Even though you can not see air pressure it is always there and has real weight. It is made up primarily of oxygen and nitrogen molecules. Air is made up of 78 percent nitrogen and 21 percent oxygen and less than 1 percent other gases in t he lowest part of the atmosphere where we live. At higher parts of the atmosphere the make up of gases is slightly different due to a number of factors.

Air molecules are constantly moving. They speed up with increases in temperature and slow down when cooled off. The speed at which they move is very fast. Nearest to the Earth’s surface air gas molecules move at an astonishing speed of approximately 1,090 miles per hour. When the air is warmed up they move faster and slow down when the air is cooled.

The impact of the moving molecules is what actually causes air pressure. Air pressure depends on how many molecules are in a given space and how fast they are moving. Heating the air increases its pressure and cooling the air decreases its pressure. Putting air into less space will increase its pressure and allowing air to expand into a larger space will decrease pressure.

Gravity has an impact on air pressure. Pressure is greater at lower levels of elevation because the weight of air above is pushing down on it. Pressure at higher elevations is lower because less weight is pushing down.

At sea level the average air pressure is 14.7 pounds per square inch. At 1,000 feet of elevation the pressure is decreased to 14.1 pounds per square inch. All the way up at 18,000 feet of elevation the pressure is approximately 7.3 feet per square inch. That there shows how air pressure decreases with elevation change due to less weight pushing down. Higher elevations often times have different cooking times when preparing foods due to this.

References were taken from The Weather Book by Jack Williams.