The presence of intervening clouds in the optical channel is a major factor limiting high availability of Free-Space Optical Communications (FSOC) systems for airborne-to-airborne or airborne-to-ground links. Modern Earth-observing satellite systems are a valuable source of information for characterizing the horizontal stratification of cloud structures that represent the most disruptive source of channel fades. We have created a tool for the estimation of Cloud-Free Lineof- Sight probability based on geographical location, altitude of the participants, the range between them, and the time of year. Using this tool, we explore several scenarios related to previous FSOC work. This tool can be used to develop concept-of-operations for future Air-to-Air and Air-to-Ground FSOC.