Theoretical and Experimental Studies of Atmospheric Factors Affecting Daylight.

In this thesis, the spectral power distribution of daylight is investigated theoretically, by calculations on the scattering of light in a model atmosphere, and experimentally, by the analysis of measured daylight spectra. The study begins with a brief discussion on the structure of the sun, the extraterrestrial solar spectrum, and those constituents of the earth's atmosphere which affect the spectrum of daylight; a comprehensive review of the atmospheric aerosol is included. The theory of the single scattering of light by air molecules and aerosol parti. cles is presented, and some of the computer programs which were written to evaluate the scattering parameters of aerosol particles are listed. The spectral power distributions of direct sunlight, for a range of solar zenith angles, and skylight, for the zenith sun only, are calculated by combining the extraterrestrial solar irradiance with a model atmosphere which contains air molecules, aerosol particles and ozone. The effect of variations in the aerosol content and the ozone content of the atmosphere are investigated. Daylight spectra, which were measured under various atmospheric conditions, are analysed; variations in the spectrum due to changes in the solar zenith angle, the distribution and amount of cloud, and the ozone content of the atmosphere are examined, with particular reference to the ultraviolet region. Empirical relations between the relative power and the solar zenith angle, and between the proportion of ultraviolet radiation in the spectrum and its correlated colour temperature are presented. Mean spectra are derived and are compared with the CIE standard daylights D[6500] and D[5500]. Finally the possibility of long-term variations in the spectrum of daylight is investigated by comparing recent spectra with those measured 10 years earlier