THE ENVIRONMENT
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since mid-1988. Tide data are collected from nine gauges in the territory and wave data are measured at Waglan Island, for operational warning of sea flooding in Hong Kong. For safety of aviation, wind conditions at the airport and its vicinity are monitored continuously by a network of anemometers.
Weather prediction requires constant meteorological data exchange with other countries through dedicated international meteorological telecommunication circuits. The observa- tory computer system handles messages of about three million characters of coded weather data each day of the year through various links. Weather chart plotting as well as dis- semination of warnings and forecasts have been computerised since 1988.
In recent years, the vagaries of the weather were better understood through experience gained in analysing and interpreting numerical weather prediction models run at the Royal Observatory and major meteorological and research centres. Previously, 24-hour fore- casts were probably the limit for reasonable accuracy. Recently, however, with the use of numerical model outputs, useful weather outlooks beyond the first day were issued when- ever possible and were in the main satisfactory.
The Year's Weather
In 1989, altogether 11 tropical cyclones traversed the South China Sea. Seven of them required the hoisting of tropical cyclone warning signals in Hong Kong. Contributing 439.4 millimetres of rainfall in May, Typhoon Brenda ranked as the 10th-wettest storm - on record. Although the rainfall associated with tropical cyclones amounted to 646.4 millimetres, which was 14 per cent above average, the annual rainfall in 1989 was only 1 944.6 millimetres, 13 per cent below the normal of 2 224.7 millimetres.
Although it rained on 19 days in January, the rainfall amounted to only 23.9 millimetres. This is slightly below the normal figure of 26.9 millimetres for January.
February was a very dry month. The total rainfall was only 2.7 millimetres and is the eighth lowest on record for February.
It was drier than usual in March, with a mean relative humidity of 75 per cent, the fifth lowest on record for the month.
With only seven days in April without rain, the total duration of bright sunshine in the month was 71.1 hours which is the ninth lowest on record for April. A trough of low pressure reached the south China coast during the evening of April 22, bringing showers and thunderstorms to the territory. Rainfall was especially heavy in the New Territories where 25 cases of flooding were reported.
May was a very eventful month in terms of the weather in Hong Kong. Rainfall was much above normal and tropical cyclone warning signals were hoisted for the first time in the year. The approach of Typhoon Brenda necessitated the hoisting of the No. 8 Gale or Storm Signal which had not been raised since August 1986. The monthly total rainfall of 771.9 millimetres is the fourth highest on record for May. Most of the rain in the month was associated with an active trough of low pressure at the beginning of the month and with Typhoon Brenda which affected the south China coastal areas from May 19 to 21. On May 2, 104.8 millimetres of rain fell between noon and 1 pm. This is the highest hourly rainfall ever recorded for May. The heavy rain resulted in widespread flooding and landslips over the territory, bringing severe disruptions to road, rail and air transport. Rainfall of 322.8 millimetres on May 20 associated with Typhoon Brenda is the second highest daily rainfall on record for May. During the passage of Brenda a total of six people was killed, one reported missing, 62 injured and 50 people made homeless. In addition, torrential rain associated with Brenda resulted in 100 cases of landslips and 118 cases of