Understanding Your Skin's Best Friend, The UV Index: What It Means And Why It Matters
The UV Index (UVI) is a number that tells you how strong the sun's ultraviolet (UV) rays are on a given day. These rays are invisible, but they can damage your skin and eyes.
The UV Index was first introduced in the early 1990s by Environment Canada (essentially the equivalent of the United States' National Oceanic and Atmospheric Administration) but it's now used worldwide, thanks to a version developed by the World Health Organization and the World Meteorological Organization. Their goal? To help people protect themselves from too much sun exposure, which can lead to sunburns, skin aging, eye damage, and even skin cancer.
(MORE: Are You Making A Sunscreen Mistake?)
Several things come into play when calculating the UV Index, including:
Time of day: UV rays are strongest when the sun is highest—usually between 10 a.m. and 4 p.m.
Time of year: UV levels are higher in spring and summer.
Latitude: Places closer to the equator usually get stronger UV rays.
Altitude: Higher elevations get more UV exposure.
Ozone levels: The ozone layer helps block UV rays, so thin ozone means higher UV.
Cloud cover: Some clouds can block UV rays, but thin or scattered clouds don't do much.
Meteorological agencies like the National Weather Service in the U.S. (or other national weather services around the world) use a standardized set of formulas to calculate and release the UV Index each day.
(MORE: Do You Need Sunscreen Indoors?)
The UV Index is a scale from 1 to 11+, and the higher the number, the greater the risk of harm from unprotected sun exposure. Here's a quick guide:
0-2 (Low): Safe to be outside. Minimal sun protection needed.
3-5 (Moderate): Some risk. Shade, sunscreen, and sunglasses are smart.
6-7 (High): Higher risk. Stay in the shade during midday, wear SPF 30+.
8-10 (Very High): Take extra precautions. Cover up, reapply sunscreen often.
11+ (Extreme): Max protection needed. Unprotected skin can burn in minutes.
The scale goes to 11+ to show that UV levels can sometimes go beyond what was once considered the maximum. Think of 11+ as the red zone—super intense sun that requires serious protection. It's especially common in tropical regions or at high altitudes, but it can happen anywhere under the right conditions.
The UV Index is like a daily sun-safety alert. It helps you know when it's time to put on sunscreen, wear a hat, or take a break in the shade. So next time you check the weather, don't skip the UV number, it's your skin's best friend.
Weather.com lead editor Jenn Jordan explores how weather and climate weave through our daily lives, shape our routines and leave lasting impacts on our communities.
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