A physical quantity representing the strength of solar radiation, called solar radiation intensity. The unit is Joule/cm2·min, which is the solar radiation energy projected vertically onto the unit area in a unit time. The intensity of solar radiation in the upper atmosphere depends on the altitude angle of the sun, the distance between the sun and the earth, and the time of sunshine. The greater the solar altitude angle, the greater the intensity of solar radiation. Because the same beam of light has the smallest irradiation area when it is directed, and more solar radiation is obtained per unit area. On the contrary, when it is obliquely incident, the irradiation area is larger and the solar radiation per unit area is less.
The altitude angle of the sun varies with time and place. During the day, the solar altitude angle is noon greater than morning and evening; summer is greater than winter; low latitudes are greater than high latitudes. The distance between the sun and the earth means that when the earth revolves around the sun, the distance between the sun and the earth keeps changing due to the elliptical orbit. The intensity of solar radiation obtained on the earth is inversely proportional to the square of the distance between the sun and the earth. When the earth is located at the perihelion, the solar radiation is greater than the perihelion. According to research, when the earth passes the perihelion at the beginning of January, the solar radiation obtained per unit area of the earth's surface is 7% more than when it passed the apocalypse at the beginning of July. The intensity of solar radiation is proportional to the duration of sunlight. The length of sunshine time varies with latitude and season.
