What is the UV Index?

The UV-Index

The UV Index describes the expected daily peak level of the erythemal UV irradiance at ground level.

Similar to the Richter scale in earthquakes, the UV Index is to be understood as an open-top scale, which gives guideline values for the UV irradiance. The higher the UV Index, the higher the UV irradiance and the faster a sunburn can occur when skin is not protected.

The UV Index has been defined by the WHO and is uniform worldwide. Therefore, for example, a UV Index of 7 in Germany means exactly the same as the same value in Kenya or Canada.

Influencing factors

The UV Index depends mainly on the position of the sun; it therefore varies most with the season, the time of day and the geographic latitude. The total ozone concentration in the atmosphere, the cloudiness and the altitude of a location also play a role.

Light sky cover hardly reduces the UV Index. In contrast, at particular sky cover situations it can even increase in the short term due to additional scattered radiation, as compared with the UV Index with a clear sky.

In Germany, values of 8 to 9 are reached in summer, and even 11 at high altitudes in the southern German mountain regions. Values of 12 and higher can occur at the equator.

UV Index as a guide for sun protection measures

The UV Index scale is divided into different areas, for which different protection recommendations apply. The UV Index is therefore not only a measure of the expected UV exposure, but gives also recommendations as to what sun protection measures should be taken at what UV Index values.

UV Index - measured or modelled

The UV Index is based either on values measured at ground level or on satellite measurement data of the UV irradiance. In the latter case, the UV Index is calculated, i.e. modelled. For Germany, BfS publishes measurement data from the UV monitoring network and UV Index forecasts for 10 forecast areas based on these network data. The German Weather Service (DWD) publishes modelled forecasts of the UV Index based on satellite data.

The UV Index during the day

The UV Index can also be presented as a variable quantity over the day. In this case, the actually measured daily increasing and decreasing erythemal UV irradiance is displayed as UV Index. Strictly speaking, this is not correct according to the UV Index definition, but this helps to illustrate the UV irradiance changing over the day. The daily UV Index variation at the individual stations of the UV monitoring network is published at Current daily graphs and at the BfS Geoportal.