I'll admit to having felt a twinge of relief when facing my daily makeup and beauty regimen on an overcast day. I felt like I could skimp on the sunscreen, maybe even leave my sunglasses at home. Surely, I thought, these clouds were blocking enough UV radiation to keep me safe. It was like nature's PARASUN! Turns out I was wrong.
While I knew that UV rays can penetrate clouds, I wasn't aware of how much. An article in American Scientist explains that "clouds are generally better at blocking visible light than UVA," and that measurements of UVB at the Mauna Loa Observatory back in the 90s showed levels as much as 29.8% above modeled clear-sky levels. That's right, sometimes UV levels on a cloudy can be significantly higher than on a sunny, clear day!
How can this be? According to the National Weather Service Climate Prediction Center, "UV radiation can and does reflect off the sides of towering cumulus clouds. Such conditions result in actual enhancements of surface UV radiation."
So if you feel like you're never quite safe from UV rays, you're right. So even if it's overcast, put on the sunscreen and take your PARASUN with you. You'll be protected from sneaky UV radiation, and prepared if it rains.