Air pressure, also known as atmospheric pressure, refers to the weight of the Earth's atmosphere pressing down on everything at the surface. The average air pressure at sea level is 101.325 kPa (1013.25 millibar), or 14.696 lbs/sq² (1.0333 kg/cm²). Basically, this means hundreds of pounds of pressure are pressing at you from all sides and at all times. We are able to survive this pressure because our bodies (and all life) evolved in an environment where it is omnipresent. If the pressure is increased or decreased too much, we perish.

With pressure, as with a lot of things, it's all relative. Air pressure is why high-pressure machinery, such as hydraulics, require high structural integrity to contain the material — pressure has a tendency to equalize. It is also partially responsible for the phrase "nature abhors a vacuum," meaning a vacuum is hard to sustain under the pressure of Earth's atmosphere.

Atmospheric pressure varies slightly over the Earth's surface, and variations in pressure are responsible for the weather that we are so familiar with. Low pressure are associated with storms, tornadoes, and hurricanes. Sometimes the air pressure at sea level can drop as low as 870 millibars, 85% of average air pressure. This only happens during the most severe storms. Pressure variations on the Earth surface cause wind: high pressure areas have a tendency to seek out low pressure areas, causing gusts.

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. At 16 km (10 mi), slightly higher than the cruising altitude of a typical jet liner, pressure is only 1/10th what it is at air level. Because low air pressure can be very unpleasant for humans (due to an absence of oxygen), all areas of aircraft that contain passengers are artificially pressurized. At 31 km (19 mi) 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 (62 mi), the international definition for outer space, the air pressure approaches zero.