Data: Tropospheric Emission Monitoring Internet Service; Map: Erin Davis/Axios Visuals
In San Francisco, it's easy to forget you can sunburn when it's cloudy — and that you don't need to feel the sun's hot rays to get a sunburn.
Reality check: 80% of solar UV radiation can penetrate light cloud cover — and infrared heat is different from UV radiation (which you can't feel), according to the EPA.
Driving the news: In the summertime, UV rays are at their worst at high noon, with San Francisco averaging a 9.2 UV index on cloudless days from June through August.
That's when it's especially important to limit time in the sun and cover up.
Why it matters: UV radiation causes sunburn and is a risk factor for skin cancer.
A UV index of 6-7 is the "high" range, 8-10 is "very high," and 11 and up is "extreme," which can lead to unprotected skin and eyes burning within minutes.
Remember that next time you're about to head out without lathering on some sunscreen!
Between the lines: Altitude, latitude, cloud cover, ozone level, time of day and seasons all factor into the UV index.