<P> Earth receives energy from the Sun in the form of ultraviolet, visible, and near - infrared radiation . Of the total amount of solar energy available at the top of the atmosphere, about 26% is reflected to space by the atmosphere and clouds and 19% is absorbed by the atmosphere and clouds . Most of the remaining energy is absorbed at the surface of Earth . Because the Earth's surface is colder than the photosphere of the Sun, it radiates at wavelengths that are much longer than the wavelengths that were absorbed . Most of this thermal radiation is absorbed by the atmosphere, thereby warming it . In addition to the absorption of solar and thermal radiation, the atmosphere gains heat by sensible and latent heat fluxes from the surface . The atmosphere radiates energy both upwards and downwards; the part radiated downwards is absorbed by the surface of Earth . This leads to a higher equilibrium temperature than if the atmosphere were absent . </P> <P> An ideal thermally conductive blackbody at the same distance from the Sun as Earth would have a temperature of about 5.3 ° C. However, because Earth reflects about 30% of the incoming sunlight, this idealized planet's effective temperature (the temperature of a blackbody that would emit the same amount of radiation) would be about − 18 ° C. The surface temperature of this hypothetical planet is 33 ° C below Earth's actual surface temperature of approximately 14 ° C . </P> <P> The basic mechanism can be qualified in a number of ways, none of which affect the fundamental process . The atmosphere near the surface is largely opaque to thermal radiation (with important exceptions for "window" bands), and most heat loss from the surface is by sensible heat and latent heat transport . Radiative energy losses become increasingly important higher in the atmosphere, largely because of the decreasing concentration of water vapor, an important greenhouse gas . It is more realistic to think of the greenhouse effect as applying to a "surface" in the mid-troposphere, which is effectively coupled to the surface by a lapse rate . The simple picture also assumes a steady state, but in the real world, there are variations due to the diurnal cycle as well as the seasonal cycle and weather disturbances . Solar heating only applies during daytime . During the night, the atmosphere cools somewhat, but not greatly, because its emissivity is low . Diurnal temperature changes decrease with height in the atmosphere . </P> <P> Within the region where radiative effects are important, the description given by the idealized greenhouse model becomes realistic . Earth's surface, warmed to a temperature around 255 K, radiates long - wavelength, infrared heat in the range of 4--100 μm . At these wavelengths, greenhouse gases that were largely transparent to incoming solar radiation are more absorbent . Each layer of atmosphere with greenhouses gases absorbs some of the heat being radiated upwards from lower layers . It reradiates in all directions, both upwards and downwards; in equilibrium (by definition) the same amount as it has absorbed . This results in more warmth below . Increasing the concentration of the gases increases the amount of absorption and reradiation, and thereby further warms the layers and ultimately the surface below . </P>

Where does the greenhouse effect occur in the atmosphere