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Optical Radiation

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Optische Strahlung

What is the UV Index?

The UV Index (UVI) is a measure of the maximum level of UV radiation intensity relevant to sunburn, induced by the sun on a horizontal surface at a given day.

It has been stipulated on an international level to specify the UVI in whole numbers from 1 to 10. Higher values are indicated as 11+. This agreement is respected when BfS publishes the UVI forecast and the estimated daily values of the UVI on the BfS web pages. However, intraday UVI values, or current UVI values are frequently published, sometimes even with decimal places. Although this is not correct according to the UVI definition, it may be helpful to illustrate the radiation exposure occurring at the moment or in the course of a day.

UVI – a standardized quantity worldwide

A UVI of 7 in Germany represents the same as in Kenia or Canada. At the equator the UVI on sea level is about 12 with cloudless skies. In Germany, the UVI reaches values of up to 8 in summer, or even more at high altitudes of Southern German montane areas. The higher the UVI value, the less time it takes for unprotected skin to exhibit sunburn.

UVI – influencing factors

Since the UVI mainly depends on the sun elevation, it varies most heavily with the season and geographic latitude. The total ozone concentration in the atmosphere (not to be confused with the ground-level ozone concentration in the case of "summer smog"), the sky cover (cloudiness) and the altitude of a location also play a part. Slight sky cover reduces the UVI only marginally. Particular sky cover situations may even cause the UVI to increase as a result of additional stray radiation, compared to the UVI obtained for cloudless skies.

Reflexion increases intensity

The UVI always refers to a horizontal area, and reflexions on sand and snow, therefore, are only of minor influence on the UVI level when the reflecting area is below the reference level on which the incident UV radiation is measured, since the reflected radiation is not recorded by the measurement system in this case. However, radiation exposure on snow, on water and at the beach on lucid sand areas is mostly higher than specified by the UVI, due to the reflected UV radiation. Particular attention should therefore be paid to sufficient sun protection in such situations.

UV Index and recommended protection measures
UV-IndexExposureProtection measures
1 - 2lowNo protection required.
3 - 5moderate

Protection required:

6 - 7high
8 - 10very high

Extra protection:

  • avoid being outside during midday hours!
  • make sure you seek shade!
  • shirt, sunscreen, sunglasses and hat are a must.
11 and higherextreme
State of 2018.04.11

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