![]() There have been instances of very high UV levels under broken clouds, which can enhance UV levels by reflection from their sides. While UV levels may be highest under cloudless skies, they can also be high with cloud cover. "Measurement and Forecasting of Ultraviolet Index in Hong Kong"įactors affect the accuracy of UV index forecast.Both sunburn and tanning are signs of skin damage. Recent studies showed that over-exposure to UVA may lead to darkening, aging and wrinkling of the skin, as well as potential development of skin cancer. In the past, the harmful effect of UVA is often neglected by people as it is much less powerful to induce sunburn than UVB. Effect of UVA radiation on healthĮxcessive exposure to UVB may cause sunburn and is one of the major causes of skin cancer. For the same reason, the difference in UVA intensity between summer and winter is smaller. This phenomenon is more prominent during the few hours after sunrise and before sunset when the sun elevation is low. As a result, the variation of UVA is less than that of UVB during the same time interval. Since the wavelength of UVA is longer than that of UVB, UVA is less scattered by air molecules as compared to UVB. Over 98% of the UV radiation reaching the ground is UVA and less than 2% is UVB. UV radiation reaching the earth's surface consists of UVA and UVB only, as all UVC is absorbed by the atmosphere. UV radiation can be classified according to their wavelength (in unit of nanometre, nm) as UVA (315-400nm), UVB (280-315nm) and UVC (100-280nm). Only limited data validation has been carried out. Therefore, 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. Ultraviolet (UV) radiation comes from the Sun and on reaching the Earth after passing through the atmosphere it may have been modified by a number of factors.For example, the data of 7:15 is the mean UV index of 7:01-7:15 the data of 8:00 is the average UV index of 7:01-8:00. The UV index data is the mean UV index for the past 15 minutes or the past hour. ![]() The forecast will be updated as necessary in case of weather changes, till 10:00 a.m. The Observatory issues an UV index forecast for the next day at 5:00 p.m. ![]()
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