Has Canada Experienced More Frequent Adverse or Fair Weather Since 1953?

Climatology and trends in adverse weather and fair weather occurrence in Canada are discussed in this study. The occurrence frequencies of freezing precipitation (FZ), blowing snow (BS), fog, and low ceiling (LC) events are analyzed to assess adverse weather trends; while the no-weather (i.e., no precipitation or visibility obscuration) event occurrence and bright sunshine hours are analyzed to assess fair weather trends. The analysis is mainly based on hourly weather observations at 95 Canadian stations, and on bright sunshine observations at around 60 sites, for the period 1953-2004. All data time series were subject to a homogenization procedure prior to the trend analysis, to diminish the effects of artificial step-type changes (due to station relocation or instrument/observer changes, etc.) on the estimate of climatic trend.

The results show that the frequency of freezing precipitation has increased almost everywhere in the Canadian region north of 50°N, especially in spring and autumn, and also in southern British Columbia and the Great Lakes area in spring. Significant decreases were only identified in central Prairies in spring and in southern British Columbia (BC) and the Great Lakes area in winter. On the contrary, blowing snow occurrence has decreased significantly in all seasons at almost all locations across Canada (with the only exception of southern BC, which was identified to have increased blowing snow occurrence in spring). It was also shown that fog occurrence has significantly increased in the Prairies and southern Yukon-Northwest Territories, but decreased in eastern Canada (east of 95°W) and southern BC, in all four seasons. Since a fog occurrence is also a low ceiling (LC) occurrence by definition, the patterns of changes in the LC occurrence have substantial similarity to those of fog occurrence, but the changes are generally less significant. In particular, the Great Lakes area was identified to have experienced more frequent LC occurrence (especially in spring and winter) but less frequent fog occurrence. The frequency of the no-weather event was identified to have increased significantly in southern Canada, but decreased in the north. The increase is most extensive in spring and summer, while the decrease is most extensive in autumn (it extends to northern Ontario in winter and further to the Prairies in autumn). The bright sunshine trend is similar to the no-weather trend.

Comparison of the no-weather trend with the adverse weather trends above indicates that the increase in no-weather occurrence (i.e., decline in weather occurrence) in southern Canada is at least partly due to a decline in freezing precipitation and blowing snow occurrences in the Prairies-southern BC, and due to a decline in blowing snow and fog occurrences in the east coast, the Great Lakes area, and southern BC. While the decline in no-weather occurrence (i.e., increase in weather occurrence) in northern Canada is largely due to an increase in the occurrence of freezing precipitation (and probably other types of precipitation). This agrees very well with the reported climatic changes in Canada, namely, more frequent cyclone activity and increased precipitation amount in northern Canada due to a northward shift of the storm track. It was also inferred that the reported temperature increase in Canada has probably resulted in more times that the temperature is just right for freezing precipitation to occur.