Atmosphere of Earth surface. The refractive index of air depends on temperature, giving rise to refraction effects when the temperature gradient is large. An example of such effects is the mirage. "Atmospheric circulation" is the large-scale movement of air through the troposphere, and the means (with ocean circulation) by which heat is distributed around Earth. The large-scale structure of the atmospheric circulation varies from year to year, but the basic structure remains fairly constant because it is determined by Earth's rotation rate and the difference in solar radiation between the equator and poles. The first atmosphere consisted of gases in the solar nebula,
Atmosphere of Earth 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. If the entire mass of the atmosphere had a uniform density from sea level, it would terminate abruptly at an altitude of . It actually decreases exponentially with altitude, dropping by half every or by a factor of 1/e every , the average scale height of the atmosphere below . However, the atmosphere is