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Everytime I check the weather news and storms are announced, the UV index is very often (if not always) "very high" or "extreme", why is that?

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What is such a "storm" in Your area? (Montreal, Canada, Isuppose) Do You have low pressure areas drifting over Your region often? – Georg Jul 22 '11 at 15:53
Also, how much high, compared to the average UV received, say, at equator at midday (or whenever the sun is at the zennith)? there ought to be some reference measurement to gauge this extra UV radiation – lurscher Jul 22 '11 at 16:07

UV index depends upon time of day, time of year, latitude, altitude and cloudiness. An index value of "10" corresponds to noon in early summer on a clear day in Toronto, Canada.

This question was asked in July by a user from Montreal, Canada, when UV index is high.
Storms do not increase the UV index.

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+1. Note that this question was asked on July 22. There is a correlation between midsummer and the UV index in Montreal. The UV index is rather high at Montreal's latitude from mid May to mid August, and that is when storms (presumably meaning thunderstorms) hit Montreal. – David Hammen Oct 31 '14 at 13:50

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