Calculation of the Natural Illumination of the Earth’s Surface at Different States of Cloud Cover

Resumo

An analytical expression which approximates experimental data on the natural daily illumination of the Earth’s surface in a wide (0–90°) range of the angular height of the Sun is suggested. The coefficients of this expression for the direct, scattered, and total components of natural light at different states of the Earth’s atmosphere and underlying surface are determined. The relationship between the solar angular height in the period from sunrise to sunset and the local time at an arbitrary reference point on the Earth’s surface is defined in the equatorial geocentric coordinates at arbitrary latitude and day of the year. Analytical expressions of the dependence of the direct, scattered, and total components of the natural illumination of the Earth’s surface on the local time for any day of the year and latitude are given. The results can be used in studies of unconscious responses of a human body to diurnal variations in the solar radiation characteristics, as well as in studies of visual perception.