What is the UV Index?

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what's the uv index
what's the uv index

What is the UV Index? Sunburn happens faster when your skin is not protected. The UV Index is a guide to answering the question what sun protection measures should be taken and when. The link between UV irradiance and SPF, or sunscreen protection factor,

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.

The UV Index describes the expected daily peak level of the erythemal UV irradiance at ground level.The higher the UV Index, the faster a sunburn can occur when skin is not protectedThe UV Index is a guide to answering the question what sun protection measures should be taken and when.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 on the time of day, the season, the position of the sun and altitude. Total ozone concentration and cloudiness also play a role. In contrast, light sky cover lowers the UV Index. Black or dark sky coverage increase it in short bursts. In Germany, summertime values of 8 to 9 are reached regularly; at high altitudes in southern German mountain regions, values of 12 and higher can occur as well.

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The UV Index is primarily dependent on the position of the sun; it thus varies most with the season, time of day, and geographic latitude. The total ozone concentration in the atmosphere, cloudiness, and altitude of a location also play a role. Light sky cover barely reduces the UV Index’s value. In contrast, at particular sky cover variations it can even increase in the short term due to additional scattered radiation as compared with a clear sky. In Germany, values of 8 to 11 occur in summertime and values of up to 12 happen at high altitudes in some mountainous regions.

UV Index as a guide for sun protection measures

UV Index is used to determine how much sunlight we can expect to get, and it helps us determine the number of hours of protection needed. Although UV Index is not a measure of exposure, it gives recommendations on sun protection measures.

The UV Index is not just one number, it’s split into four categories and provides different recommendations depending on the level. So, not only does it estimate how much exposure you’re going to get, but it also tells you what sun protection measures should be applied at what level.

UV Index – measured or modelled

The UV Index is based either on measurements taken at ground level or satellite data. In the latter case, the UVI is calculated and published. For Germany, the BfS publishes measured data from a UV Monitoring Network and the forecasts for 10 forecast districts are published based on those data. The German Weather Service (DWD) publishes modeled forecasts of the UVI which come from satellite data.

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The UV index is based either on values measured at ground level or on satellite data. Based on the former, the UV Index is modeled, i.e. determined by a formula that takes into account factors such as cloud cover and time of day. For Germany, we publish UV monitoring network data and our UV Index forecast for ten forecast regions according to these network measurements. The German Weather Service (DWD) publishes modelled forecasts of the UV Index which are determined by data from satellites.

The UV Index during the day

The UV Index is a type of scale that describes how much solar radiation or the intensity of sunlight is hitting an area. This is usually shown as a quantifiable quantity. With the daily increasing and decreasing erythemal UV irradiance changes over time, this measure also helps to illustrate this change. It’s done with the variability at different locations of the UV monitoring network, which is published at UV-Index Aktuell (in German) and at the BfS Geoportal.

The UV Index also has a variable quantity that can change day by day. This helps to show how the changing UV irradiance affects people each day. The daily UV Index variation is reported at UV-Index Aktuell and with the BfS Geoportal.