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SSI's Optical Communications team has had many encounters with fog. Rain makes optical communication links difficult, due to the scattering properties of drops of water in the optical path. Fog tends to cause even more problems than rain, due to the large number of very fine water droplets.

On a typical night, fog forms where the coast meets the Pacific Ocean and is blown inland and over the top of the Santa Cruz mountains. The Skyline Boulevard test site is in the Santa Cruz mountains, and is quite often host to very dense fog as the temperature drops below the dew point with nightfall. To date, this phenomenon has severely interfered with three 10 km OpComms tests.

The OpComms team has gained some very basic weather prediction knowledge from studying this area. Real-time weather information can be viewed at an Amateur Radio operator’s home weather station [1], as well as many other locations in the area. Typical fog forming conditions involve a temperature and dew point within 4-5 degrees Fahrenheit. It is also important to look for unfavorable wind conditions (those that blow wet air from the ocean into the mountains) and dropping barometric pressure, which tends to herald cloudy weather.

For the most recent test, this problem was addressed by moving the time window earlier in the day, setting up in daylight, and completing the test before the fog rolled in. This time frame seems to be a successful compromise between fog formation and the optical need for darkness, but is still not immune to inclement weather.

Cloud formation may pose problems for optical communication satellites, due to the requirement of a clear path to the receiver from a point in space. The extent of this problem has not yet been determined.