In terms of Free Space Optical (FSO) telecommunications, either terrestrial or surface to space, clouds and fog in the atmosphere can limit system performance. NOAA/NESDIS has developed several experimental products derived from GOES Imager infrared channels that describe low clouds and fog, giving the bases of low clouds as well as the thickness of the low cloud and fog layers. In the spring of 2001, Terabeam deployed a network of weather instruments in San Francisco to take advantage of the climate and hilly terrain to study optical properties of fog and low clouds. The instruments include three visibility sensors at various elevations, one ceilometer, and a weather station. Cloud ceiling data from the GOES experimental product have been compared to ceilometer data up to 25,000 ft and in situ visibility data at three different elevations below 1000 ft. One of the challenges of this comparison is the highly localized meteorology of the San Francisco Bay area combined with the relative coarseness of the GOES pixels. This paper will present preliminary results of correlations between ground-based visibility measurements and the GOES fog products.