Air pressure

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Air pressure is the pressure we experience due to the weight of the atmosphere. Since air is a fluid, this pressure takes the form of a force exerted uniformly, in all directions, against everything. The typical air pressure at sea level is one atmosphere (1 atm, 760 millimeters of mercury, or 14.7 pounds per square inch). Since the pressure is the result of all of the air above the observer, as one climbs higher, the fraction of the atmosphere that is above one decreases, so the air pressure decreases. For example, the air pressure on top of Mount Everest is less than the air pressure on top of Denali, and 67% less than at sea level.

Air pressure is used by modern industry, in the form of pneumatic controls. Hydraulic controls are similar to pneumatic ones, but use fluid pressure instead of air pressure, and are more precise due to the effectively incompressible quality of the fluids used.

There are a number of incredibly simple ways to demonstrate the pressure of air or other gases. One is to blow up a balloon. The pressure inside the balloon is greater than the outside air pressure, so it exerts an outward force, making the balloon expand. Other experiments, some safe and others less so, can involve mixing baking soda with a mild acid such as lemon juice or vinegar. But the simplest experiment, often done as a prank, is to shake a bottle or can of carbonated beverage. This causes the carbon dioxide to go out of solution.

Biblical literalist view

Some Biblical literalists believe that

... before the Flood, there was a "firmament", or vapor canopy, separating the waters of the heavens from the waters of the earth. Because all that water was pressing down on the Earth's atmosphere, the air pressure in Biblical times would have been several times greater than it is today. Some creation scientists credit the longevity of the patriarchs to the greatly increased air pressure — much as an astronaut living in a pressurized space station ages more slowly than his twin at home.
There are a few problems with this: There is no scientifically plausible mechanism that would cause the much denser water to lie above the air. There is no evidence suggesting that greater air pressure increases longevity. The pressure inside a spacecraft is not higher than normal. Also, the phrase "ages more slowly than his twin at home" suggests a confusion with the "Twin Paradox" of relativity. The relativistic age difference is about 10 milliseconds per month.