A study of the division of global irradiance into direct beam and diffuse irradiance at seven Canadian sites

Canadian hourly global and diffuse irradiation data and associated daily surface meterological data of humidity, temperature and snow depth for the years 1977-1984 are analyzed. These data have been measured at Edmonton, Goose Bay, Montreal, Port Hardy, Resolute, Toronto and Winnipeg. Hourly values of the clearness index k(t) and diffuse index k(d) are sorted into bivariate histograms according to their numerical values. Different histograms are established for different ranges of the three variables: solar elevation, atmospheric precipitable water, and snow depth for each station. Properties of the different histograms are compared using standard statistical procedures. It is found that the division of global irradiation into direct beam and diffuse irradiation is correlated with the four variables K-t, precipitable water, solar elevation, and snow depth. II is also found that many, but not all, of the differences between data from the same station at different times and between different stations can be attributed to conditions associated with differences in these four variables. The data show evidence that the division of global irradiation into direct and diffuse irradiation can depend upon the properties of the clouds beyond how these clouds are characterized by the four variables.