The amount of solar energy reaching the Earth’s atmosphere is 1.94 calories or 1370 watts per square meter per minute. This number is known as the solar constant. However, up to 70 percent of this incoming radiation can be blocked by the atmosphere and cloud cover. The amount that actually reaches the surface varies according to weather conditions, the amount of particulate matter and water vapor in the air, the time of day and season of the year (i.e., the angle of incidence of the radiation in relation to the surface), the earth’s distance from the sun, and the amount of solar activity occurring.

Figure 3 What happens to solar energy? The biggest share of solar energy reaching the earth is absorbed at the surface.