What is UV index?   Meaning of UV index 

 Introduction to UV radiation and UV index What is UV index? Meaning of UV index
UV index scale and corresponding exposure categories
Meaning of UV index The UV index is a measure of the effect of solar ultraviolet radiation
on human skin. The higher the UV index, the greater the potential for
damage to the skin and eye, and the less time it takes for harm to
occur. For details of the calculation of UV index, please click here for the
Observatory's technical report "Solar Ultraviolet Index in Hong Kong
1999 ­ 2003" (pages 2­3) [top]
UV index scale and corresponding exposure categories The UV index and the corresponding
exposure level as categorized by the
World Health Organization are shown in
the table on the right. UV Index Exposure Level 0­2 Low 3­5 Moderate 6­7 High 8­10 Very High >= 11 Extreme The higher the UV index, the more likely the damage to skin. For
Hong Kong, UV index can often exceed 10 on a sunny day in the
summer. The figure below shows the diurnal variation of UV index
during a typical sunny day. During the period with clouds and rain, the UV index will be lower.
The effect of showers on the UV Index is shown in the example
below. The showers around noon time drove down the UV index but
it rose to the level of 8 to 9 in the afternoon when the showers eased
off. The Hong Kong Observatory has been measuring and disseminating
UV index since 1999. In 2006, it started to provide forecast of the
maximum UV index for the next day. When the index is measured or
forecast to be 11 or above, the Observatory will also advise the
public to avoid prolonged exposure under the sun. [top]