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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

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If you check the process of storm creation, you can understand that.

The atmospheric pressure of storm's outside is very high, and the pressure of eye of storm is very low.

Because of the atmospheric pressure of storm's outside is very high, the sky is very clear.

enter image description here

So the UV index goes high.

Link Added. Transmittance and Opacity

(For someone who can't understand the reason why if the sky is clear, UV index goes high.)

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In this picture, High pressure area should be more far away from the eye. Sorry for the small image. – 4545454545SI Jul 22 '11 at 16:05
again, why sky very clear implies UV index going high? – lurscher Jul 22 '11 at 16:06
Is their any problem on my answer? – 4545454545SI Jul 22 '11 at 16:47
Who clicked down vote? Is their any problem? could you explain me why??? – 4545454545SI Jul 22 '11 at 16:56
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@David: Umm--- if the sky is clear for light, especially blue light, then it is clear for UV. The scattering of visible and UV light is similar. – Ron Maimon Sep 4 '11 at 3:28
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