THE ENVIRONMENT
291
Chek Lap Kok was relocated to Tuen Mun late in the year, as a first step towards providing more localised weather information to the densely populated new towns. In collaboration with the Guangdong Meteorological Bureau, the observatory operated an automatic weather station at Huangmao Zhou, an island 40 kilometres to the south of Lantau Island to monitor vital information when tropical cyclones approached from the south.
Rainfall figures have always been important in Hong Kong because of the water supply and because of the susceptibility of slopes to landslips. A dense network of about 96 raingauge stations all over Hong Kong provided adequate coverage for information on rainfall intensity and distribution. Up-to-the-minute rainfall figures from 20 automatic raingauges were relayed to the observatory primarily for monitoring flooding and landslips in the territory. Rain intensity within 500 kilometres of Hong Kong was monitored by a digital radar system which also produced objective short-term rainfall estimates at selected places. A lightning detection system located cloud-to-ground lightning within a range of about 130 kilometres and a spheric recorder was used to register thunderstorm activity within about 100 kilometres.
Besides land-based observations, upper-air atmospheric conditions were measured by radiosondes carried by balloons launched at King's Park Meteorological Station. For a hemispheric overview of weather systems, three-hourly high resolution satellite cloud pictures from the Japanese Geostationary Meteorological Satellite were received in Hong Kong.
For operational warning of sea flooding in Hong Kong, tide data were collected from nine gauges in the territory and wave data were measured at Waglan Island. For safety of aviation, wind conditions at the airport and its vicinity were monitored continuously by a network of anemometers and a doppler acoustic radar at Lei Yue Mun on the southeastern approach to the airport.
Weather prediction requires constant meteorological data exchange with other countries through the dedicated international meteorological telecommunication circuit. The obser- vatory computer system handled messages of about three million characters of coded weather data each day of the year through this circuit. These messages constituted the data set on which weather forecasts were produced. They were decoded, plotted, analysed normally at three-hour intervals.
In recent years, weather vagaries were better understood through experience gained in analysing and interpreting numerical weather prediction models run at major mete- orological and research centres. Previously, 24-hour forecasts were probably the limit for reasonable accuracy. This year, using numerical model outputs, useful weather outlooks beyond the first day were issued whenever possible and they were in the main satisfactory.
Geophysical, Oceanographic and other Services
The Royal Observatory carried out geophysical and oceanographic studies to provide the demanded investigatory and advisory services from engineering consultants undertaking major civil or coastal development projects.
During the year, a method to forecast waves and swells in Hong Kong waters was developed. Microprocessor-based data logging systems were designed and fabricated to store tidal information on site. Plans were made to analyse tide levels recorded at all tide stations with a view to refining tidal predictions for Hong Kong.
For earthquake monitoring and engineering seismicity, a network of three short-period seismometers was operational at Cheung Chau, High Island and Tsim Bei Tsui. Long- period seismographs recorded tremors from all over the world. Strong motion accelero-