What Does the UV Index Really Mean?
We get sunlight every day, but some days the UV rays are stronger than others. You know this intuitively if you’re prone to sunburn: you probably don’t wear sunscreen if you’re going for a walk in the winter, but put it on for an afternoon hike in the summer. The UV index is a way of gauging the strength of the sun’s rays rather than relying on intuition.
What is UV Index?
The UV Index measures the ultraviolet (UV) radiation from the Sun at Earth’s surface and serves as an indicator of potential damage to our skin from the sun’s rays.
UV radiation and therefore UV index values are higher when the sun is at a higher angle to the sky. This means that outside the tropics, the UV index is highest in the summer at noon. (The angle of the sun is less in winter, even in the middle of the day.)
Other factors include:
- Altitude (ultraviolet is stronger at higher altitude)
- Cloudiness (the thinner the clouds, the more UV penetrates)
- Latitude (the farther you are from the equator, the less UV reaches the ground)
- Ozone (which can absorb ultraviolet light; the amount of ozone in the air depends on weather conditions)
- Reflection from the ground (for example, ultraviolet radiation can be reflected from snow)
The UV Index takes into account both UVA radiation (which can cause thickening and wrinkling of the skin and cancer risk) and UVB radiation (which is known to cause sunburn and sunburn, and may also contribute to cancer risk). The SPF numbers on a sunscreen refer to its UVB protection, and the fine print describing the sunscreen as “broad spectrum” indicates that it also protects against UVA rays.
How to find out the UV index?
Because the UV index varies with the season, time of day, and weather conditions, it is often reported along with the weather. If your local weather report or app doesn’t include it, you can look up today’s UV Index at sites like Project Sunscreen .
Carrot Weather even provides a “sunscreen reminder” notification in the morning if the UV index is predicted to be high later in the day. By default, a reminder is sent if the UV index is above 6, but you can set it to whatever value you want. As someone who is prone to occasional sunburn , I really appreciate this feature.
Even if you don’t check the UV index every day at home, it can still be useful when traveling. I will never forget the time I brought Pennsylvania-appropriate sunscreen (SPF 15) to Wyoming. It did not help. If I had known that I could look at the UV index, I could have avoided the scalding sunburn.
What do the different ratings mean?
You can think of the UV Index as a scale from 1 (very little UV expected) to about 10. In some cases, it can go over 10, with 11+ being defined as “extreme” UV exposure.
Here is how the World Health Organization describes the different levels:
- UV Index 1-2: No sun protection required.
- UV index 3-7: Moderate to severe UV exposure. Sun protection recommended.
- UV index 8 or more: very strong UV radiation. Additional sun protection recommended.
By “protection” WHO means any means of protection from the sun. These include sun-protective clothing and hats, sunscreen, sunglasses, and being able to stay in the shade.
Instead of giving detailed advice (sunscreen at 3, sunglasses at 5), it’s a good idea 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 can put on some sunscreen and wear a hat. But when the UV index is very high, you can change your schedule to stay out of the sun at noon and you will use a high SPF sunscreen and reapply religiously.