What the UV Index Actually Means

We get sunlight every day, but some days the ultraviolet rays are stronger than others. You know this intuitively if you’re prone to burning: You probably skip the sunscreen if you’re going for a walk in the winter, but slather it on for a midday summer hike. The UV index is a way of putting a number on how strong the sun’s rays are, rather than relying on a gut feeling.

What is the UV index?

The UV index measures ultraviolet (UV) radiation from the sun at the level of the Earth’s surface and is meant as an indicator of the potential for sun damage on our skin.

Ultraviolet radiation, and thus UV index numbers, are higher when the sun is at a higher angle in the sky. Outside of the tropics, this means the UV index is highest during midday in the summer. (The angle of the sun in winter is shallower, even in the middle of the day.)

Other factors include:

• Altitude (UV is stronger at higher altitude)
• Cloud cover (the thinner the clouds, the more UV penetrates)
• Latitude (the further you are from the equator, the less UV reaches the earth)
• Ozone (which can absorb UV; the amount of ozone in the air varies with the weather conditions)
• Ground reflection (UV can reflect off of snow, for example)

The UV index accounts for both UVA radiation (which can cause skin thickening and wrinkling, as well as cancer risk) and UVB radiation (which is known for causing sunburns and suntans, and can also contribute to cancer risk). The SPF numbers on sunscreen refer to its protection against UVB, and the fine print describing a sunscreen as “broad spectrum” indicates that it also protects against UVA rays.

How do you find out the UV index?

Since the UV index changes based on the time of year, time of day, and weather conditions, it’s often reported alongside the weather. If your local weather report or app doesn’t include it, you can look up today’s UV index on sites like Project Sunscreen.

Carrot Weather even provides a “sunscreen reminder” notification in the morning if the UV index is forecast to be high later in the day. The default is to send a reminder if the UV index is above 6, but you can set that to whatever value you want. As a person who is prone to accidental sunburns, I appreciate this feature very much.

Even if you don’t look up the UV index every day at home, it can be useful when you’re travelling. I’ll never forget the time I brought Pennsylvania-appropriate sunscreen (SPF 15) to Wyoming. It did not do the job. If I had known I could look up the UV index, I could have been spared a scorching sunburn.

What do the different ratings mean?

You can think of the UV index as a scale from 1 (very little UV radiation expected) to roughly 10. In some cases, it can go higher than 10, with 11+ being defined as “extreme” UV exposure.

Here is how the World Health Organisation describes the different levels:

• UV index of 1-2: No sun protection is required.
• UV index of 3-7: Moderate to high UV exposure. Sun protection is recommended.
• UV index of 8 or more: Very high UV exposure. “Extra” sun protection is recommended.

By “protection,” the WHO is referring to any and all ways you might protect yourself from the sun. Those include sun-protective clothing and hats, sunscreen, sunglasses, and choosing to stay in the shade.

Rather than giving granular advice (sunscreen at 3, sunglasses at 5) the recommendation is to be aware of the UV index and be more vigilant about protecting yourself from the sun as the numbers get higher. For example, at a moderate level, you might apply some sunscreen and throw on a hat. But when the UV index is very high, you might want to rearrange your schedule so you won’t be out in the sun at midday, and you’ll use a high-SPF sunscreen and reapply it religiously.