Sunlight makes all life possible, but there’s a major downside. Along with visible light, we get doused with ultraviolet rays, which are higher energy than visible violet light — high enough to harm our skin, eyes, and immune system. They can even damage DNA and cause skin cancer. Major bummer for us sun lovers.
While some insects and birds can see UV, we have no way to see or feel the harmful rays out there. (When you’re hot outside, that’s courtesy of infrared rays, which are lower energy than visible red light.)
Since we can’t see it, we need to rely on the UV index (weather and specialty sites have that info every day). But there's a lot more that affects UV exposure — there's altitude, snow, water, sand, shade, and more.