How much sun on a cloudy day

Investigators have known since that clouds can have paradoxical effects on incident UV radiation. In more than a dozen studies since then, every data set includes at least some examples of what is known as cloud enhancement of UV.

For people hoping to avoid skin photoaging and cancer, this can be a confounding characteristic. How much increase in radiation?

It depends how you look at it. Although the mechanisms aren't yet entirely clear, the degree of enhancement can be significant. In various other studies, the range has been reported as a few percent up to 50 percent.

According to Sabburg, "In our latest research [soon to appear in the Journal of Atmospheric Research] , we use new equipment and refine our methodology, and the highest UVI [an index of skin reddening] enhancement we found was 25 percent.

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.

And as it turns out, even more so on those cloudy days! It is important to perform regular skin checks yourself and visit a dermatologist every year — especially here in Florida. If you notice changes to your skin or if it is time for your annual skin check, contact our office for an appointment.

Skip to content. Pay Your Bill. Insurance Accepted. Search for:. Fact or Myth? Do cloudy days really create stronger UV Rays?

So yes! Ultraviolet rays scatter more strongly in the atmosphere than visible light, so they can literally bounce off the air and then to you even if you are in the shade. If you can see blue sky, you are getting scattered UV.

Another sunscreen indicator is the international UV index. At about 3, you should definitely wear sunscreen. For instance, today in Portland, our UV index is 1 low. Sun-protective clothing is a great choice. The more covered up you are, the less sunscreen you need. You can protect your skin and stay stylish, too. Want to learn more? Sign up for our newsletter and receive great skincare tips and sun savvy tricks.