UV radiation monitoring: UV index and UV dose

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UV radiation
monitoring:

UV index
UV dose

Main data product pages: Erythemal UV index  |  Daily erythemal UV dose

 

 
Introduction to
UV radiation
monitoring

UV radiation
UV & health
UV index
forecasts of
UV index
UV dose
example:
1 June 2002
links

 

UV index

The erythemal UV index -- usually simply called the UV index (UVI) -- is an estimation of the UV levels that are important for the effects on the human skin, where 1 unit equals 25 mW/m2. It is usually given for local solar noon, when the Sun is highest in the sky, and it is valid for clear-sky conditions: effects of clouds shielding part of the UV radiation are not taken into account.

The erythemal UV index is an artificial quantity derived from the erythemal irradiance, which is an integration of the UV irradiance at the ground weighted by the CIE spectral action function. The CIE action spectrum is a model for the susceptibility of the caucasian skin to sunburn (erythema). It is proposed by McKinlay & Diffey (1987) and adopted as a standard by the Commission Internationale de l'Éclairage (International Commission on Illumination).

Of the global UV radiation at the ground, 94% is UV-A, 6% is UV-B.
Of the erythemal UV irradiance, however, 17% is UV-A, 83% is UV-B.

CIE action spectrum
The CIE action spectrum is a model for the susceptibility of the caucasian skin to sunburn (reddening of the skin; erythema). It was proposed by McKinlay & Diffey (1987) and adopted as a standard by the Commission Internationale de l'Éclairage (International Commission on Illumination).

===> Some more info on action spectra

UV spectrum in De Bilt
Ultraviolet spectrum measured with the Brewer Spectrophotometer at De Bilt (Netherlands) on 1 June 2002, a completely cloud-free day. Also drawn is the CIE erythemal action spectrum. The multiplication of these two gives the erythemal UV spectrum, and the surface below this graph (shown in yellow) is the UV index. The value of the UVI from this measurement is 6.3.
figure by Marc Allaart, KNMI, De Bilt

Ground-based measurements such as the one from the previous graph give UVI values for these specific sites at these specific moments in time only. In order to obtain the UVI for all locations, it must be computed from total ozone amounts, in combination with the solar zenith angle (SZA): the angle from which the Sun shines. The UV index is usually given at noon of the local solar time: the moment when the Sun is highest in the sky (i.e. in the zenith); this moment is therefore not noon of the local time zone.

Ground-based measurements of the UVI and simultatiously measured total ozone column (TOC) values have resulted in a parametrisation of the UVI as function of TOC and SZA, both at local solar noon. The TOC at local solar noon is determined from satellite observations in combination with data assimilation, which uses meteorological fields (wind, temperature, pressure) to obtain a global ozone field at local solar noon. The SZA at local solar noon depends on the latitude and the day of the year.

The UV index thus is a measure for the amount of UV radiation valid for clear-sky conditions and at local solar noon. This does not say much about the erythemal UV dose: the total amount of UV that actually can reach the human skin during a day.

 

 


last modified: 26 June 2017
data product contact: Jos van Geffen & Ronald van der A
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