The earth's atmosphere exerts a force on everything within it. This force, divided by the area over which it acts, is the atmospheric pressure. The atmospheric pressure at sea level has an average value of 1,013.25 millibars. Expressed with other units, this pressure is 14.7 lb per square inch, 29.92 inches of mercury, or 1.01 × 105 pascals. Atmospheric pressure decreases with increasing altitude: it is half of the sea level value at an altitude of about 3.1 mi (5 km) and falls to only 20% of the surface pressure at the cruising altitude of a jetliner. Atmospheric pressure also changes slightly from day to day as weather systems move through the atmosphere.
The earth's atmosphere consists of gases that surround the surface, and like any gas, the atmosphere exerts a pressure on everything within it. A gas is made up of molecules that are constantly in motion. If the gas is in a container, some gas molecules are always bouncing off the container walls. When they do so, they exert a tiny force on the walls. With a sufficient number of molecules, their impacts add up to make a force that can easily be measured. Dividing the total force by the area over which it is measured gives the gas pressure. Anything else the gas touches will also have this pressure exerted on it. Thus anywhere we go within the earth's atmosphere we can detect atmospheric pressure.
Atmospheric pressure decreases as one climbs higher in the atmosphere, and increases the closer one gets to the earth's surface. The reason for this change with altitude is that atmospheric pressure at any point is really a measure of the weight, per unit area, of the atmosphere above that point. At sea level, for example, the pressure is 14.7 pounds per square inch. This means that a slice of the atmosphere in the shape of a long, thin column, with a one square inch base and as tall as the top of the atmosphere (at least 120 mi or 200 km), would have air within the column weighing 14.7 lb (6.7 kg). At a higher elevation, such as the top of a 10,000 ft (3,048 m) mountain, one is above some of the atmosphere. Here the atmospheric pressure is lower than at sea level, because there is less air weighing down from above. A person feels this sort of pressure effect when they dive to the bottom of a lake or deep swimming pool. As the diver descends deeper into the water, more and more water lies overhead. The extra water exerts an increasing pressure that the diver can feel on his or her skin (and especially on the eardrums).
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 the 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.
Atmospheric pressure is measured by a barometer, of which there are several designs. The first barometer was made by Evangelista Torricelli in 1643, using a column closed at one end and partially filled with mercury. The column was placed vertically in a small pool of mercury with the open end downward. In this arrangement, the mercury does not run out the open end. Rather, it stays at a height such that the pressure exerted by the suspended mercury upon the pool will equal the atmospheric pressure on the pool. The mercury barometer is still in common use today (this is the reason pressure is still given the units "inches of mercury" on weather reports). Modern barometers include the aneroid barometer, which substitutes a sealed container of air for the mercury column, and the electronic capacitance manometer, which senses pressure electronically.
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