1.1 Direct solar radiation, scattered radiation and total radiation
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.
Solar radiation passes through the atmosphere, and part of it reaches the ground, which is called direct solar radiation; the other part is the absorption, scattering and reflection of atmospheric molecules, dust and vapor in the atmosphere. A part of the scattered solar radiation returns to the universe; the other part reaches the ground, and 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.
1.1.1 Direct radiation
The change of direct radiation is related to the solar altitude angle, atmospheric transparency coefficient, altitude and latitude.
①The increase of the solar altitude angle increases the direct radiation; the decrease of the solar altitude angle decreases the direct radiation
②The atmospheric transparency coefficient increases, and the direct radiation increases; the atmospheric transparency coefficient decreases, and the direct radiation decreases
③When the altitude increases, the direct radiation increases; when the altitude decreases, the direct radiation weakens.
④ At high latitudes, direct radiation increases; at low latitudes, direct radiation weakens.
1.1.2 Scattered radiation
When solar radiation encounters air molecules or tiny particles in the atmosphere, when the diameter of these particles is smaller than the wavelength of the electromagnetic waves that make up solar radiation, part of the energy in the solar radiation will propagate from the particle in all directions in the form of electromagnetic waves. The phenomenon is called scattering. The energy propagated through the scattered form is called scattered radiation. Scattering only changes the direction of radiation, so that part of the solar radiation cannot reach the ground. In the scattering process, the tiny particles do not absorb solar radiation and increase their own internal energy. Theory and practice show that the distribution of scattered wave energy is related to the incident wavelength, intensity and particle size, shape and refractive index. For example, the shorter the radiation wavelength, the stronger the scattering. Among visible light, violet light and blue light have the shortest wavelengths, so the scattering is the strongest. Usually, the blue sky seen in sunny weather is actually the result of the intense scattering of purple and blue light by tiny particles in the atmosphere. It is precisely because of the scattering effect of the atmosphere on the solar radiation that the sky becomes bright and blue, otherwise it will be pitch black. Scattering is only applicable to tiny particles. If the diameter of the particles in the atmosphere is close to or exceeds the wavelength of the radiant light, diffusion will occur. When the incident light is white light, it will still be white light after diffusion. This is why the sky is often grayish white in fog or when the air is turbid.
Assuming that half of the scattered radiation returns to space, the other half does not consider atmospheric absorption. The intensity of the scattered radiation reaching the ground mainly depends on the following factors.
①The increase of the solar altitude angle increases the scattered radiation; the decrease of the solar altitude angle decreases the scattered radiation
②The transparency of the atmosphere increases, and the scattered radiation decreases; the transparency of the atmosphere decreases, and the scattered radiation increases
③Half the sky has clouds and half is cloudless, and the scattered radiation reaches the maximum.
④High altitude, enhanced scattered radiation; low altitude, weakened scattered radiation
1.1.3 Total solar radiation
Solar radiation is greatly weakened when it passes through the atmosphere. The main reason is the absorption, scattering and reflection of solar radiation by the atmosphere. When solar radiation passes through the atmosphere, almost all ultraviolet rays below 0.29um are absorbed, and there is very little atmospheric absorption in the visible light region. There is a strong absorption band in the infrared region. The main substances in the atmosphere that absorb solar radiation are oxygen, ozone, steam and liquid water, followed by carbon dioxide, methane, nitrous oxide and dust. Clouds can strongly absorb, reflect and scatter solar radiation, while also strongly absorbing solar radiation reflected from the ground.
The sum of direct solar radiation and scattered radiation that reaches the ground after the weakening of the atmosphere is called total solar radiation. On a global average, the total solar radiation only accounts for 45% of the solar radiation reaching the upper boundary of the atmosphere. The total radiation amount decreases with the increase of latitude, and increases with the increase of altitude. It is the largest around noon in a day and zero at night; summer is big and winter is small during the year.