UV solarmeter with me, but had I brought one, the intensity of the ultraviolet-B rays at the top of Black Butte the day Allie and I hiked there in 2008 would likely have been a dangerous 400 or higher. It was summer, there was open sky all around, the air was clear and dry, and it was about noon. It’s hard to get a bigger dose of UVB in Oregon, unless you add being on fresh snow on a mountain peak like Mt. Jefferson (elev. 10,500 feet), which you can see prominently in the photo.
A reading over 400 represents an extreme level of sunburn risk. The normal summertime range of UVB in the Willamette Valley is 280 to 350, depending on the clarity of the atmosphere and the density of the ozone layer overhead. The sun’s elevation above the horizon is the single most important factor in jacking up UVB intensity. The higher the sun, the bigger the potential burn.
All UV energy can be dangerous, but UVB rays are the ones that mainly affect the outer layers of the skin, causing sunburn, premature aging of the skin and—most ominously—skin cancer. These rays are strongest during the summer months, especially between 11 a.m. and 4 p.m. UVA radiation penetrates deep into the skin and is especially associated with skin aging. There is also UVC radiation that is very dangerous biologically, but fortunately is almost totally filtered by the earth’s atmosphere.
While it’s true that you can burn on a cloudy day, it helps to apply common sense before the sunscreen when you want to spend time outdoors. Overcast skies cut the overall UV intensity by about half. A good hat with UPF of at least 15 and a good pair of sunglasses is then about all you need to avoid problems. If it brightens or the sun comes out, then so should the sunscreen, if it’s between 10 and 4. I go to considerable lengths to avoid using sunscreen—primarily by simply staying out of the sun during midday. The thought of just hanging out on a sunny beach is anathema—even if there was such a thing in Oregon.