Chances are you’ve heard of the UV index—and alerts about it may even pop up on your weather app on especially sunny days. But you may be confused as to what those numbers really mean and how understanding them can help you better protect your skin.

“The UV index is a way to convey the risk of sun damage by putting a number on it,” says David J. Leffell, M.D., professor of dermatology and surgery at the Yale School of Medicine. “Its main purpose is to keep sun protection top of mind for people as they plan outdoor activities.”

So if the weather is warming up in your area or you’re heading to the beach or the slopes for spring break, understanding the UV index can help guide you to smarter sun-safety strategies.  

How the UV Index Is Calculated

Ultraviolet rays from the sun are a known cause of premature aging of the skin—wrinkles and sagging—and skin cancer. In 1994, the National Weather Service and the Environmental Protection Agency developed the ultraviolet index as a means of helping to quantify how strong the sun’s UV rays are at any given time.

Sunlight intensity (and consequently, the level of the UV index) varies according to time of day, cloud cover, ozone levels, altitude, time of year, ground surface (snow, sand, water, pavement), and amount of cover (such as buildings and trees that provide shade). The calculation is a complex formula that takes into account all these factors. Checking the UV index can help you plan when and how to safely spend time outdoors.

Reading the Numbers

Depending on where you are and what the weather is doing, the UV index can read from 1 (Low) to 11+ (Extreme). The highest readings—regardless of location or time of year—will be at midday when the sun’s UV radiation is at its peak intensity. 

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You can search for the UV index at any location in the U.S. on the EPA’s website, or download the agency’s UV index app.

A reading of 1 or 2 is Low, 3 to 5 is Moderate, 6 to 7 is High, 8 to 10 is Very High, and 11 or more is Extreme. The idea is that the higher the number forecasted for a given day, the more diligent you need to be about protecting yourself.

At the Extreme end of the UV scale (which you’ll commonly find at midday during the summer in places like South Florida and Arizona), experts warn that the sun’s rays will be so intense that unprotected skin can burn in a matter of minutes. “Those are the days when you may even want to stay indoors or only go out early or late in the day, especially if you have fair skin,” Leffell says.

But don’t be lulled into a false sense of security by lower numbers. “You can still get burned even on a cloudy day when the UV index is only 2 if you don’t protect yourself,” says Darrel Rigel, M.D., clinical professor of dermatology at the NYU School of Medicine. “Cloud cover definitely dials back the intensity of the rays, but it’s not that nothing is getting through.”

The same goes for the colder months. While the UV index will be significantly lower on average in winter than in summer, unprotected skin is still vulnerable to damage when readings are at Low to Moderate levels.

And because snow reflects the sun’s rays, the UV index on a sunny day at a ski area can be as high as—or higher than—it is at the beach (especially when you also factor in the effect of altitude). Snow reflects as much as 80 percent of UV rays (and UV increases by about 2 percent for every 1,000 feet of elevation), compared with 15 percent for sand and 10 percent for water, according to the EPA. 

Cover Up Accordingly

Even on days when the UV index is in the Low to Moderate range, the expert recommendation is still to cover any exposed skin with a broad-spectrum SPF 30 or higher sunscreen.

And as the UV index climbs, you should step up your sun protection—being vigilant about reapplying sunscreen every 2 hours or immediately after swimming or heavy sweating, and adding sun-protective clothing (shirts, pants, rash guards, and wide-brimmed hats) to the mix.