A unit of barometric pressure that is taken to be the standard pressure of the Earth's atmosphere at sea level: The average atmospheric pressure at sea level is approximately 14.7 pounds per square inch.

With an increasing altitude, the atmospheric pressure decreases. For example, at 30,000 feet, approximately the height of Mt. Everest, the air pressure is 4.3 pounds per square inch.

The atmospheric pressure at any point in an atmosphere is due solely to the weight of the atmospheric gases above the point concerned.