Transmittance index characterization at two solar measurement stations in Brazil

This paper constructs probability density distributions of direct normal, diffuse horizontal, and global horizontal irradiance for 1-min clearness indexes kb, kd, and kt, respectively. Previous studies on the clearness index, conditioned to the optical air mass, are analyzed and distributions of these three indexes, using data from two solarimetric stations in Brazil, are presented. The pattern found in probability density distributions, in general, matches the patterns found in the literature. The 50th percentile (P50) of kb values from Sao Paulo city (a location with high levels of pollution) is 0.22, and from the countryside region of the state of Sao Paulo (a location with expected low levels of pollution), it is 0.55. Regarding kt values, P50 is 0.57 and 0.72 for these two regions, respectively. We conclude that different atmospheric conditions result in a different dependence relation between probability density distribution and air mass increases. These conditions include not only cloud formation, but also pollution levels and the influence of relative humidity. These specific results endorse the need for local radiation measurements to the characterization of few-minute average of transmittance indexes.