ENG-1977 — Page 267

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THE ENVIRONMENT

and aircraft. The coded meteorological information is passed to a computer system for sorting, decoding, printing and archiving. Automatic message switching of meteorological data between Hong Kong, Peking, Bangkok and Tokyo is done by computer.

Both visible and infra-red photographs are received daily from polar-orbiting weather satellites. More of these pictures will soon be received several times a day from a geostationary meteorological satellite launched by Japan during the year.

The tropical cyclone warning service is one of the observatory's most important functions. Tropical cyclones are tracked by radar, satellite pictures and from aircraft and ship reports. Once a tropical cyclone has formed and moved into the area between latitudes 10°-30°N and longitudes 105°-125°E, the observatory prepares bulletins on the position, intensity and movement of the tropical cyclone, and issues 24-hour forecasts. These statements are disseminated to the public through the media, to shipping companies and airlines, and to neighbouring countries. Objective forecasts of tropical cyclone movements are made four times a day by computer. These fore- casts are used by the observatory and also are sent to other countries. This year, objective forecasts were computed for the additional area of latitudes 10°-20°N and longitudes 125°–135°E.

When tropical cyclones approach Hong Kong, warnings are distributed by visual signals, telephone, radio and television. Information and forecasts are broadcast at frequent intervals, along with advisory bulletins and precautionary announcements. The observatory displays signals for various stages of alert when tropical cyclones come within 600 kilometres of Hong Kong. The signals are based on the forecast or actual affect of the tropical cyclone on Hong Kong. If the centre of a tropical cyclone comes within 400 kilometres of Hong Kong, it can normally be seen on the observatory's radar installations mounted on top of Tate's Cairn, 580 metres above mean sea level. Time-lapse films of the radar display are taken during tropical cy- clones and, this year, a video time-lapse system was used in the Central Forecasting Office to record and play back the radar display in real-time.

Instruments and Measurements

The Seismology Section of the observatory operates six seismographs in a specially- constructed cellar at the Tsim Sha Tsui headquarters. These instruments record long and short period vibrations transmitted through the ground. On average, tremors from about 800 earthquakes all over the world are detected and analysed each year. Other tremors from underground nuclear explosions, storm microseisms, local blasting or pile-driving -- also are recorded. Hong Kong lies just outside the circum- Pacific seismic belt and has not suffered significant earthquake damage since 1918. However, an average of two to three minor tremors are felt each year by residents in certain locations, specially in high-rise buildings. One such tremor, which occurred on May 12, 1977, registered an intensity of three to four on the modified Mercalli Scale of 12.

The observatory takes part in the Tsunami Warning System for the Pacific region. Tsunamis are seismic sea waves caused by earthquakes. A special warning is issued whenever an intense earthquake is recorded with an epicentre in the Pacific Ocean or the South China Sea.