The atmosphere that surrounds the earth can be considered a reservoir of low-pressure air. Its weight exerts a pressure that varies with temperature, humidity, and altitude. For thousands of years, air was considered weightless. This is understandable, since the net atmospheric pressure exerted on us is zero. The air in our lungs and the blood in our cardiovascular system has an outward pressure equal to (or perhaps slightly greater than) the inward pressure of the outside air. Since we feel no pressure, we are unaware of the air's weight. The weight of the earth's atmosphere pressing on each unit of surface constitutes atmospheric pressure, which is 14.7 psi (1101,300 Pa or 0.1013 MPa) at sea level. This pressure is called one atmosphere.
