The incident radiant energy emitted by the Sun, which reaches a unit horizontal area of the Earth's surface. The term is a contraction of incoming solar radiation. About 99.9% of the Sun's energy is in the spectral range of 0.15– 4.0 micrometers. About 95% of this energy is in the range of 0.3–2.4 μm; 1.2% is below 0.3 μm and 3.6% is above 2.4 μm. The bulk of the insolation (99%) is in the spectral region of 0.25–4.0 μm. About 40% is found in the visible region of 0.4–0.7 μm and only 10% is in wavelengths shorter than the visible. Energy of wavelengths shorter than 0.29 μm is absorbed high in the atmosphere by nitrogen, oxygen, and ozone.

Insolation depends on several factors: (1) the solar constant—that is, the amount of energy that in a unit time reaches a unit plane surface perpendicular to the Sun's rays outside the Earth's atmosphere, when the Earth is at its mean distance from the Sun; (2) the Sun's elevation in the sky; (3) the amount of solar radiation returned to space at the Earth-atmosphere boundary; and (4) the amount of solar radiation absorbed by the atmosphere and the amount of solar radiation reflected at the lower boundary of the Earth. Insolation is commonly expressed in units of watts per square meter, or calories per square centimeter per minute, also known as langley/min. For instance, the mean value of the solar constant has been estimated as 1368 W/m² (∼1.96 ly/min), and the average insolation in summer for a midlatitude clear region could be 340 W/m² (700 ly/day), while for a cloudy region it is only about 120 W/m² (250 ly/day).Atmosphere Terrestrial radiation