composed of such gases as argon, carbon dioxide, helium and hydrogen. Typically, the weather of the Earth is caused by processes that occur within the lowest 20 km of the atmosphere. This includes such phenomena as fog, wind, rain, storms, snow, tornadoes and clouds.
Air and consequently, the atmosphere, do have weight. This weight decreases as you go up within the atmosphere. When gravity acts on the air, the air exerts a force upon the Earth called pressure. The typical pressure at sea level is 1013.25 millibars or 14.7 pounds per square inch. A millibar is a unit that is used to report the atmospheric pressure. Do you think that air pressure varies with respect to air temperature? Take a few moments to think about this.
NOAA National Oceanic and Atmospheric Administration