Photogrammetrically Determined Cloud-Free Lines-of-Sight at Columbia, Missouri

Book excerpt: The probability of a cloud-free line-of-sight was determined at specified elevation angles and directions by using a camera with a 180 degree lens and infrared film to produce high-quality photograms of the sky. Three years of hourly data were used to analyze the probability with respect to the parameters of azimuth, elevation angle, season, time of day, sky cover, cloud type, and sunshine. A very unique probability surface as a function of sky cover and elevation angle of the line-of-sight, the two most important parameters, was constructed. The persistency of a cloud-free line-of-sight was determined by utilizing a summer season of photograms taken at five minute intervals. The persistency was analyzed for diurnal variations up to six hours of consecutive observations. (Author).