Clear and cloud-free line-of-sight probabilities have been derived from analysis of: (1) Whole-sky photographs, (2) visual sky cover observations, (3) jointly observed sky cover and sunshine, (4) satellite observations and (5) in-flight observations. The most accurate probabilities of cloud-free lines-of- sight through the entire atmosphere can be obtained from a model derived from several thousand whole-sky photographs and visual sky cover observations. The most accurate probabilities of both cloud-free and haze-free conditions through the entire atmosphere, or protions of the atmosphere, or portions of the atmosphere, can be obtained from the in-flight observations.

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