So, you want to protect your skin from sun damage. The UV index is where to start.

But what do the numbers mean, and how should you respond to them? Our expert, Lauren Leavitt, a dermatology physician's assistant, explains.

How can I use the UV index?

“I am a mom of two young boys who are very active, and we always look at the UV index on the Weather Channel app,” Leavitt said. “That can help guide you to your level of sun protection.”

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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. It factors in time of day, time of year, latitude, altitude, ozone levels and cloud cover.

Her guidance is simple: any UV index above a level three means it’s time to respond and do something to protect your skin. 

“Once it gets beyond three, at that point, you're going to start to see the UV do more long-term potential damage and burning,” she said.

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(MORE: Understanding your skin’s best friend, the UV index: what it means and why it matters)

There are several types of ultraviolet radiation. UVB enters the skin’s outermost layers, can cause tans and sunburns and is able to damage your skin all year round.

The UVB level is highest between 10 a.m. and 4 p.m., so during that time, that's when our skin has the highest risk of burning, she explains.

How do clouds affect the UV index?

“When the clouds come out, it feels like it's cooler out or like the sun is taking a break, but that's not true,” Leavitt says. “Up to 80% of UV can get through the clouds, and we see people come in with sunburns all the time when it's cloudy.”

The great thing about the UV index, though, is that it already compensates for this, and the number you get still reflects any cloud cover.

(MORE: When it's cloudy, you can still get a sunburn)

Leavitt explains that if you’re seeing any shadows or if it's bright enough to see outside, it's bright enough to need some sort of sun protection. 

What should I do if the UV index is high?

Once that UV index hits a level three or above, sunscreen is definitely the first step.

“But I also like to layer that with just what I like to call effortless protection, which is hats and clothes, being in the shade, that kind of thing,” Leavitt said.

And it’s important to keep an eye on the index throughout the day.

“As the day goes on, and my kids and I are out at the pool or at the beach,” she said. “I remind myself and my husband to recheck our UV index on the Weather Channel app because it changes.”

Content writer Ada Wood enjoys exploring the stories that science and climate teach us about our natural world and how it influences the way we live in it.