Solar radiation refers to the electromagnetic waves and particle streams emitted by the sun into the universe. The solar radiation energy received by the earth is only one-two billionth of the total radiation energy emitted by the sun into space. Changes in solar activity and the distance between the sun and the earth will cause changes in the solar radiation energy of the upper boundary of the earth’s atmosphere.
After the solar radiation passes through the atmosphere, its intensity and spectral energy distribution change. The solar radiation energy reaching the ground is much smaller than the upper boundary of the atmosphere. The energy distribution in the solar spectrum is almost extinct in the ultraviolet spectrum, reducing to 40% in the visible spectrum and increasing to 60% in the infrared spectrum.
In addition to the changes in the sun itself, the differences that affect the solar radiation energy are mainly related to the following factors.
(1) The difference between the solar altitude and azimuth angle. The solar radiation energy increases with the increase of the solar altitude angle. It gradually increases after sunrise, the maximum is at noon, and it begins to decrease again in the afternoon, and it is zero at night. This is actually the reason why the sun’s altitude and azimuth have changed.
(2) The different transparency of the atmosphere increases, and the total radiation increases. The sky is cloudless, not blocked by the sun, and the total radiation is greater than when there are clouds.
(3) Seasons and latitudes are different. Seasons are caused by the angle of 23°27′ between the rotation axis of the earth and the rotation axis of the earth’s orbit around the sun. The earth rotates once a day from west to east around the “earth axis” that passes through its own south pole and north pole. Each revolution is a day and night, so the earth rotates 15° per hour. In addition to its rotation, the earth also revolves around the sun once a year in an elliptical orbit with a small eccentricity. The normal of the earth’s rotation axis and the orbital plane is always 23.5°. When the earth revolves, the direction of the axis of rotation remains the same, always pointing to the north pole of the earth. Therefore, when the earth is in different positions of the orbit, the direction of the sun’s light projected on the earth is also different, thus forming the four seasons on the earth.
The amount of solar radiation obtained in different latitudes is different. For example, in the tropics, the sun can get the most heat during the year; the cold sun is very low and there are long polar nights, so it gets the least heat. In the upper boundary of the earth’s atmosphere, the total amount of daily radiation in the northern hemisphere is the largest during the summer solstice, and the distribution from the pole to the equator is relatively even; during the winter solstice, the total amount of daily radiation in the northern hemisphere is the smallest, within the polar circle is zero, and the difference between north and south is the largest. The situation in the southern hemisphere is the opposite. At the vernal and autumnal equinox, the distribution of total daily radiation is proportional to the cosine of latitude. In the area between the Tropic of Cancer and the Tropic of Cancer, the total daily radiation is the largest twice in a year, with small annual changes. The maximum annual change occurs in summer and the minimum occurs in winter. The higher the latitude, the greater the change in total daily radiation. The total amount of solar radiation decreases with the increase of latitude, and increases with the increase of altitude.
(4) The orbit of the sun and the earth at different distances from the earth around the sun is elliptical, and the sun is located at one of the two focal points. Therefore, the distance between the sun and the earth is changing all the time. Every year, it passes through the perihelion on January 2 to 5, and passes through the aphelion on July 3 to 4. The intensity of solar radiation received on the earth is inversely proportional to the square of the distance between the sun and the earth.
(5) The difference between day and night is the largest around noon during the day, and zero at night; the summer is big and winter is small in one year.
(6) The thickness of the atmosphere is different. The solar radiation reaching the ground is mainly affected by the thickness of the atmosphere. The thicker the atmosphere, the more severe the absorption, reflection and scattering of solar radiation, and the less solar radiation reaching the ground.