Air pressure basically refers to the volume of air in a particular environment, with greater volumes creating higher pressures. On the earth's surface, for example, it is known as "atmospheric pressure" and refers to the weight of the earth's atmosphere pressing down on everything. Changes in pressure can impact the temperature, weather patterns, and cause physiological problems for people and animals. This pressure can even impact the performance of a basketball or similarly inflated object.
On the earth, the average air pressure at sea level is 1.03 kilograms per square centimeter (kg/cm2) or 14.7 pounds per square inch (psi); this is commonly measured in bars, in which atmospheric pressure is about 1 bar. This means that hundreds of pounds of pressure are pressing on everyone from all sides, at all times. Humans and other animals are able to survive this pressure because their bodies evolved on the surface where it is natural. If the pressure increases or decreases, it can result in discomfort or even death.
Changes in Pressure and Weather
Atmospheric pressure varies slightly over the earth's surface, and variations in pressure are responsible for various types of weather. Low pressure systems are associated with storms, tornadoes, and hurricanes. Sometimes the air pressure at sea level can drop as low as 870 millibars, which is about 85% of average air pressure. This only happens during the most severe storms. Pressure variations on the earth's surface cause wind: as high pressure air moves toward low pressure areas, creating gusts.
Various Pressures at Different Altitudes
On the top of Mt. Everest, the tallest mountain on earth, the air pressure is just about a third of what it is at sea level. Humans at high altitudes often experience discomfort, such as ear popping, due to differences in their internal and external pressures. At 16 kilometers (km) or almost 10 miles above the surface, slightly higher than the cruising altitude of a typical jet liner, pressure is only 1/10th what it is at sea level. Because low air pressure can be very unpleasant for humans, due to low oxygen content, all areas of aircraft that contain passengers are artificially pressurized. In the event of a rupture in an airplane's fuselage, unsecured items may be "sucked" out of the craft as the high pressure air within it rushes out into the low pressure environment outside.
Higher Altitudes and Outer Space
At 31 km or about 19 miles above the earth's surface, in the stratosphere, the air pressure is only 1/100th what it is at sea level. From this level on, the atmosphere quickly deteriorates into nothingness. Above 100 km or just over 62 miles above the surface, the international definition for outer space, the pressure approaches zero and nearly becomes a vacuum. Humans cannot exist unprotected in such a low-pressure environment.
Other Uses for Pressure
Air pressure is used in a number of commercial applications and consumer products. Pressurized air can be utilized in hydraulic machinery that uses the expansion of air to move different parts. Aerosol canisters use pressurized air to make different chemicals spray out, as the high pressure naturally rushes out into the lower atmospheric pressure when used. Basketballs and similar objects, like tires, are inflated so that they bounce or support other objects. Under-inflation can leave them weakened or less effective, while excess pressure can cause them to burst or more easily rupture during use.
Beneath sea level on the earth, pressure becomes increasingly greater. Under the ocean, for example, high pressure in the water can crush a person or object that is designed to withstand the atmospheric pressure of the planet. Special suits and vehicles are needed to withstand such high pressures.