Lightning, hailstorms ... and a blanket of smog

Hong Kong experienced particularly tumultuous weather yesterday as a hot, hazy, smog-ridden early afternoon gave way to thunderstorms with major lightning - even hailstorms - in some districts.

The day began fine. Early readings of the air pollution index in Tung Chung were at around 25, considered low, but the hot weather caused by the presence of Typhoon Kaemi near Taiwan caused it to soar to 136 by 4pm, according to Environmental Protection Department officials.

In Central and Western District, the index reached a high of 113 around 3pm before a heavy downpour caused it to plummet to one-third of the level by 5pm.

Sai Kung residents saw 1,550 lightning strikes in two hours from 3pm and also experienced a 15-minute hailstorm.

Lee Kwok-lun, scientific officer with the Hong Kong Observatory, said public reports told of hail the size of 'a fingernail'.

'Because the thunderstorm is a system of convection, it is very strong and will also lead to the formation of hailstones in the atmosphere,' Mr Lee explained. 'It is not that rare. Between 1967 and now, we have had more than 30 hailstorms in Hong Kong.'

Convection is the term meteorologists use to describe the rise of heat and moisture, especially by updrafts and downdrafts in an unstable atmosphere.

Typhoon Kaemi, which is heading towards the mainland coast, caused subsidence or 'sinking air' in Hong Kong, Mr Lee said.

'There are very few clouds in a subsidence area and so it gets very sunny and hot,' he said. 'Because it was too hot, when the temperature over the landmass got very high, it triggered some showers and thunderstorms near 4pm.'

The maximum temperature recorded at the observatory was about 34 degrees Celsius, but Tuen Mun, Sai Kung and Chek Lap Kok saw highs of 37 degrees.

An EPD spokesman added that the light wind conditions before the thunderstorms 'hindered the dispersion of pollutants in Hong Kong and the Pearl River Delta Region and the sunny and hot weather conditions promoted the photochemical formation of ozone'.

The Department of Health warned people engaged in outdoor work or activities to take care against heat stroke by drinking plenty of water and avoiding caffeine and alcohol.

The Home Affairs Department opened nine temporary night heat shelters.

The EPD spokesman said the 'fine and very hot' weather was likely to continue today, causing the air pollution levels to remain high. Air quality in the area should improve gradually as the typhoon and its strong winds hit the mainland early tomorrow, he said.