But those values are with respect to expected clear-sky UV. Compared with the level of attenuation usually seen when clouds are present, such measurements can actually be 50 to 75 percent higher than predicted, says Sabburg.
And therein lies a conundrum for those who work or recreate outdoors and depend on UV forecasts. No national forecast based on the World Health Organization's numerical scale for UV takes enhancement into account. Indeed, although several mention the possibility on their Web sites, the calculations instead assume that clouds reduce UV exposure.
The U. National Weather Service and Environmental Protection Agency, for example, figure 89 percent transmission for scattered clouds, 73 percent transmission for broken clouds and 32 percent transmission for overcast conditions.
So how do we get more rather than less? Several studies suggest that reflection off the sides of cumulus clouds is one mechanism by which UV radiation can become focused. Sabburg and Joe Wong also with the University of Southern Queensland have also postulated that refraction and scattering of direct and diffuse radiation could result in markedly increased enhancement. Thus cloud conditions that include cirrus clouds thin enough not to completely obscure the solar disk, along with lower-altitude cumulus clouds, may lead to the perfect UV storm.
Just how common is cloud enhancement? The various studies have found that between 1. Most often the enhancement lasts for 10 minutes or less—not a concern for the sun worshiper—but it has been known to persist for an hour. The problem, of course, is predicting something that is influenced by systems as dynamic as clouds.
Therefore, it is important to wear sunscreen and protective clothing whenever you are outdoors or working near a window, especially between the hours of 10am to 4pm. I was browsing on line for some information since yesterday night and I at last found this! This is a magnificent webpage by the way, except it seems a little hard to read in my i phone. You definitely come with good posts.
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