The valleys of the Wasatch Front of northern Utah experience their greatest concentrations of nitrogen oxides, carbon monoxide and particulates from November through February. Ozone reaches its highest concentrations in June through August. Highest concentrations of nitrogen oxides and carbon monoxide occur during the morning and evening rush-hour periods of traffic. Ozone reaches its peak during early afternoon. During the spring and fall months, there are minimal short-lived episodes of pollution. During these transition seasons, sufficient circulation exists due to weather systems to preclude significant pollution episodes.
Surface visibility is reduced due to the pollution in the atmosphere. Back in the coal burning days, Salt Lake City recorded 29 days in January 1944 of haze and smoke that reduced the visibility to six miles or less. Additionally, 25 days of haze and smoke were noted in November 1937, and 26 days in November 1944. As in the case of heavy fog, November through February are the "pollution" months. An even 100 days of haze and smoke was noted in this period during the late fall and winter of 1943-1944.
Since 1948, there has been an average of 41 days of haze and smoke during November through February. December and January average 13 days, February eight days, and November seven days.
Much of the information for this section originally appeared in the copyrighted book Utah's Weather and Climate, edited by Dan Pope and Clayton Brough, in 1996. UCCW Directors have received permission from the copyright owners of this book to reproduce such information on its website and to revise and updated it where appropriate.