ENG-1974 — Page 258

THE ENVIRONMENT

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decoded, plotted and analysed at the Royal Observatory. Special weather bulletins are broadcast for shipping and for fishermen, and all aircraft leaving Hong Kong are given briefings, written forecasts and weather charts.

One of the most important functions of the Royal Observatory is to issue warn- ings of tropical cyclones. Whenever a tropical cyclone is located within the region bounded by latitudes 10°-30° north and longitudes 105°-125° east, warnings for shipping are generally issued every three hours. These provide information on the strength of the circulation, the position and movement of the centre and the forecast position 24 hours ahead. Reports from ships and reconnaissance aircraft and cloud pictures received at the Royal Observatory direct from meteorological satellites together with radar observations are used to locate the centre and evaluate the intensity of the tropical cyclone.

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When Hong Kong is threatened, warnings are widely distributed by visual signals, telephone, radio and television. Advice and recommended precautions are broadcast at frequent intervals whenever signals are displayed. The Royal Observatory also issues strong monsoon, thunderstorm, heavy rain, fire hazard, frost and low temperature warnings whenever necessary.

The observatory's weather radar station at Tate's Cairn is equipped with a 30- millimetre radar for detecting showers and local rainstorms and a 100-millimetre radar for locating large tropical disturbances up to 240 nautical miles away. A new iso-echo device was fitted to the larger radar in 1973 to facilitate the real-time estima- tion of the intensity of rainfall. This equipment now provides valuable additional information for rainfall forecasting, as well as for hydrological applications.

The observatory is responsible for Hong Kong's Time Service. Six pip signals from a special crystal clock, accurate to 0.05 of a second, are broadcast every 15 minutes on a frequency of 95 MHz and are relayed by broadcasting and television stations. With effect from January 1972, the time kept by the Hong Kong Time Standard was changed to Universal Co-ordinated Time (UTC). This new time system has been adopted by international agreement and is based on an atomic time standard which provides a more uniform time scale than that based upon astronomical stand- ards. The UTC will never differ by more than 0.7 of a second from the astronomical time. To ensure this, step adjustment to UTC of one second was made on January 1, 1974 in accordance with international agreement.

Twelve seismometers are operated by the observatory. The department prepares bulletins of all earthquake tremors recorded and participates in the Pacific Tsunami Warning Service. Hong Kong lies just outside the circum-Pacific seismic belt and has not suffered serious earthquake damage since 1918. However, an average of two to three tremors may be felt each year by residents in certain locations such as on balconies of high buildings. During the year three such tremors occurred. The strongest was of intensity four to five in the Modified Mercalli Scale on November 19.

Geomagnetic measurements, which ceased in 1941 and resumed in 1971, were regularly made at the geomagnetic station near Tate's Cairn, where magnetic varia- tion was also recorded. This was made possible with a donation from the Nuffield