ИСТИНА

For nearly half a century (1964-2010 gg.) the observations of natural light of the earth's surface are held in the Meteorological Observatory of Moscow State University. This is the longest series of continuous recording of natural light in the world, and for the territory of Russia it is the only one. Based on these observations, the daily course of the total illumination (EQ) throughout the year under a clear sky and at average cloudy conditions were analyzed. The estimation of impact of various climatic factors on the illuminance was done: solar elevation as the main astronomical factor causing the periodic change in earth’s surface illuminance, cloudiness as a factor leading to disruption of cyclicity, surface albedo as a seasonally acting factor. The focus is on changing light conditions throughout the day. In the absence of clouds in December and January, when the sun rises no more than 15 degrees above the horizon, the total illuminance average per hour in midday is about 17 klx. During June and July when the sun elevation h is maximum (56-57o at midday) EQ reaches 90-94 klx. With the increase of the sun elevation the contribution of direct lighting in EQ increases too: for h = 10-15o it is less than 50%, and at h = 50-55° - more than 70%. The upper and middle clouds passes up to 75-80% EQ on average, low clouds may reduce the natural light up to 90% compared with the clear sky conditions. Climatic norms of total illumination are at 35-40% less than the EQ under a clear sky. The influence of snow albedo is the most pronounced during continuous cloud cover when natural illuminance can increase up to 30% in compare of warm period. Based on the results of observations the time of onset critical light conditions and period of use of natural light in the design of lighting in buildings in Moscow region are calculated.