UV Light

At one time or another, most of us have proved empirically, and painfully, the old mother's tale that it's possible to get sunburned on a cloudy day. On average, clouds do reduce the amount of ultraviolet A and B radiation that reaches the Earth's surface and our skin, but it far from stops the damaging rays. Indeed, clouds are generally better at blocking visible light than UV.

Under partly cloudy conditions a phenomenon sometimes called the "broken-cloud effect" can come into play, resulting in higher UV levels than a clear sky would produce, and so a greater risk of sunburn - or worse. A survey conducted at six U.S. sites in 1994 found that cumulus clouds could raise surface UV-B measurements by 25 percent, and in 2004 Australian researchers reported that the specific UV-B frequencies associated with DNA damage were up to 40 percent stronger under somewhat cloudy skies.

Why does this happen? Scientists aren't positive, but there seem to be two key mechanisms here: (1) UV rays bouncing off the sides of dense clouds, and (2) rays getting redirected as they pass through wispier clouds. Conceivably (as an American Scientist article suggested last year), a combination of thin refracting clouds up high and puffy reflecting clouds down low could result in a major UV boost at ground level. Throw in an aggravating factor or two - say, a blanket of snow to knock the rays around some more - and you're on the bullet train to sunburn city.