Solar radiation that reaches the earth's surface also called solar irradiation, solar exposure, solar insolation, or insolation. It is measured by the amount of solar energy received per square metre. Amount of solar energy received over the planet’s surface varies according to season, latitude, transparency of the atmosphere (including cloudiness), and aspect or ground slope. In the long-term forecast it basically shows the contribution of the future cloudiness regime for for a certain location. Therefore, the presented forecast can also be considered as an inverted long-term forecast of cloudiness.