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 the Moon and potassium, which are not found in the atmospheres of Earth, Mars, or Venus. At sea level on Earth, each cubic centimeter of the atmosphere contains approximately molecules; by comparison the lunar atmosphere contains fewer than molecules in the same volume. On Earth, this is considered to be a very good vacuum. In fact, the density of the atmosphere at the Moon's surface is comparable to the density of the outermost fringes of Earth's atmosphere, where the International Space Station orbits. The elements sodium and potassium have been detected in the Moon's atmosphere using Earth-based spectroscopic methods, whereas the isotopes