Estimating Probabilities of Cloud-Free Fields-of-View From the Earth Through the Atmosphere

Abstract

A model for estimating the probability of obtaining a cloud-free field-of-view as a function of ground-observer-reported total sky cover is presented for earth-to-space viewing applications. The development of the model and examples of its application are described. The model was developed to extend the cloud-free line-of-sight research reported earlier to cover finite field-of-view applications and is based on a subset of the same photographic archive. The archive comprises 2805 whole-sky photographs taken at Columbia, Missouri at 0900, 1200 and 1500 CST during a 39-month period. A grid of 185 annular sectors was used in the analysis of the whole-sky photographs. Statistics of the fractional cloud cover in each sector were computed as a function of total sky cover. Probability estimates were derived and are presented in a matrix format of cloud-free fraction versus reported sky cover for zenith centered areas of 10°, 50°, 90°, 130° and 170° in angular diameter. When the matrix for a given field-of-view is multiplied by a column vector representing the frequencies of observed sky cover at a station, an estimate of the climatic probability that any specified fraction of the field-of-view will be cloud free is obtained. Results of a limited study of the variability of cloud-free field-of-view probabilities as a function of elevation angle are also presented for fields-of-view of 10° and 20°. The results are compared to the cloud-free line-of-sight results.