I haven't yet found a definitive answer. I'd suggest that "visibility thresholds are based on stopping distances". I did find some related website: 1) http://www.highways.gov.sk.ca/docs/travelers_info/winterroad_terminology.asp; 2) http://www.mtq.gouv.qc.ca/en/securite/hiver/terminologie.asp; Mark Chandler

Jim, regarding your thresholds for descriptive visibility conditions, I think I may have found a reference for you: Shepard, F.D., Reduced Visibility Due to Fog on the Highway, Synthesis of Highway Practice 228, TRB, 1996. And see Chapter Two, Table 1 "International Classification of Visibility", page 4. I'll replicate the table: visibility Description less than 40 m Dense fog 40 - 200 m Thick fog 200 - 1000 m Fog 1 - 2 km Mist (if mainly due to water droplets) Haze (if mainly due to somode or dust) 2 - 4 km Poor visibility 4 - 10 km Moderate visibility 10 - 40 km Good visibility over 40 km Excellent visibility best regards, Jim Mark

The following was provided by Andy Stern, Mitretek astern@mitretek.org 703-610-1754 The official National Weather Service (NWS) definition of a blizzard does include a visibility component (http://www.nws.noaa.gov/directives/010/pd01005013d.pdf). In this case "frequent visibilities must be below a quarter mile for 3 hours or more." Heavy snow and blowing snow conditions are also defined as having "occasional visibility to a quarter mile or less for 3 hours or more." There are several "non precipitation warnings" issued by the NWS (http://www.nws.noaa.gov/directives/010/pd01005015e.pdf). Again, for public consumption, visibilities below a quarter of a mile is an important threshold: Dust Storm Warning: "Widespread or localized blowing dust reducing visibilities to a quarter mile or less." Dense Fog Advisory: "Widespread or localized fog reducing visibilities to a quarter mile or less" Dense Smoke Advisory: "Widespread or localized smoke reducing visibilities to a quarter mile or less." In general NWS forecasts (http://www.weather.gov/directives/010/pd01005003a.pdf), visibility restrictions such as fog, haze, smoke, blowing snow, blowing dust and volcanic ash can be included when visibilities are forecast to be less than or equal to one quarter of a mile. Visibility restrictions may be included in the forecast text for visibilities greater than a quarter mile up to 6 miles based on local user requirements. Areal coverage terms that are used to describe visibilities include patchy, areas and widespread: Areas: Areal coverage of non-measurable, non-convective weather and/or restrictions to visibility affecting 25 to 50 percent of a forecast zone. Patchy: Areal coverage of non measurable, non-convective weather and/or restrictions to visibility affecting less than 25 percent of a forecast zone. Widespread: Areal coverage of non-measurable, non-convective weather and/or restrictions to visibility affecting more than 50 percent of a forecast zone. Hope that this helps. If there are additional specific questions, please let me know.