noun
ultraviolet index; UV index
A measure of the strength of ultraviolet radiation from the sun, commonly used in weather forecasts and health advisories.
今日の紫外線指数は非常に高いので、日焼け止めを塗ってください。
Today's UV index is very high, so please apply sunscreen.
紫外線 refers to ultraviolet radiation itself, while 紫外線指数 is the numerical index measuring its intensity.
Compound of 紫外線 (shigaisen, 'ultraviolet rays') and 指数 (shisū, 'index').