UV index or clear-sky ultraviolet index is the effective UV irradiance reaching the Earth's surface (1 unit equals 25 mW/m2).
The clear-sky UV index is based on the CIE action spectrum for the susceptibility of the caucasian skin to sunburn (erythema) and it is valid for cloud-free conditions at local solar noon.
The ultraviolet region of radiation covers the wavelength range 100-400 nm and is divided into three bands: UVA (315-400 nm), UVB (280-315 nm) and UVC (100-280 nm).
As sunlight passes through the atmosphere, all UVC and approximately 90% of UVB radiation is absorbed by ozone, water vapour, oxygen and carbon dioxide. UVA radiation is less affected by the atmosphere. Therefore, the UV radiation reaching the Earth’s surface is largely composed of UVA with a small UVB component.