Measured shortwave sky radiance in an urban atmosphere

An extensive data set of measurements of the shortwave sky radiance at Toronto, Canada, was compiled by the authors in 1984. A pyroelectric radiometer with a half-angle field of view of 5 degrees was used to make a total of more than 100,000 sky radiance measurements. This paper presents an analysis of almost 40,000 of these measurements in the form of a set of plots showing means and percentiles of sky radiance as a function of position in the sky; atmospheric clearness index, k t ; diffuse fraction, k; and solar zenith angle, θ z . The data show (i) that, in the circumsolar region, the highest observed values of sky radiance occur for partly cloudy sky conditions characterized by intermediate values of k and k t , (ii) that, in the regions of the sky away from the solar position, sky radiance tends to increase monotonically both with increasing k and with increasing k t , (iii) that there is a progressive transition with increasing k, from the horizon brightening for skies with low diffuse fractions to horizon darkening for skies with diffuse fractions approaching unity, (iv) that the shape of the sky radiance distribution is a strong function of the elevation of the sun in the sky, and (v) that the sky radiance is highly variable, with a positively skewed probability density function.