Ultraviolet index

The UV index ( UVI ) indicates the sunburn effective solar irradiance and varies with the clouds, with the sun ( ie with latitude, day and year ), with the thickness of the ozone layer, as well as with height. In the German space noon UV index between five and eight are common in the months of May to August. At 10 and 16 clock, the values ​​are typically about half as large.

Gradation and protection

The UV index is also frequently reported in the weather forecasts. It is to be expected for that day maximum. The various UVI values ​​are recommendations for the protection against UV radiation associated with ( behavioral measures, sunscreen ). In consideration of UVI and individual photosensitivity ( skin type, acquired self-protection ) man should be able to select the most appropriate protection measures:

Since it is not specified,

  • Whether with consistent application of the recommended protective measures a permanent outdoor living without skin damage is possible or
  • Whether additional periods of stay must be limited in the sunshine,
  • Whether the same measures are also required for very short periods outdoors
  • And because the skin type is excluded,

Can be used the UV index by man only as a comparative indication, similar to how he has learned to estimate at which weather data (temperature, wind speed ) which prevents clothes frostbite during a period of residence and outdoor activity.

If the individual minimum erythema dose (MED, expressed in Jm- 2) a person known to be with the UV Index ( UVI ), the time to calculate (in minutes) to reach a first currently visible skin redness, sunburn time, as follows: Amount MED divided by a half times the UV index. In a MED of 240 Jm- 2 and a UV index of 8, for example, results in a sunburn time of 20 minutes. Because skin damage already occurs at a dose that still does not produce a sunburn, an unprotected outdoors should be significantly shorter than the sunburn time remain.

UVI values ​​during the year

Typical UVI values ​​of selected locations in the course of a year (maximum values ​​, ie at noon, each 21 a month ):

  • The intensity of UV radiation depends primarily on the sun angle to the earth and thus strongly influenced by latitude, year and time of day from.
  • Extreme UVI values ​​above 10 are found only near the equator and in areas with reduced ozone layer.
  • Depending on the cloud cover also significantly lower values ​​may result.

Definition

The UV index is an internationally agreed benchmark. The spectral irradiance of a horizontal surface is integrated weighted with the action spectrum defined by the CIE:

The spectrum of radiation of 298 nm and rated short-wave factor 1, wave radiation increasingly less radiation of 328 nm is about even with factor 0.00151356 and radiation of 400 nm after all, only by a factor of 0.0001216186. Thus, values ​​are usually to about 300 mW/m2. To get the UV Index as a handy dimensionless quantity, is still divided by 25 mW/m2.

Finally, the appropriate warnings for maximum UV index one day is defined as the highest 30 -minute average.

The weight function takes into account the strongly wavelength dependent damaging effect of radiation. While it is in the range of 250 to 400 nm defined ( CIE S007/G-1998 document ), but since the ultraviolet intensity at the earth steeply beyond 300 nm, it is sufficient for the application of UV index from the intensity of radiation in range 290 to 400 nm to measure.

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