The main factors affecting ground solar irradiance

1. Sun altitude

That is, the altitude angle at which the sun is above the horizon. It is often represented by the angle between the sun’s rays and the horizon, that is, the incident angle θ. When the angle of incidence is large, the sun is high, and the irradiance is also large; conversely, the angle of incidence is small, the sun is low, and the irradiance is also small.

Since the earth’s atmosphere absorbs, reflects and scatters solar radiation, the proportion of infrared, visible and ultraviolet rays in the light rays also changes with the height of the sun. When the height of the sun is 90°, in the solar spectrum, infrared light accounts for 50%, visible light accounts for 46%, and ultraviolet light accounts for 4%; when the height of the sun is 30°, infrared light accounts for 53%, visible light accounts for 44%, and ultraviolet light accounts for 3%; when the sun height is 5°, infrared light accounts for 72%, visible light accounts for 28%, and ultraviolet light accounts for nearly 0.

The altitude of the sun is constantly changing throughout the day. It is the lowest at sunrise in the morning, at 0°; it gradually increases after that, and reaches the highest at noon, at 90°; in the afternoon, it gradually decreases, and at sunset, it decreases to 0°. The altitude of the sun is also constantly changing throughout the year. This is because the earth is not only spinning, but also revolving around the sun. The Earth’s rotation axis is not perpendicular to the orbital plane, but always maintains a certain inclination. The angle between the axis of rotation and the plane normal of the orbital orbit is 23.5°. In the first half of the year, the sun rises day by day from low latitude to high latitude, until noon on the summer solstice, reaching the highest point of 90°, and thereafter, it decreases day by day until the winter solstice, when it reaches the lowest point. This is why the summer is hot, the winter is cold, and the day is warmer at noon than in the morning and evening.

For a certain ground plane, the energy is attenuated more because the light travels longer through the atmosphere when the sun is low. At the same time, because the light is projected onto the ground plane at a smaller angle, less energy reaches the ground plane; otherwise, more energy is reached.

2. Air quality

Due to the presence of the atmosphere, solar radiation will be greatly attenuated before reaching the ground. The magnitude of this attenuation is closely related to the length of the solar radiation energy passing through the atmosphere. The longer the sun’s rays travel in the atmosphere, the more energy is lost; the shorter the distance, the less energy is lost. Usually, when the sun is at the zenith, that is, vertically illuminating the ground, the distance of the atmosphere that light travels through is called 1 atmospheric mass. When the sun is in other positions, the atmospheric mass is greater than 1. For example, at 8-9 o’clock in the morning, there are about 2-3 atmospheric masses. The more atmospheric mass, the longer the sun’s rays travel through the atmosphere, the more attenuated they are, and the less energy reaches the ground. Therefore, we define atmospheric mass as the ratio of the distance the sun’s rays travel through the atmosphere to the distance the sun’s rays travel through the atmosphere when the sun is at the zenith. For example, when this value is 1.5, the air mass is called 1.5, usually written as AM1.5. Outside the atmosphere, the air mass is 0, usually written as AM0.

3. Atmospheric transparency

In the upper boundary of the atmosphere and the plane perpendicular to the light, the solar irradiance is basically a constant; but on the surface of the earth, the solar irradiance is constantly changing. This is mainly due to the difference in the degree of atmospheric transparency. Atmospheric transparency is a parameter that characterizes the degree to which the atmosphere transmits sunlight. In clear and cloudless weather, the transparency of the atmosphere is high, and more solar radiation reaches the ground. When the sky is cloudy or dusty, the transparency of the atmosphere is very low, and less solar radiation reaches the ground. It can be seen that the transparency of the atmosphere has a great relationship with the amount of cloudiness in the sky and the amount of impurities such as dust contained in the atmosphere.

4. Geographical latitude

The solar radiation energy gradually weakens from low latitudes to high latitudes. What is the reason for this? Assuming that the atmospheric transparency at high latitudes and low latitudes is the same, the comparison is made under such conditions, as shown in Figure 1.

The main factors affecting ground solar irradiance
Figure 1 – Relationship between solar radiation flux and geographic latitude

Take the noon time of the spring equinox, when the sun shines vertically on the earth’s equator at point F, and assume that 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 AB of the atmosphere that the sunlight needs to pass through the point B is longer than the distance CD of the atmosphere that the sunlight needs to pass through the point D, so the vertical radiation flux at point B will be smaller than that at point D. The vertical radiant flux is greatest at point F at the equator because sunlight travels the shortest path EF in the atmosphere. For example, St. Petersburg, which is located at a high latitude (60° north latitude), can only obtain 335kJ of heat per year in an area of 1cm2; in Beijing, the capital of China, because it is located in the middle latitude (39°57′ north latitude), it can get 586k] of heat; in the low-latitude Sahara region, it can get up to 921kJ of heat. It is for this reason that the equatorial region has a hot climate all year round, and the four seasons are lush green, while near the Arctic Circle, it is cold all year round, covered in silver and snow, like two different worlds.

5. Sunshine time

This is also an important factor affecting the terrestrial solar irradiance. If there are 14 hours during the day in a certain area, 6 hours are cloudy and the sun is out at 8 hours. Then, it is said that the sunshine time of that day in the area is also called sunshine hours) is 8h, the longer the sunshine time, the more total solar radiation the ground gets.

6. Altitude

Elevation is measured by the mean sea level. The higher the altitude, the more transparent the atmosphere, and therefore the higher the amount of direct solar radiation.

In addition, the distance between the sun and the earth, topography, topography, etc., also have a certain influence on the solar irradiance. For example, the average temperature of the earth at perihelion is 4°C higher than that at aphelion, and at the same latitude, the temperature of the basin is higher than that of the plain, and the sunny slope is hotter than the shady 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.