8. THUNDERSTORMS IN EDMONTON AND CALGARY


In section 1, subsection c, data from Hage (1990) indicated that Edmonton, with 12 tornadoes between 1890 and 1989, was the hands-down leader over Calgary where only 3 tornadoes had struck during that century. Since 1989 Edmonton has had 3 more, Calgary none.

The number of times during the past 15 years that Calgary and Edmonton were struck by large hail, that is, at least walnut size and thus damaging to weathered shingles, was carefully estimated. This was accomplished by perusing the storm reports listed by the Insurance Bureau of Canada (1996) while comparing them with the detailed tabulations of hail events in the severe-summer-weather lists issued annually by the ALWC from 1982 to 1994 and, since then, by similar lists from the now separate Southern and Northern Alberta Environmental Services Centres located in Calgary and Edmonton. Since 1982 it was determined that Calgary had 17 storms with large hail while Edmonton had 14. However, during the summers of '91 through '96, Calgary has had 12 of their 17 storms while Edmonton has had only 6 of 14. The severe weather coordinator at the Southern Alberta Environmental Services Centre (Dudley, 1996) has reported that this situation may support suggestions by the insurance industry that Calgary is being struck much more frequently by damaging hail. A similar increase has not been experienced in Edmonton where visits by walnut sized or larger hail have averaged approximately 1 per year during both the 80's and 90's. Devastating hailstorms with tennis-ball-sized hailstones have struck both cities twice during recent decades; Calgary in '81 and '91 and Edmonton in '69 and '87 (Charlton et al., 1995).

Major hail swaths passing through Calgary during the period 1957-1973 were shown in maps contained in Climatic Summaries of Hailfall in Central Alberta (Wojtiw, 1975). During those 17 years, Calgary lay within the cloud seeding area of the Alberta Hail Project. The maps indicate that Calgary was struck by walnut sized hail on between 13 and 20 occasions, or roughly once per year. These events showed no trend during the 17 years. Edmonton was never within the jurisdiction of the Alberta Hail Project. Between 1974 and 1985, the final decade of the hail suppression project, Calgary also lay outside the project area and, unfortunately, the reported hail swaths could not be interpreted in terms of whether or not they dispersed large hail on Calgary or Edmonton. Finally, the '57-'73 hailstorm information, discussed above, gave further credence to the possibility that Calgary, with an average of two storms with large hail per year since 1991, is experiencing a remarkable run of severe hailstorms.

Recently, Charlton and Kachman (1997) estimated the life expectancies of asphalt shingles exposed to weathering and a normal mix of hailstorms for both Edmonton and Calgary. They determined that asphalt shingles will last an average of 14 years in Edmonton and 11 years in Calgary if they can resist fracturing by walnut sized hailstones until late in their life. Improved asphalt shingles which would resist fracturing by golfball sized hailstones after 15 years of weathering were found to have hypothetical average lifetimes of 27 years in Edmonton and 17 years in Calgary. These lifetimes were based upon official weather observations at the airports which, averaged over several decades, indicated that Edmonton airports averaged 2.0 hailfalls (pea size or larger) per year while Calgary Airport averaged 4.1 hailfalls per year. Unfortunately, the mix of hail size categories reported by farmers in central Alberta had to be used in the shingle lifetime estimates so the strong possibility that Calgary receives a much higher proportion of pea sized hail, because of its altitude, than does Edmonton could not be taken into account.

A Severe Thunderstorm Climatology for Alberta

(Paruk and Blackwell, 1994) based on '82 through '91 reports by volunteer weather watchers and other members of the public was recently published by researchers at the Northern Alberta Environmental Services Centre. The objective of the research was to adjust the number of reports per county for population density to yield realistic provincial maps of various severe weather parameters. Direct comparisons of values for Calgary and Edmonton should not suffer from anomalies found in some sparsely monitored counties. For the ten year period, Edmonton had 2.3 severe thunderstorm events per year per 1000 km2 while Calgary had 1.6. Similarly, Edmonton had 0.8 hail events (walnut or larger) per 1000 km2 while Calgary had 0.7. Edmonton had 0.5 heavy rain events (30 mm in an hour) per year while Calgary had 0.4. Severe wind events associated with thunderstorms were 0.3 per year per 1000 km2 in both cities. Winter wind events associated with Chinooks in the foothills and cold frontal passages throughout the province were not under consideration. As expected, tornado sitings during the decade were much more frequent in Edmonton (0.8 per year per 1000 km2) than in Calgary (0.1 per year 1000 km2) but in both cities the tornadoes seldom did damage to more than a few trees or one or two buildings, the exception being the Edmonton tornado.

Finally, it is hoped that the severe hailstorms in Calgary during the 90's will become a meteorological aberration like the Edmonton tornado appears to have been, and if not, may Calgarians discover hail resistant roofing before the insurance companies modify their coverage to reduce the stupendous insured losses.

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