It is an exponential expression of the amount of radiation in the ultraviolet region that reaches the surface when the sun is at It's maximum, and provides a forecast of the expected risk of overexposure. The UV index is usually expressed as a number from 1 to 11, with the higher the number, the stronger the UV light and the greater the effect on the skin. A low UV index (1-2) means the risk of UV rays is low, while a high UV index (9-11) indicates that the skin needs protection when exposed to UV rays.