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UV Index definition

Ozone absorbs so strongly in the UV-C (λ < 280nm) that solar radiation in these wavelengths does not reach the earth's surface. As the wavelength is increased through the UV-B range (280nm < λ < 315nm) and into the UV-A (315nm < λ < 400nm) ozone absorption becomes weaker, until it is undetectable at about 340nm. Figure below shows several spectra including the spectrum of solar radiation measured above the atmosphere. The fractions of solar energy above the atmosphere in the UV-B and UV-A ranges are approximately 1.5%% and 7%% respectively. The figure also shows spectra measured at the ground near mid-day on two very clear days in mid-June. (These two spectra are almost indistinguishable). The fine curve (D) is computed from the extraterrestrial spectrum (A) and the absorption by the average measured ozone amount for the two days. It is above the measured ground-level spectra, but otherwise almost identical to them. One may conclude that for these two clear days, the ground level spectra are mostly the result of ozone absorption with a relatively constant extra attenuation of about 40%% presumably due to air molecules and background aerosols. Figure thus represent the simplest case of ozone determining the ground level solar UV radiation. In general there is a great deal of variability in the UV due to clouds, ground albedo, aerosols and other atmospheric constituents.

Graph of spectral irradiance versus wavelength
The solar spectrum above the atmosphere (A), and at the ground level (B, C), ozone absorption (D), and the erythemal action spectrum (E).

Changes in UV radiation can be expressed in a variety of ways, and the question of which particular measure is most relevant to a given biological effect is often complex and difficult to resolve. A similar question refers to what wavelengths should be considered. An action spectrum, for a particular biological effect, expresses the effectiveness of radiation at each wavelength as a fraction of the effectiveness at a certain standard wavelength. For UV induced erythema (sunburn), the action spectrum adopted by most international organizations is the CIE (Commission Internationale de l'Éclairage, International Commission on Illumination) action spectrum (E), using the method described by McKinlay, A.F. and B.L. Diffey ("A reference action spectrum for ultraviolet induced erythema in human skin, in Human Exposure to Ultraviolet Radiation: Risks and Regulations", pp. 83-87, Elsevier, Amsterdam, 1987):

E(λ) = 1 when 250 nm < λ < 298 nm,
E(λ) = 100.094(298-λ) when 298 nm < λ < 328 nm,
E(λ) = 100.015(139-λ) when 328 nm < λ < 400 nm.

For a fairly wide range of atmospheric conditions, the CIE weighted irradiance changes by approximately 1.2%% for a change of 1.0%% in the ozone value.

The UV-Index itself is an irradiance scale computed by multiplying the CIE irradiance in watts m-2 by 40. Thus the clear sky value at sea level in the tropics would normally be in the range 10-12 (250-300 mWm-2) and 10 is an exceptionally high value for northern mid-latitudes. This scale has been adopted by the WMO and WHO and is in use in a number of other countries. UV intensity is also described in terms of ranges running from low values (0-2) to medium (3-5), high (6-7), very high (8-10) and extreme (11+).

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