Buzz
While sunlight graces us daily, its ultraviolet (UV) intensity varies. If you're prone to sunburns, you know this instinctively: you might skip sunscreen on a chilly winter walk, but you'll certainly apply it before a midday summer hike. The UV index quantifies how potent the sun's rays are, offering more than just a gut feeling.
What exactly is the UV index?
The UV index quantifies the ultraviolet radiation from the sun that reaches the Earth's surface and serves as a gauge for the risk of skin damage from prolonged exposure.
UV radiation, and consequently UV index values, are stronger when the sun is positioned higher in the sky. For regions outside the tropics, this typically means that the UV index peaks at midday during summer. (In winter, even midday sun has a lower angle.)
Other influencing factors include:
Altitude (UV radiation increases with elevation)
Cloud cover (thinner clouds allow more UV to pass through)
Latitude (the farther from the equator, the less UV reaches the surface)
Ozone (which absorbs UV radiation; its concentration varies depending on weather)
Surface reflection (UV can bounce off surfaces like snow)
The UV index considers both UVA radiation (which contributes to skin aging, thickening, and cancer risks) and UVB radiation (which causes sunburns, tans, and also raises cancer risk). Sunscreen SPF numbers reflect UVB protection, while the term “broad spectrum” indicates that the sunscreen also shields against UVA rays.
How can you check the UV index?
The UV index fluctuates throughout the year, day, and depending on weather, which is why it’s commonly included in weather reports. If your local forecast or app doesn’t show it, you can always check today’s UV index on websites like Project Sunscreen.
Carrot Weather even sends a “sunscreen reminder” notification in the morning if a high UV index is predicted later in the day. The default setting triggers the reminder when the UV index reaches 6 or higher, but you can customize it to your preferred threshold. As someone who often gets accidental sunburns, I find this feature incredibly helpful.
While you might not check the UV index daily at home, it can be a lifesaver while traveling. I’ll never forget the time I packed Pennsylvania-level sunscreen (SPF 15) for a trip to Wyoming. It wasn’t nearly enough. Had I known about the UV index, I could have avoided a painful sunburn.
What do the different UV index values indicate?
Think of the UV index as a scale ranging from 1 (indicating minimal UV exposure) to about 10. In some situations, it can exceed 10, with 11+ categorized as “extreme” UV exposure.
The World Health Organization outlines the different levels of UV exposure as follows:
UV index between 1-2: No sun protection necessary.
UV index between 3-7: Moderate to high UV exposure. Sun protection is advised.
UV index of 8 and beyond: Very high UV exposure. 'Extra' sun protection is advised.
When the WHO mentions 'protection,' they are referring to various methods to shield yourself from the sun. This includes wearing sun-protective clothing and hats, applying sunscreen, wearing sunglasses, and staying in the shade.
Instead of providing specific instructions (e.g., sunscreen at index 3, sunglasses at index 5), the recommendation is to stay mindful of the UV index and be increasingly cautious with sun protection as the numbers rise. For instance, during moderate levels, applying sunscreen and wearing a hat might suffice. But at very high levels, it might be best to adjust your schedule to avoid midday sun, use high-SPF sunscreen, and reapply it frequently.
