The UV index is a measure of the solar UV (ultraviolet) intensity at the Earth's surface relevant to the effect on human skin. The skin-damaging UV radiation is governed by the erythemal action spectrum. This spectrum has been adopted by the International Commission on Illumination (CIE) to represent the average skin response over the solar UV spectrum.
The standard way to calculate UV index, recommended by the World Meteorological Organization (WMO) and the World Health Organization (WHO), is to measure the intensity of solar UV radiation at different wavelengths up to 400 nanometres. Multiply these UV intensities by the weights at the corresponding wavelengths in the erythemal action spectrum to reflect the human skin's response to each wavelength. Sum up the products above to obtain the total erythemally weighted UV intensity in mW/m2. Multiply the total erythemally weighted UV intensity by 0.04 to obtain the UV index, e.g. if the maximum intensity of the 15-min average radiation at mid-day is 200mW/m2, the UV index will be 8.
The value of UV Index is closely related to the solar zenith angle, ozone amount, clouds, suspended particulate and height.
UV Index indicates the strength of the UV radiation. The higher the number, i.e. the stronger the UV radiation, the more likely the damage to skin within short period of time
For the purpose of simplicity, UV index is divided into different levels as below:
Download "Global Solar UV Index - A Practical Guide"
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