Study of Cloud-to-Ground Lightning in Quebec: 1996-2005

Using data from Hydro-QuEbec, a spatio-temporal summary study of cloud-to-ground lightning in Quebec (45 degrees-53 degrees N; 81 degrees-65 degrees W) for the 1996-2005 period was performed on a sample of close to four million lightning strokes. The annual number of lightning strokes and the ratio of negative to positive lightning (76: 24) do not differ significantly from one year to the next. Despite the fact that there was an average of 239 lightning days per year, the lightning strokes were concentrated over a period of a few days. Between 1996 and 2005, 50% of the total annual lightning was distributed over 11 days, 75% over 25 days, and 90% over 44 days. Overall, the peak in the average annual cycle occurs on 15 July. Between 1996 and 2002, the number of days with at least one positive lightning stroke remained higher than the number of days with at least one negative lightning stroke. This tendency reversed from 2003 until 2005. Most of the annual lightning occurred during June, July and August. The average minimum number of lightning strokes per hour occurred at approximately 14: 00 UTC, and the maximum number occurred at 21: 00 UTC. The ratio of positive lightning to negative remained constant throughout the day. Both the density and the number of lightning days were mapped for the 10-year period. The spatial distribution of lightning indicates a higher density in the southern and western parts of the study area with an average of 0.52 to 1.27 lightning strokes km(-2) yr(-1). The St. Lawrence Lowlands ecoregion receives the greatest number of lightning strokes annually (from 0.73 to 1.27 km(-2) yr(-1)). The spatial distribution of the number of lightning days per year is approximately the same as that of the density. The same two gradient axes can be observed crossing from north to south and from east to west. The spatial distribution of the percentage of positive lightning strokes varies considerably in the area, ranging from 0 to 65% depending on the location. While the St. Lawrence Lowlands ecoregion has the highest density and highest number of lightning days, it also has the lowest number of positive strokes. Additional research must be done to establish a correlation between our results and environmental variables, such as topography and vegetation, as well as the spatial variations of lightning and instances of forest fire.