THE ENVIRONMENT
294
January and February are cloudier, with occasional cold fronts bringing in cold northerly winds. Temperatures can drop below 10 degrees Celsius in urban areas. Sub- zero temperatures and frost occur on high ground and in the New Territories infrequently.
March and April can be mild and pleasant but humid. Fog and drizzle sometimes disrupt air traffic and ferry services because of reduced visibility.
The months from May to August are hot and humid with occasional showers and thunderstorms, particularly in the morning. Afternoon temperatures often exceed 31 degrees, but at night temperatures generally stay around 26 degrees.
Hong Kong is most likely to be affected by tropical cyclones in September, although tropical cyclones of varying strength are not unusual at any time between May and November. On average, about 31 tropical cyclones form over the western North Pacific and the China Seas every year, and about half of them reach typhoon strength with maximum winds of 118 kilometres per hour or more.
When a tropical cyclone is about 700 to 1 000 kilometres south-east of Hong Kong, the weather is usually fine and exceptionally hot, but isolated thunderstorms sometimes occur in the evening. If the centre of the tropical cyclone comes closer to Hong Kong, winds will increase and rain may become heavy and widespread. The heavy rain may last a few days, and landslips and flooding sometimes cause considerably more damage than the winds.
The mean annual rainfall ranges from about 1 300 millimetres at Waglan Island to more than 3 000 millimetres around Tai Mo Shan. About 80 per cent of the rain falls between May and September. August is the wettest month with an average rainfall of 391.4 millimetres at the Hong Kong Observatory and, on average, rain falls on more than half of the days. The driest month is January with only 23.4 millimetres of rain and six rain days on average.
Severe weather phenomena in Hong Kong include tropical cyclones, strong winter and summer monsoon winds, monsoon troughs and thunderstorms with associated squalls. Waterspouts and hailstorms occur infrequently, while snow and tornadoes are
rare.
Climatological data are given in the Appendices.
The Year's Weather
The year 2002 was the second warmest on record. The mean temperature of 23.9 degrees was 0.9 degree above normal. The first few months of the year was warmer than usual because fewer cold surges had affected Hong Kong. In May and June, the monsoon trough which normally brings a lot of rain to Hong Kong was quite inactive, resulting in more days with sunny weather and hence higher temperatures. While the first half of the year was exceptionally warm, the temperature in the second half of the year was near normal.
The tropical cyclone season started much later than usual, with the first tropical cyclone warning signal issued on August 3. Only three tropical cyclones affected Hong Kong in 2002, about half the normal figure. All three had originated in the South China Sea. During the year, none of those tropical cyclones originating over the western North Pacific entered the South China Sea to affect Hong Kong. This was due largely to the development of an El Niño, which was characterised by above normal sea surface temperatures in the equatorial eastern and central Pacific. The