Bulletin updated at 00:01HKT 05/Dec/2008

The maximum UV Index for today will be about 5.

• UV Index recorded today

• Ultraviolet radiation (UV) comes from the Sun and on reaching the Earth after passing through the atmosphere it may have been modified by a number of factors(please refer to here for details).
• The factors include: cloud amount, cloud type, rain, fog, haze, ozone, altitude and ground surfaces. In respect of clouds, 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.
• Regarding altitude, the atmosphere gets thinner as we go up, and absorbs less UV. With every 1 000 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.
• Thus, while there are uncertainties in the accurate forecasting of weather elements such as clouds, rain, fog and haze, there are further uncertainties associated with where you are and how high. For these 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.
• The Observatory issues an UV Index forecast for the next day at 5:00 p.m. every day. The forecast will be updated as necessary in case of weather changes, till 10:00 a.m. the next day.

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