Some little-known facts about lightning
Wednesday, February 2, 2011
A recent study shows that the southwest tip of Ontario had the highest average density of lightning strikes in a 10-year period, with the town of Courtright holding the prize for the national lightning hot spot
by HENRY HENGEVELD
Courtright, in southwestern Ontario, gets more lightning per year than any other spot in Canada.
That's one of the conclusions coming out of a study into 10 years of data on cloud-to-lightning flashes across the country. The study, recently published as two articles in the Canadian scientific journal Atmosphere-Ocean, was undertaken by Environment Canada researchers William Burrows and Bohdan Kochtubajda. Burrows and Kochtubajda examined almost 24 million lightning flashes that were recorded between 1999 and 2008 by a network of 83 lightning detection sensors scattered across Canada.
They divided the country into squares, each 20 by 20 km, then looked at how many of those lightning flashes occurred in each of these squares, when they occurred and what their characteristics were. The researchers estimate that they captured at least 80 per cent of the cloud-to-ground flashes that occurred, but did not include cloud-to-cloud flashes, since these are weaker and harder to detect.
A number of little facts emerge from this study, some very useful, others less so but just interesting to know. For example, on average there are more than 2.3 million cloud-to-ground lightning flashes across Canada each year. In some years such as 2008, the numbers reach almost three million.
Courtright experiencing an average flash density of 2.8 flashes per square kilometre, won the prize for the national lightning hot spot, while Windsor was a close second at 2.7 flashes per square kilometre per year.
In marked contrast, Burrows and Kochtubajda found a small pocket of unusually low lightning activity southeast of Georgian Bay. This they attribute to the orographic effects of the Niagara Escarpment, where air tends to sink in the lee of the escarpment.
While the highest average lightning density over the 10 years was in the southwest tip of Ontario, in some years activity was actually highest in other spots. For example, the highest one-year density recorded over the decade – 10.3 flashes per square km per year – occurred in a grid cell over Lake Erie southeast of St. Thomas.
Nearby St. Thomas recorded a peak of eight flashes while Leamington registered one year with 7.3 flashes. All of these are located in regions where lake breezes from different sources frequently converge. Where they meet, these breezes push local air upward, adding to any local updraft from heat convection and enhancing the probability of the development of thunderstorm activity.
By comparison, the highest one-year cloud-to-ground lightning flash density for Western Canada, which occurred in southern Manitoba, was only five flashes per square km per year.
In some parts of the country, such as the Pacific coast and over southern Nova Scotia, cloud-to-ground lightning can occur year-round. However, most of the rest of the country has a lightning season that decreases in length from south to north.
The authors of the study defined the lightning season for a given location as the period of time that includes 99 per cent of all lightning flashes occurring there. They found that the season for southern Ontario extends from mid-March to early November. One of the key reasons for such a long lightning season in this region is the frequent movement of warm humid air masses into the region from the Gulf of Mexico. In contrast, in more northerly regions of Canada, the season begins in late May and ends in late August or early September. With the exception of southern Manitoba, the Prairies also have a shorter season.
Not surprisingly, southern Ontario was also the region with the most days per year with lightning. The record in this case went to Harrow, which averaged 36 lightning days per year. This number varied considerably from year to year, with 50 lightning days for one year occurring along the north shore of Lake Erie, near Highgate.
The data also suggest that most lightning activity in southern Ontario occurs between late afternoon and evening, and the least between six and nine in the morning. However, between 30 and 50 per cent of the lightning occurs between 10:30 p.m. and 10:30 a.m.
Much of this can be linked to movement of thunderstorms into a given region from elsewhere, often in squall lines linked to frontal systems, rather than to local convection activity.
Most flashes involve electrical flow from negative charge centres in clouds to the ground, but slightly more than 10 per cent involve positive flashes that occur between positively charged clouds (usually at higher elevations) and the ground.
Over Eastern Canada, about half of the lightning flashes involve multiple strokes, while the average lightning stroke has a peak current of 15,000 to 30,000 amps and about one per cent of the strokes exceed 100,000 amps. Most of these intense strokes occur in Western Canada.
One might wonder what value such information on lightning may have, other than to respond to scientific curiosity and provide fodder for idle chatter about weather trivia.
However, the authors remind us that lightning, on average, kills 10 Canadians per year, and injures between 92 and 164. In Canada, it also causes property damage and interruptions in the electrical power transmission and distribution system that cost between $600 million and $1 billion each year. Furthermore, lightning causes 45 per cent of all forest fires, many in remote regions where they can quickly get out of control.
Electrical grid managers and forest fire fighters will find such lightning information very useful, particularly if the risks can be predicted. However, the data will also provide one more factor for insurance adjustors, among others, to include in their property loss risk assessments – and hence in calculating insurance rates. BF
Henry Hengeveld is Emeritus Associate, Science Assessment and Integration Branch/ACSD/MSC, Environment Canada
