Part two of solar irradiance and characteristics
2. Air quality
Due to the presence of the atmosphere, solar radiation energy will be greatly attenuated before reaching the ground. The magnitude of this attenuation is closely related to the length of the solar radiation energy travel through the atmosphere. The longer the distance the sun’s rays travel through the atmosphere, the more energy is lost; the shorter the distance, the less energy is lost. Usually, the distance of the atmosphere through which the light passes when the sun is at the zenith, that is, when it illuminates the ground vertically, is called 1 atmospheric mass. When the sun is in other positions, the air mass is greater than 1, for example, at 8-9 o’clock in the morning, there are about 2 to 3 air masses. The more atmospheric mass, the longer the distance the sun’s rays travel through the atmosphere, the more attenuated, and the less energy that reaches the ground. Therefore, we define atmospheric quality as the ratio of the distance of the sun’s rays through the atmosphere and the distance of the sun’s rays through the atmosphere when the sun is at the zenith. For example, when the value is 1.5, the air quality is called 1.5, usually written as AM1.5, outside the atmosphere, the air quality is 0, usually written as AM0.
3. Atmospheric transparency
On the plane where the upper boundary of the atmosphere is perpendicular to the light, the solar irradiance is basically a constant; but on the surface of the earth, the solar irradiance is constantly changing, which is mainly caused by the difference in the degree of transparency of the atmosphere. （What is solar irradiance？）Atmospheric transparency is a parameter that characterizes the degree to which the atmosphere penetrates the sun’s rays. In clear and cloudless weather, the transparency of the atmosphere is high, and more solar radiant energy reaches the ground. When there are a lot of clouds and fog or windy sand and dust in the sky, the transparency of the atmosphere is very low, and less solar radiation energy reaches the ground. It can be seen that the transparency of the atmosphere is greatly related to the amount of cloud in the sky and the amount of impurities such as dust in the atmosphere.
4. Geographic latitude
The solar radiation energy gradually weakens from low latitudes to high latitudes. Assuming that the atmospheric transparency of high-latitude regions and low-latitude regions are the same, compare them under these conditions, as shown in Figure 2. Take the vernal equinox at noon, when the sun is shining vertically on point F on the earth’s equator, suppose there are two other points B and D on the same longitude. The latitude of point B is higher than the latitude of point D. It can be clearly seen from the figure that the distance of the atmosphere required by sunlight to reach point B is longer than the distance CD of the atmosphere required by sunlight to reach point D, so the vertical radiation flux at point B will be smaller than that at point D. The vertical radiant flux at point F is the largest on the equator, because the sunlight traverses the shortest path EF in the atmosphere. For example, St. Petersburg at high latitudes (60°N latitude) can only get 335kJ of heat per year on an area of 1cm2; while in my country’s capital Beijing, because of its mid-latitude (39°57′N latitude), it can get 586k] In the Sahara area at low latitudes, you can get up to 921kJ of heat. It is for this reason that the equatorial zone has a hot climate throughout the year and green in all seasons, while near the Arctic Circle, it is cold all year round, covered in silver and covered with snow and ice, like two different worlds.
5. Sunshine time
This is also an important factor affecting the solar irradiance on the ground. If there are 14 hours during the day in a certain area, of which 6 hours are cloudy, and the sun shines at 8 hours, then the sunshine time of that day in the area is said to be 8 hours. The longer the sunshine time, the more total solar radiation that the ground receives.
The higher the altitude, the higher the transparency of the atmosphere and the higher the direct solar radiation. In addition, the distance between the sun and the earth, topography, topography, etc., also have a certain impact on solar irradiance. For example, the average temperature of the earth at perihelion is 4℃ higher than that at aphelion. Another example is that at the same latitude, the temperature in the basin is higher than that of Pingchuan, and the sunny slope is hotter than the shaded slope.
In short, there are many factors that affect the solar irradiance on the ground, but the size of the solar irradiance in a specific area is determined by the combination of these factors.