Skip to main content
Hong Kong Observatory Brand Hong Kong - Asia's world city
GovHK Traditional Chinese Simplified Chinese Search Search Site Map Contact Us
red dot
Print Version
Print Version PDF Version
UV Radiation Information

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.


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
Very High
>= 11

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.





Last revision date: <27 Dec 2012>