After the solar radiation passes through the atmosphere, its intensity and spectral energy distribution change. The energy of solar radiation reaching the ground is much smaller than that of the upper atmosphere. On the solar spectrum, the energy distribution is almost extinct in the ultraviolet spectrum region, reduced to 40% in the visible spectrum region, and increased to 60% in the infrared spectrum region.
In the upper boundary of the Earth's atmosphere, the total daily radiation is the largest in the Northern Hemisphere during the summer solstice, and the distribution from the polar to the equator is relatively uniform; during the winter solstice, the total daily radiation in the northern hemisphere is the smallest, with zero in the polar circle, and the largest difference between the north and the south. The situation is reversed in the southern hemisphere. During the spring equinox and the autumn equinox, the distribution of total daily radiation is proportional to the cosine of latitude. The area between the Tropic of Cancer and the Tropic of Capricorn has the largest total daily radiation twice a year, with small annual changes. The higher the latitude, the greater the change in total daily radiation.
The distribution of global annual total radiation reaching the surface is basically band-shaped, and is only destroyed in low latitudes. In the equatorial region, due to cloudy conditions, the total annual radiation is not the highest. In the subtropical high belt of the northern and southern hemispheres, especially in the desert regions of the mainland, the total annual radiation is relatively large, and the maximum is in northeastern Africa.
