ENG-1975 — Page 246

THE ENVIRONMENT

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strength of the circulation, the position and movement of the centre and the 24-hour forecast position. Reports from ships and reconnaissance aircraft are received at the Royal Observatory, as well as cloud pictures obtained direct from meteorological satellites. These reports are used to locate the centre and evaluate the intensity of the tropical cyclone.

When tropical cyclones approach Hong Kong, warnings are widely distributed visual signals, telephone, radio and television. Information, advice and recom- mended precautions are broadcast at frequent intervals. If the centre of a tropical cyclone comes within 240 nautical miles of Hong Kong, it can be located and moni- tored by the observatory's radars mounted on the top of Tate's Cairn. An isoecho device has been fitted to the radar to facilitate the real-time estimation of the intensity of rainfall associated with tropical cyclones. This equipment also provides valuable information for heavy rain and thunderstorm forecasting, as well as 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 radio 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 standards.

Instruments and Measurements

The seismological section of the observatory operates 12 seismographs in a specially constructed cellar. These sensitive instruments can record vibrations of both long and short periods transmitted through the ground. On average, tremors from about 800 earthquakes occurring all over the world—are detected and analysed each year. Other tremors resulting from underground nuclear explosions, storm micro- seisms, local blasting or pile driving are also registered by the seismographs. 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, but none were reported in 1975.

The observatory prepares bulletins on all earthquake tremors recorded and participates in the Tsunami Warning System for the Pacific area. Tsunamis are seismic sea waves and are caused by earthquakes. Whenever an intense earthquake is recorded, with its epicentre anywhere in the Pacific Ocean or the South China Sea, a special message is sent to Honolulu and Tokyo where Tsunami warnings can be issued.

Geomagnetic measurements, which ceased in 1941 and resumed in 1971, are now made regularly at the geomagnetic station near Tate's Cairn, where magnetic variation is also recorded. This is a joint project by the University of Hong Kong and the Royal Observatory, made possible by a donation from the Nuffield Foundation.