THE ENVIRONMENT
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Apart from manual observation, a network of automatic weather stations telemeters real-time weather data to the Central Forecasting Office at the Observatory Headquarters from Cheung Chau, King's Park, Ta Kwu Ling, Lau Fau Shan, Tsing Yi, Sha Tin, Tuen Mun, Wong Chuk Hang, Tai Po Kau, Sai Kung, Tseung Kwan O, Sha Lo Wan and Waglan Island. These stations provide a comprehensive coverage on regional weather variation. For the safety of aviation, wind conditions at the airport and its vicinity are monitored continuously by a network of anemometers. In collaboration with Guangdong Meteorological Bureau, the observatory also operates an automatic weather station at Huangmao Zhou, an island 40 kilometres south of Lantau Island and strategically located for monitoring tropical cyclones approaching from the south.
Rainfall information has always been important in Hong Kong because of its effects on water resources management and also because of the susceptibility of slopes to landslips in heavy rain. A dense network of 143 raingauge stations provides information on the spatial distribution and intensity of rainfall in various parts of Hong Kong. Sixty-nine of these stations are equipped to provide up to the minute information every five minutes. Rain cloud development, movement and intensity within 512 kilometres of Hong Kong are monitored by 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 thunderstorm activity within a range of about 100 kilometres. Assimilation of all information is essential for assessing the likelihood of flooding and landslips.
For the real-time monitoring of floods in north-western New Territories, water-level and rainfall information at Kam Tin is telemetered directly to the Observatory Headquarters. The system was scheduled to include 12 more stations by the end of 1992. Tide data are collected from eight gauges in the territory for operational warning of sea flooding. Wave information for forecasting the sea state of local waters is obtained in real time from a wave recorder at Waglan Island. Numerical models are used to predict the likelihood of storm surges in tropical cyclone situations. Storm surge warnings are issued when a significant rise in sea level is forecast.
Besides surface observations, upper-air 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 from the Japanese Geostationary Meteorological Satellite. The satellite imageries, which cover east Asia and the western Pacific, greatly facilitate the monitoring of weather systems when they are outside the range of the radar system.
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, the Hong Kong-Beijing circuit, the Hong Kong- Tokyo circuit and the Hong Kong-Bangkok circuit.
In recent years, the vagaries and evolution of weather have become better under- stood through experience gained in analysing and interpreting the outputs of numerical weather prediction models run at the Royal Observatory and other major meteorolog- ical centres abroad. With the continuing advances in the accuracy and reliability of numerical model outputs, useful weather outlooks several days ahead can now be issued with confidence.