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. The highest ever hourly rainfall recorded at the Hong Kong Observatory is 109.9 millimetres which occurred during the rainstorm on 8 May 1992.
Rainstorms often bring about serious road flooding and traffic disruptions.
Flooding occurs when the rainfall rate is so large that natural or artificial drainage is insufficient to drain away the fast accumulating water on the ground. Floods are usually fairly transient in the urban area but may last up to a few hours in those rural areas with large catchment and gentle slopes, such as the plains in the northern New Territories. Hong Kong is mostly of hilly terrain. During heavy and prolonged rain, natural slopes may fail and therefore pose considerable threat to people living on the foot-hills and on these slopes. Due considerations are given to both the rain intensity and duration when landslip warning, rainstorm warning signals and special announcements on flooding are issued.