What Is the UV Index?
Updated 2026-01-10
How the scale works
The UV index is an open-ended scale, typically shown from 0 upward, that measures the strength of ultraviolet radiation from the sun at a given place and time. Values of 0–2 are considered low risk, 3–5 moderate, 6–7 high, 8–10 very high, and 11 or above extreme. The higher the number, the less time it takes for unprotected skin to start burning.
What drives a high UV index
UV index is driven mainly by the sun's angle in the sky, which is why it peaks around midday and is highest closer to the equator and at higher altitudes. Cloud cover reduces UV somewhat but doesn't block it entirely — it's possible to get a significant UV dose on an overcast day, especially with reflective surfaces like snow, sand or water nearby.
Protecting yourself
At UV index 3 and above, sun protection is generally recommended: seeking shade during peak hours, wearing protective clothing and a hat, and using sunscreen on exposed skin. At very high or extreme levels, minimizing time outdoors during the middle of the day is the most effective protection, since burning can occur in a matter of minutes.