Earth's atmosphere consists of gases that surround the surface of the planet. Like any gas, which is made up of molecules that are constantly in motion, the atmosphere exerts a force or pressure on everything within it. This force, divided by the area over which it acts, is the atmospheric pressure. The atmospheric pressure at sea level—considered the mean atmospheric pressure—has an average value of 14.7 pounds per square inch or 29.92 inches of mercury (as measured by a barometer). This means that a one-inch-square column of air stretching from sea level to about 120 miles (200 kilometers) into the atmosphere would weigh 14.7 pounds.
Atmospheric pressure decreases with increasing altitude. The reason for this change with altitude is that atmospheric pressure at any point is a measure of the weight, per unit area, of the atmosphere above that point. Higher altitudes have a lower atmospheric pressure because there is less atmosphere weighing down from above. At an altitude of about 3.1 miles (5 kilometers), the atmospheric pressure is half of its value at sea level.
Atmospheric pressure is closely related to weather. Regions of pressure that are slightly higher or slightly lower than the mean atmospheric pressure develop as air circulates around Earth. The air rushes from regions of high pressure to low pressure, causing winds. The properties of the moving air (cool or warm, dry or humid) will determine the weather for the areas through which it passes. Knowing the location of high and low pressure areas is vital to weather forecasting, which is why they are shown on the weather maps printed in newspapers and shown on television.