Variation in incident light with time of day and cloud conditions at Hurst Spit, UK South Coast

Spectral irradiance of incident light was measured throughout the day during 6 days from December 1995 to March 1996. Variations in incident light with zenith angle were recorded by shading the cosine collector, allowing light from only a 10 degree zenith angle band to reach the sensor. On bright days total irradiance showed an asymmetrical pattern with high mean levels in the afternoon for equivalent zenith angles of the sun. This may be attributed to morning haze and low cloud which gradually cleared. Spectra contained a higher proportion of blue light in the morning and late afternoon, and during overcast periods. For periods of changeable cloud cover, good correlation was found between the natural log of blue to red spectral ratios and total irradiance adjusted for sun zenith angle. Lower blue to red ratios were found in the 60-90 degree bands, and higher ratios in the 0-50 degree bands, reflecting the high zenith angle of the sun and the presence of haze and cloud near the horizon. Comparison with model output show reasonable agreement at medium sun zenith angles in March, but more variable fit at high zenith angles in December and January.