Heavy-Fog Regions in the Conterminous United States¹

The heavy-fog statistics for 256 first-order weather stations were utilized to update analyses of the geographic distribution of f o g within the conterminous United States. The survey shows that heavy fog (visibility one-fourth mile or less) occurs more than 20 days a year at approximately 50 percent of the first-order weather stations (of which 229 are air terminals) and that the mean heavy-fog frequency reported per station appears to be generally higher than it was 30 yr ago. The geographic distribution of heavy fog is shown in two ways-by a conventional isopleth analysis and by fog climatic regions with characteristic frequency distributions.