What is the UV index?
The UV index is a measure for the intensity of the UV radiation from the Sun at the surface of the Earth.
Fig. 1: UV index categories as defined by the World Health Organisation
The UV index was created to inform the public about the expected level of UV radiation at the surface of the Earth.
Originally, the UV index was formulated independently in several countries and used in programs for public information about UV radiation. Its definition has later been standardized and published as a joint recommendation by the World Health Organisation (WHO), the World Meteorological Organisation (WMO), the United Nations Environmental Program (UNEP) and the International Commission on Non-Ionizing Radiation Protection (ICNIRP), in order to raise public awareness about the potential detrimental effects on health from solar UV exposure and to alert people of the need to adopt protective measures.
Fig. 2: The yellow line shows the intensity of the solar light at the top of the atmosphere between 280 and 400 nm. The blue line represents the intensity on a horizontal surface at the ground, typically on a clear summer day noon at mid latitude. The line in cyan is the CIE Reference Action Spectrum for Ultraviolet Induced Erythema of Different Human Skin Types. When the spectrum at the ground is multiplied with this action spectrum the result (red line) is the effective irradiation at the ground. These values are integrated between 280 and 400 nm and multiplied by 40 to yield the UV-index (indicated by the red-shaded area).
The UV index is defined as the effective irradiance on a horizontal surface obtained by integrating the spectral irradiance weighted by the Commission Internationale de l’Eclairage (CIE) reference action spectrum for UV-induced erythema on the human skin up to and including 400 nm and normalized to 1.0 below 298 nm (see also Fig. 2).
The UV Index is a number describing the effect of the intensity of the solar radiation on the unprotected skin. The different classes of the UV index and their effect on sunburn are summarised in the table below.
| Color | UV-Index | UV-Intensity | Skin burns |
|---|---|---|---|
| 1 | low | slowly | |
| 2 | |||
| 3 | moderate | easily | |
| 4 | |||
| 5 | |||
| 6 | high | quickly | |
| 7 | |||
| 8 | very high | very quickly | |
| 9 | |||
| 10 | |||
| 11+ | extreme | almost immediate |
Today's UV index at Uccle can be found here.