This element was computed using snowfall data from the National Climatic Data Center's United States Climatology database. The extreme date is the absolute first or last date of measurable snowfall within a 29 year period from 1961 to 1990. If no snow was observed for at least one-half of the years with available data, then the station was desginated "RARE SNOW". If no snow was observed for any of the years then the station was designated "NO SNOW".