Abstract
Relative frequencies of cloud-free lines-of-sight were determined at specified elevation angles and directions by utilizing data from photographs taken with a camera with a 180° (fish-eye) lens and infrared film to produce high-quality photographs of the sky. Four summers of hourly daytime data were used to find relative frequencies as functions of viewing angle, sky cover, sunshine and cloud type. Persistence and recurrence relative frequencies, comparisons between “clear” and cloud-free lines-of-sight, and a general method for estimating probabilities of cloud-free lines-of-sight for any location are presented and discussed.
