What Exactly Is The UV Index?

What Exactly Is The UV Index?

By Michael Pollak

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What's the real deal with that number that appears daily in your weather app?

Amidst all the mortgage ads and clickbait, you see it almost every time you pull up your Weather.com forecast – the UV index. But what does the number actually mean?


What is the UV Index?

The UV Index provides a forecast of the expected risk of overexposure to UV radiation from the sun, and it’s expressed on a scale of 0 (low) – 11 (extremely high). Just when you thought red and the number 10 would equal the greatest danger, purple at 11 is all out insanity.

Each number comes with a guide to how to protect yourself, as well as how quickly you’re likely to experience skin damage.

What factors go into the number chosen and who picks it?

The U.S. National Weather Service uses a fancy computer model that relates the ground-level strength of solar UV radiation to four things: forecasted ozone layer strength, forecasted cloud cover, location elevation, and general ground observations. If you really want to geek out on it, the EPA has the detailed calculation method here.

So the risk actually has nothing to do with how hot it is outside?

Nope! That is why it’s important to wear sunscreen year round, even in the winter. All of the above factors are still factors, no matter the temperature. We might not feel the heat of hte sun during winter or get sunburns (caused by UVB rays; think 'B' for burning), but UVA rays still exist and penetrate clouds and windows (think 'A' for aging).

How do UV rays affect my health?

We know we belabor this point, but in short, nothing good comes of it. Skin cancer is the big one – unfortunately, there are now more new cases of skin cancer are diagnosed in the U.S. than new cases of breast, prostate, lung, and colon cancer combined. And one in five Americans will develop skin cancer in their lifetime.

UV exposure and time in the sun also contributes to premature aging. While part of this is certainly natural and comes with living and enjoying life, up to 90 percent of the visible skin changes commonly attributed to aging (fine lines, wrinkles, age spots, etc.) are caused by the sun.

How can we protect ourselves?

The good news is that unprotected exposure to UV radiation is the most preventable risk factor for skin cancer, so we can act on this today. Keep applying your sunscreen daily and your skin will thank you in the weeks and years to come. Our best advice? Don't worry about the UV index number in terms of sunscreen application. Know that during the heat of the summer day, especially if you're outside, reapply liberally. And on cloudier days (or in less sunny seasons), know that UV rays still make it through clouds to cause those fine lines and wrinkles that many people ask us for help on minimizing.

As for beyond that, we recommend taking a look at the guide for each number and following those instructions.

What’s the Shadow Rule and what else do I need to know?

An easy way to tell how much UV exposure you’re getting is to look for your shadow. If your shadow is taller than you are (in the early morning and late afternoon), your UV exposure is likely to be lower. If your shadow is shorter than you are (around midday), you are being exposed to higher levels of UV radiation. The shorter the shadow, the higher the UV exposure. Protect your eyes with some quality shades and liberally apply sunscreen of SPF 30 or greater. Remember that chemical-based sunscreens take 20 – 30 minutes to truly start protecting you, so make sure you apply before you head out the door.

Oh wow, okay and what is the UV Index today?

Take a guess before you click here. Hint – this time of year, it’s high just about everywhere in the US.

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