1. The first 40 to 50 mi (64–80 km) above the earth contains 99% of the total mass of the earth's atmosphere and is generally of a uniform composition, except for a high concentration of ozone, known as the ozone layer, at 12–30 mi (19–50 km).
2. Because of the pull of gravity the density of the atmosphere and the pressure exerted by air molecules are greatest near the earth's surface (about 1 gram per 103 cc and about 106 dynes per sq cm, respectively). Air pressure decreases quickly with altitude, reaching one half of its sea-level value at about 18,000 ft (5,500 m).
3. The density of air at sea level is about 1.2 kg/m³(1.2 g/L). The atmospheric density decreases as the altitude increases. This variation can be approximately modeled using the barometric formula. More sophisticated models are used by meteorologists and space agencies to predict weather and orbital decay of satellites.
4. There is no definite boundary between the atmosphere and outer space. It slowly becomes thinner and fades into space. The Kármán line, at 100 km (62 miles or 328,000 ft), is also frequently regarded as the boundary between atmosphere and outer space.
5. The temperature of the Earth's atmosphere varies with altitude;The average temperature of the atmosphere at the surface of Earth is 15 °C (59 °F)
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