THE ENVIRONMENT
tropical cyclones approach from the south. For the safety of aviation, wind conditions at the airport and its vicinity are monitored continuously by a network of anemometers.
Rainfall information has always been important in Hong Kong because of its relation to water supply and also because of the susceptibility of slopes to landslips in heavy rain. A dense network of about 147 raingauge stations over Hong Kong provides comprehensive coverage for information on rainfall intensity and distribution. Sixty-nine of these stations are equipped to provide up-to-the-minute information essential for assessing the likelihood of flooding and landslips. Rainfall intensity within 512 kilometres of Hong Kong is monitored with a digital radar system which also produces objective short-term rainfall estimates at selected places. A lightning detection system locates cloud-to-ground lightning within a range of about 130 kilometres and a spherics recorder is used to register thunder- storm activity within about 100 kilometres.
Besides land-based observations, upper-air atmospheric conditions are measured by radiosondes carried by balloons launched at King's Park Meteorological Station. For an overview of weather systems, hourly high-resolution satellite cloud pictures are received in Hong Kong from the Japanese Geostationary Meteorological Satellite. The availability of hourly imageries greatly facilitates the monitoring of weather systems.
For the real-time monitoring of floods in the north-western New Territories, water-level and rainfall information at Kam Tin has been relayed directly to the observatory headquarters. Tide data are collected from eight gauges in the territory for operational warning of sea flooding in Hong Kong. Numerical models are also used to predict the likelihood of storm surges in tropical cyclone situations. Storm surge warnings will be issued if a significant rise in sea level is expected. Wave information for forecasting the sea state of local waters is obtained in real time from a wave recorder at Waglan Island.
Weather prediction requires constant meteorological data exchange with other countries. This is achieved through telecommunication circuits dedicated to the transmission of meteorological data. The Royal Observatory exchanges meteorological data with overseas centres via three international circuits, namely the Hong Kong-Beijing circuit, the Hong Kong-Tokyo circuit and the Hong Kong-Bangkok circuit.
In recent years, the vagaries of the local weather have become better understood, through experience gained in analysing and interpreting the outputs of numerical weather prediction models run at the observatory and major meteorological centres abroad. With advances in the accuracy of numerical model outputs, useful weather outlooks beyond the first day may now be issued with confidence.
The Year's Weather
Hong Kong experienced a dry year in 1991, the 10th driest year on record. Rainfall was only 1639.1 millimetres compared to a normal of 2214.3 millimetres. All months but January and October were drier than normal. Thirteen tropical cyclones occurred over the South China Sea, six of them requiring the hoisting of tropical cyclone signals in Hong Kong. In July, the No. 8 Gale or Storm Signal was hoisted during the approach of Severe Tropical Storm Brendan, the first No. 8 Signal since Typhoon Gordon in 1989.
January 1991 was warmer than normal. The mean daily minimum temperature of 15.4 degrees was the seventh highest on record for January. The weather was generally cloudy and humid. The monthly rainfall of 28.7 millimetres, was 23 per cent above the January normal of 23.4 millimetres.
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