http://www.nws.noaa.gov/asos/vsby.htm http://www.nws.noaa.gov/asos/vsby.htm Understanding Human Visibility http://www.nws.noaa.gov/asos/vsby.htm Visibility http://www.nws.noaa.gov/asos/vsby.htm http://www.nws.noaa.gov/asos/vsby.htm