On Destructive Canadian Prairie Windstorms and Severe Winters

Raw data on tornadoes and other destructive windstorms in the provincesof Alberta and Saskatchewan, Canada, are analyzed for spatial variations and timetrends. All storms and a subclass of intense storms are analyzed separately. StatisticsCanada agriculture census data are used to correct storm frequencies for the absenceof reports from unoccupied land, for sampling network drift, and for changes in farmnumbers and sizes. These corrections eliminate an apparent tornado spatial frequencymaximum in central Alberta and reveal a minor temporal frequency maximum from1906 to 1940 followed by no apparent trend in time for all intense storms. The frequencies of reports of severe winters in rural community histories of Alberta and Saskatchewan are compared with mean winter temperatures at Edmonton and with a severe winter index derived from temperature and snowfall data. All temperature data were corrected for urban heat island warming by an adaptation of Oke's proposed logarithmic relation between heat island intensity and urban population. The index identifies in correct order of importance the three most frequently mentioned severe winters of 1907, 1920 and 1936. Ten-year averages of mean winter temperatures and the severe winter index from 1882 to 2001 seem to show wavelike time changes rather than a long-term trend.[PUBLICATION ABSTRACT]