<P> The average atmospheric pressure at sea level is defined by the International Standard Atmosphere as 101325 pascals (760.00 Torr; 14.6959 psi; 760.00 mmHg). This is sometimes referred to as a unit of standard atmospheres (atm). Total atmospheric mass is 5.1480 × 10 kg (1.135 × 10 lb), about 2.5% less than would be inferred from the average sea level pressure and Earth's area of 51007.2 megahectares, this portion being displaced by Earth's mountainous terrain . Atmospheric pressure is the total weight of the air above unit area at the point where the pressure is measured . Thus air pressure varies with location and weather . </P> <P> If the entire mass of the atmosphere had a uniform density from sea level, it would terminate abruptly at an altitude of 8.50 km (27,900 ft). It actually decreases exponentially with altitude, dropping by half every 5.6 km (18,000 ft) or by a factor of 1 / e every 7.64 km (25,100 ft), the average scale height of the atmosphere below 70 km (43 mi; 230,000 ft). However, the atmosphere is more accurately modeled with a customized equation for each layer that takes gradients of temperature, molecular composition, solar radiation and gravity into account . </P> <P> In summary, the mass of Earth's atmosphere is distributed approximately as follows: </P> <Ul> <Li> 50% is below 5.6 km (18,000 ft). </Li> <Li> 90% is below 16 km (52,000 ft). </Li> <Li> 99.99997% is below 100 km (62 mi; 330,000 ft), the Kármán line . By international convention, this marks the beginning of space where human travelers are considered astronauts . </Li> </Ul>

Gases found in each layer of the atmosphere