Are UV rays really more powerful on cloudy days?

November 2, 2007

Dear Cecil:

Here in Japan, muggy summer marks the annual appearance of parasols, long gloves, and enormous hats, all to protect against dreaded UV rays. Yet rather surprisingly I see more of this on cloudy days than on sunny days. When I asked a friend why, she said it was because UV rays are more powerful on cloudy or hazy days and because people were duped by Mother Nature into believing that dark skies equals less UV exposure. Are clouds really UV magnifying glasses? Or do the Japanese need one fewer old wives' tale?

I can't really blame you for not buying this one, Kirk. Off the bat, the notion that ultraviolet radiation is worse on cloudy days sounds a lot like one of those truly moronic bits of "counterintuitive" folk wisdom that often catch on (see, e.g., cold water boils faster than hot). And what with Japan being home to some very serious devotees of neo-Victorian fashion (the curious can go search "gothic lolita"), a reasonably skeptical gaijin like yourself might well dismiss the UV scare story as an excuse for the less committed to join in the dress-up fun. But guess what? It's for real.

From the top: The ultraviolet rays that reach the earth's surface come in two basic varieties, the more prevalent UV-A and the more potent UV-B. While each seemingly plays a role in skin cancer and eye damage, it's UV-B that both helps the body produce vitamin D and causes sunburn; its intensity varies widely with time of day, time of year, and latitude. (Being closer to noon, summer, or the equator all = more.) Clouds usually block UV rays, particularly UV-B; on a really overcast day they can keep out 70 to 90 percent of the UV-B coming in.

Maddeningly enough, though, that's not where it ends. 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.

Also at work is another insidious effect involving haze. Both natural haze and the kind resulting from pollution have a redistributing effect on solar radiation: while they can block UV-B from reaching the earth directly (sometimes reducing overall levels 50 percent or more), they also scatter it all over the place, in effect turning the entire sky into a radiation source. Standing in the open on a hazy day, you may get less UV-B than if the sky were clear (on the other hand, you may not), but over in the shade haze means you'll get lots more - an ominous thought for those dwelling in the smoggier parts of the world. And compounding it all is the problem your friend mentioned, namely that generally people are less likely to take precautions against sun when it's cloudy, leaving themselves wide open to any UV-amplifying consequences.

Crying over spilt milk is lousy public health policy, of course, but it's hard not to feel a little nostalgic for the days when we had a first-rate ozone layer. Ozone's crucial in blocking solar UV-B, hence the widespread dismay on discovering we'd put a big dent in our atmospheric supply. Most authorities agree that ozone depletion has leveled off following decades of fluorocarbon bans, but it'll still take 50 years for the layer to mend - so expect more parasol weather ahead.

And while we're all here: Some will occasionally claim that since the moon reflects UV radiation, staying out too long when it's full can get you a case of "moonburn." In medical parlance, these people are known as half-wits. The moon's only 0.0002 percent as bright as the sun and reflects UV light only about half as well as it does visible light; thus, eight hours of top-strength moonlight delivers less UV-B than a second of sun.

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