Several recent studies have revealed a widespread decrease in low visibilities caused by fog. Long term decreases in fog over several decades have been reported in Canada, Argentina, and Brazil. During the period 1961 through 1990, fog decreased significantly at two major airports in the Los Angeles Basin. In the United Kingdom, a 45% decrease is seen for 28 stations when the period 1974 to 1983 is compared to the period 1950 to 1959. This decrease is attributed to the passage of the Clean Air Act in the United Kingdom in 1956. In addition to a decrease in particulate pollution, growth of urban heat islands, and long term ocean-atmosphere cycles are hypothesized to have a role in the frequency and intensity of fog. Long-term (50 year) records were examined for 13 cities in the United States to determine the extent to which low visibility frequencies have decreased.