Extreme Vs. Severe Weather

Please note that this short article represents my opinion only. It does not represent the official positions of either Environment Canada or York University. I would appreciate your comments.

More and more I find that the term ‘severe weather’ is being replaced by the far sexier term ‘extreme weather’. But do these terms refer to the same thing? Can we just drop the word 'severe' and start putting out 'extreme weather warnings'?

I think not. The AMS Glossary of Meteorology states that severe weather is, in general, any destructive weather, usually applied to such things as intense thunderstorms, tornadoes, hail, etc. Extreme, it states, is the highest, or in some cases, the lowest value of a climate element observed during a given period.

So, to give a concrete example, a tornado in southern Ontario is severe weather in that it is typically destructive and we get tornadoes pretty regularly in this neck of the woods. However, there has never been a documented F5 tornado in Canada. So, climatologically speaking, an F5 tornado would be an extreme weather event. Even F4 tornadoes are quite rare here and might be called extreme. It works the same way for other parameters. Take heavy rain. A storm that produces 50 mm of rain in an hour is severe. Rainfalls of this amount occur quite frequently in a place like southern Ontario and can cause flash flooding. However, a storm that produces 450 mm of rain (like the Harrow, ON event) would not only be severe (ie. above the severe threshold of 50 mm) but extreme since a rainfall accumulation of that magnitude occurs so infrequently in Canada. Sure, somewhere in between there must be a fuzzy line that can be statistically determined but the two terms are definitely not interchangeable if they are to make some kind of scientific sense.

Thus, statements like:

"Extreme weather events are part of Canada’s normal climate."

found on an EC web site make absolutely no sense. Extreme weather is not normal - it is the *opposite* of normal. However, severe weather *is* normal in Canada.

I’ve complained about the increasing use and abuse of the term 'extreme weather' in the company of colleagues and just get strange looks. However, I think the distinction is an important one and I encourage everyone to buck the trend, take the less exciting, less hip route, and stick with good old 'severe weather' where appropriate.

Dave Sills
*Severe* Weather Scientist

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Last update: June 5, 2000