When a tornado passes directly overhead the wind and pressure effects result in great forces which could shatter weak structures. Five tornadoes are known to have occurred in Hong Kong. Waterspouts are rather more common and were reported in local waters on 20 days during the period 1961-1997. There have been reports of waterspouts capsizing small boats in nearby waters and damaging coastal facilities in Hong Kong as they made landfall. Hail sometimes forms in well-developed thunderstorm clouds.
Hailstones are hard pellets of ice which are usually a few millimetres in diameter.
Larger hailstones have alternate rings of clear ice and frost. Large hailstones can damage crops, particular young fruits and vegetables, and can break windows, glass houses and car windscreens. During the period 1961-1997 there were 28 days in which hail was reported in Hong Kong. Although heavy rain is not uncommon at any time of the year in Hong Kong, it occurs most often during the summer months. Indeed, close to 80 per cent of the annual rainfall occurs between May and September.