There are three forms of heat transmission: conduction, convection and radiation. The sun mainly spreads its heat and particles to the vast universe in the form of radiation. This process of propagation is called solar radiation. Solar radiation is not only the fundamental way for the earth to obtain heat, but also the most important factor affecting the survival activities of human beings and all other living things and the earth’s climate change.

Solar radiation can be divided into two types. One is the light radiation emitted from the surface of the photosphere, which spreads light and heat in the form of electromagnetic waves, which is also called electromagnetic wave radiation. Solar radiation consists of visible light and invisible light that is invisible to the human eye. The other is particle radiation, which is a stream of particles composed of positively charged protons and roughly equal amounts of negatively charged electrons and other particles. Particle radiation is usually weak and its energy is unstable. It does not send any heat to the earth. It is strongest during the solar maximum period and has a certain impact on humans and the upper atmosphere of the earth. Therefore, the solar radiation introduced below mainly refers to optical radiation.

Solar radiation has to travel a long distance to reach the earth, and it has to encounter various effects. The layer of air within 10~12km from the ground is called the troposphere. The layer of atmosphere from above the troposphere to within 50km is called the stratosphere. The layer of atmosphere from above the stratosphere to about 950km is called the ionosphere. When the sun radiates its light, heat and particle flow to the earth from a distance of 1.5×10⁸km at a speed of 3×10⁵km/s, it will be interfered and blocked by the earth’s atmosphere and cannot be projected onto the earth’s surface unimpeded.

Large particles such as water molecules, small water droplets and dust in the atmosphere reflect solar radiation. Their reflectivity accounts for about 7% of the average solar constant; the reflectivity of clouds is related to cloud volume, cloud shape and cloud thickness. The reflectivity of 3000m thick altocumulus cloud can reach 72%, and the reflectivity of cumulus cloud layer is 52%. According to calculations, based on the average cloud cover of the earth being 54%, about 1/4 of the solar radiation energy is reflected by the clouds and returned to the outer space.

When solar radiation hits the earth’s atmosphere with parallel beams, it will encounter the blocking of particles such as air molecules, dust, clouds and fog to produce scattering effects. This kind of scattering is different from absorption. It does not convert solar radiation energy into the internal energy of each particle, but can only change the direction of solar radiation, so that solar radiation transmits energy in all directions on the particle, so that part of the solar radiation becomes The back radiation of the atmosphere shoots out of the earth’s atmosphere and cannot reach the surface of the earth. This is an important reason for the weakening of solar radiation energy.

Due to the existence and influence of the atmosphere, the solar radiation energy reaching the surface of the earth can be divided into two parts, one is called direct radiation, the other is scattered radiation, and the sum of these two parts is called total radiation. The part of the sun’s rays that hit the ground is called direct radiation. It is not directly projected on the ground, but the part of the sun’s rays that reach the ground through the scattering of the atmosphere, clouds, fog, water droplets, dust and other objects in different directions, called scattered radiation. The energy difference of these two kinds of radiation is very big. Generally speaking, direct radiation in the daytime on a clear day accounts for most of the total radiation, scattered radiation on a cloudy and rainy day accounts for most of the total radiation, and at night it is completely scattered radiation. The use of solar energy is actually the use of the total radiation of the sun. For most solar energy equipment, the direct radiation part of solar radiant energy is mainly used.

The total radiant energy emitted by the sun is about 3.75×10²⁶W, which is extremely huge. But only one part of 2.2 billion reaches the earth. The total radiant energy of the sun reaching the earth is about 1.73×10¹⁴kW, which is equivalent to more than 35,000 times the total energy produced by various energy sources consumed in the world in one year. Of the solar radiation energy received by the land surface, only 0.015% is absorbed by plants, and only 0.002% is used by people as fuel and food. It can be seen that the potential of using solar energy is quite large. The various situations of solar radiation to the earth are shown in Figure 1.

Read more: The huge energy of the sun