The standardized scale that tells people how strong the Sun’s ultraviolet radiation is at Earth’s surface is the UV index.
The UV index was developed to communicate the risk of sunburn and other ultraviolet effects in a simple numerical format. It represents the intensity of sunburn-producing ultraviolet radiation, weighted according to how strongly different wavelengths affect human skin. Higher values indicate greater risk and a shorter time before unprotected skin may be damaged.
The World Health Organization, World Meteorological Organization, United Nations Environment Programme, and International Commission on Non-Ionizing Radiation Protection helped promote a consistent international format in the 1990s. Weather services commonly publish daily forecasts showing the expected maximum value.
The UV index is not a temperature scale and does not measure visible sunlight. Clouds can reduce ultraviolet levels, but they do not always block them completely. Altitude, latitude, season, ozone concentration, and reflection from snow, sand, or water can also change exposure.