The Clear Sky Calculator is a model used to predict daily total radiation, based on latitude, longitude, elevation, humidity, and temperature. Notice that the predicted radiation varies slightly due to small fluctuations, caused by the absolute humidity of the air column.
The clear sky solar radiation model calculates solar radiation at the Earth’s surface based on solar radiation at the top of the atmosphere (extraterrestrial radiation) and empirical extinction coefficients for direct beam and diffuse radiation components. The extinction coefficients are calculated from empirically-derived equations that account for the physical factors that govern radiative transfer through the atmosphere, such as atmospheric composition and air mass. Extraterrestrial radiation is based on a theoretical calculation from the solar constant, and accounts for day of year, time of day, latitude, and longitude. These factors determine the position of the sun relative to the measurement site, and thus the solar radiation at the top of the atmosphere magnitude for the given site.