UV Index Today
What Is the UV Index? A Plain-English Guide
The UV index is a single number that tells you how strong the sun's ultraviolet radiation is right now. The higher it is, the faster your skin can burn.
Ultraviolet, or UV, is a type of energy that comes from the sun. You cannot see it or feel it directly, which is what makes it tricky. You can pick up a serious sunburn on a cool, breezy day because the temperature you feel has almost nothing to do with how much UV is reaching your skin.
The UV index solves that problem. It converts the invisible strength of the sun into one easy number, so you can decide at a glance whether you need sunscreen, a hat, or shade.
How the scale works
The scale starts at 0 and climbs upward. In practice, most places on Earth top out somewhere around 11 or 12, though high-altitude and tropical spots can go higher. The World Health Organization groups the numbers into five bands:
- 0 to 2, Low. Little risk. Most people can be outside safely.
- 3 to 5, Moderate. Take basic precautions like sunscreen and a hat.
- 6 to 7, High. Protection is needed. Limit midday sun.
- 8 to 10, Very High. Skin burns fast. Be careful and cover up.
- 11 and above, Extreme. Take every precaution.
For a level-by-level breakdown, see the UV index scale guide.
What changes the UV index
The reading is not the same everywhere or all day. A few things push it up and down:
- Time of day. UV is highest when the sun is directly overhead, around solar noon. Early morning and late afternoon are much gentler.
- Season. Summer sun sits higher in the sky, so UV is stronger than in winter.
- Latitude. Places near the equator get more intense UV year round.
- Altitude. UV rises roughly 10% for every 1000 meters you climb.
- Clouds. Thick cloud lowers UV, but light cloud lets most of it through.
- Reflection. Snow, water, and sand bounce UV back up at you, adding to your exposure.
How to use it day to day
The habit is simple. Before you head out, check the current UV index for your location. If it is 3 or higher, put on sunscreen and grab a hat. If it is 8 or higher, try to stay out of direct sun in the middle of the day. That is really all there is to it.
Frequently asked questions
What does the UV index measure?
The strength of the sun's ultraviolet radiation at ground level, on a scale from 0 upward.
What is a dangerous UV index?
8 or higher is very high and burns unprotected skin quickly. 11 and above is extreme.
Can the UV index be high in winter?
It is usually lower in winter, but snow reflects UV and high altitude raises it, so ski slopes can see surprisingly strong readings.