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UV index forecast
UV index forecast methodology and factors affecting its accuracy

Forecast methodology
Variation of surface UV radiation depends greatly on time of the year and time of the day. It also depends on a number of factors such as cloud cover, ozone concentration, aerosol, rain, fog, haze and so on. The methodology of UV forecast employed by the Hong Kong Observatory involves two steps:
Step 1:
Find out the clear sky UV index using an equation with a number of astronomical factors and ozone concentration as input parameters. This equation was derived using past observation data collected since 1999.
Step 2:
The UV index forecast is then obtained by multiplying the clear sky UV index by several factors, the magnitude of which depends on the forecast cloud amount, cloud type and the presence of rain, fog or haze. Details of the forecast methodology are documented in the Observatory's technical paper "Measurement and Forecasting of Ultraviolet Index in Hong Kong".

 
Factors affect the accuracy of UV index forecast

While UV levels may be highest under cloudless skies, they can also be high with cloud cover. There have been instances of very high UV levels under broken clouds, which can enhance UV levels by reflection from their sides.

There are uncertainties in the accurate forecasting of weather elements such as clouds, rain, fog and haze. Besides, there are further uncertainties associated with where you are and how high. The atmosphere gets thinner as we go up, and absorbs less UV. With every 1000 metres increase in height, the UV levels can increase by up to 12%. The amount of UV received is also influenced by the nature of ground surface. On the beach, sand on the beach can reflect some of the incoming UV, while UV levels at 0.5 metre beneath water can still be 40% as intense as at the water surface. (Reference: "Global Solar UV Index: A Practical Guide" , World Health Organization, 2002)

For the above reasons, apart from referring to the UV index predicted by the Observatory, members of the public are advised to note the sky conditions, their altitude, and their whereabouts regarding the nature of ground surface, and take the appropriate protective measures against UV.


 
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