Forecasters are now issuing day-ahead warnings on days of high risk of sunburn or other health hazards.
The Hong Kong Observatory yesterday started to release forecasts of the maximum ultra-violet (UV) index readings in urban areas for the next day. But assistant director Lee Boon-ying said the accuracy of the new service was only about 70 per cent.
'This means that for every 10 forecasts we make, seven are correct ... but we think there's a need to release such information,' Dr Lee said, adding forecasts would get more accurate when more figures were gathered.
'It's not 100 per cent reliable because there were many factors affecting the prediction. If cloud appears, it would affect the index.'
If the index is expected to hit 11 or above - the highest rating of 'extreme' on the five-category index - the observatory will recommend precautions.
The maximum UV index for today is forecast to reach 10, or 'very high'.
Dr Lee said forecasts of the UV index, which measures the level of radiation from the sun, were based on predictions of the amount and the thickness of cloud, the sun's position and the amount of haze and ozone expected to be present in the atmosphere. He said sand on beaches, water surfaces and glass walls raised UV radiation by reflection.