The solar constant value published by the World Meteorological Organization (WMO) in 1981 is 1368 watts/m2. More than 99% of the solar radiation spectrum in the upper boundary of the Earth's atmosphere is between 0.15 and 4.0 microns in wavelength. About 50% of the solar radiation energy is in the visible spectrum (wavelength 0.4 to 0.76 microns), 7% is in the ultraviolet spectrum (wavelength <0.4 microns), 43% is in the infrared spectrum (wavelength >0.76 microns), and the maximum energy is at a wavelength of 0.475 Micron. Because the wavelength of solar radiation is much smaller than the wavelength of ground and atmospheric radiation (about 3 to 120 microns), it is usually also called solar radiation as short-wave radiation, and ground and atmospheric radiation as long-wave radiation. Changes in solar activity and the distance between the sun and the earth will cause changes in the solar radiation energy in the upper boundary of the Earth's atmosphere.
Solar radiation passes through the atmosphere, and part of it reaches the ground, called direct solar radiation; the other part is absorption, scattering, and reflection of atmospheric molecules, fine dust, and water vapor in the atmosphere. Part of the scattered solar radiation returns to cosmic space, and the other part reaches the ground. The part that reaches the ground is called scattered solar radiation. The sum of scattered solar radiation and direct solar radiation reaching the ground is called total radiation.
