Surface Meteorological Measurements in Hong Kong
Introduction : Surface meteorological measurements generally refer to observations of meteorological elements made near the surface of the Earth with the aid of passive sensors such as barometers, thermometers and rain-gauges. Measurements by radars and other ground-based active sensors which detect the energy emitted by themselves are not included.
Information on atmospheric pressure, air temperature, humidity, wind speed and direction, rainfall, visibility, cloud and sea surface temperatures is used operationally in day-to-day weather analysis and forecasting. This information, together with surface observations of sunshine duration, global solar radiation, soil and grass minimum temperatures, evaporation and potential evapotranspiration, are essential for many fields of studies including climatology, hydrology, agriculture and civil engineering design.
Routine surface meteorological observations began at the Hong Kong Observatory in 1884. Initially the times of observation were 10, 16 and 22 hours local time. Atmospheric pressure, air temperature, wind speed and direction, cloud type, cloud amount and direction of motion and rainfall amount were observed. Sunshine duration was also recorded. Relative humidity was calculated from the dry- and wet-bulb temperature readings using humidity tables. In addition, observations were also made by voluntary observers at Victoria Peak, Cape D'Aguilar, Green Island and Stonecutters Island. The state of sea was observed at Cheung Chau, Waglan Island, Tai O and Green Island.
Apart from a break during the Second World War, the frequency of observations, the number of observing stations and the number of meteorological elements observed increased through the years. There were also many changes in the instruments used for making measurements especially in recent years. Description of these changes can be found in Hong Kong Observatory Technical Memorandum No. 5 "Hong Kong Meteorological Records and Climatological Notes 60 years 1884-1939, 1947-1950" for the years before 1951. Between 1951 and 1986 these information were published annually in "Meteorological Results Part I" and thereafter in "Surface Observations in Hong Kong" until 1992. From 1993 onwards, these information were published in "Summary of Meteorological Observations in Hong Kong".
Observing Stations : At present there are four stations (the Hong Kong Observatory Headquarters, King's Park Meteorological Station, Kai Tak Airport Meteorological Office and Chek Lap Kok Airport Meteorological Office) manned by the Observatory staff. Various meteorological elements are recorded continuously round-the-clock at these manned stations. Since 1984, 62 automatic weather stations were set up in Hong Kong. Among These 62 stations, 24 are "full" stations from which measurements of winds, temperature, humidity, pressure and rainfall are made continuously by the field systems; there are nine stations equipped with anemometers only; another network of eight anemometer stations located in the airport areas is dedicated for the Wind Analyzer System for aircraft safety; there is also a network of 21 rain gauge stations. The data from these automatic weather stations are transmitted in real time to the Observatory Headquarters via telephone lines or UHF radio links. To enhance the capability of these automatic weather stations, a system utilizing a remotely controlled video camera was installed at Waglan in 1989 to provide observations of the weather and visibility. In addition, co-operation between the Guangdong Meteorological Bureau and the Hong Kong Observatory to establish automatic weather stations started since 1985 when a station was set up at Huangmao Zhou, a small uninhabited island about 50 km south-southwest of Hong Kong Island. In 1996, two more automatic weather stations were established jointly by the Guangdong Meteorological Bureau and the Hong Kong Observatory, one at Tuoning Islands about 50 km to the northeast of Hong Kong Island and another at Neilingding Islands about 50 km to the northwest. Measurements at these stations are transmitted first via radio link to relay stations in Hong Kong then by telephone lines to the Observatory Headquarters. The data are then routed via the Global Telecommunications System to Guangzhou. The locations of the four manned stations, the 27 "full" automatic weather stations, and the nine anemometer stations are shown in Figure 1. The network of rain-gauge stations is described in another information sheet of this series: "Rainfall Measurements and Hydrometeorology in Hong Kong". The anemometer network associated with the wind analyzer system for aircraft operation is described in "Weather Services for Aviation", also an information sheet of this series.
Atmospheric Pressure : Kew Pattern mercury barometers are operationally in use. A digital barometer making use of ceramic pressure transducers was introduced in 1982. The accuracy of this type of digital barometer is comparable to mercury barometers. The output voltage of the transducer is adjusted electronically to give the mean sea-level pressure which is continuously displayed digitally.
Air Temperature, Dew Point and Relative Humidity : Digital thermometers with platinum resistance sensors have been used since 1982. Mercury-in-glass thermometers are used for back-up purposes. The temperature sensors are placed at a height of about 1.2 metres above ground level either in a Stevenson Screen or a thermometer shed made of palm leaves and mattress. Values of dew-point temperature and relative humidity are calculated automatically from the dry- and wet-bulb temperatures by a microcomputer. The digital thermometer also has solid state memory to record the maximum and minimum temperatures. An analog output for continuous recording of the temperature on a chart recorder is also provided.
Cloud : Observations of cloud type, amount, and height are made visually. At the Hong Kong International Airport, ceilometers and cloud searchlight are installed to facilitate accurate measurements of the height of the base of low clouds above the station.
Wind : Most meteorological stations are equipped with combined vane and cup generator anemometers with dial indicator and/or chart recorder for wind speed and direction measurement. Generally the anemometers are mounted on masts at a height of about 10 metres above the highest obstructions in the vicinity.
Airflow is computed from daily readings of a cup-counter anemometer mounted near the evaporation pans at King's Park, with the cups 0.15 metres above the rim of the pan.
Duration of Sunshine : Duration of sunshine is measured at King's Park by a Campbell-Stokes recorder.
Soil and Grass Minimum Temperatures : Soil temperatures are read from mercury-in-glass thermometers placed vertically at depths 0.05 m, 0.1 m, 0.2 m, 0.5 m, 1.0 m, 1.5 m, and 3.0 m. The grass minimum temperature is read in the morning daily from a minimum thermometer lying horizontally over ground covered with short grass.
Evaporation : Two U.S. Weather Bureau Class 'A' pans are operated at King's Park. The mean water temperature is taken to be the mean of the daily maximum and minimum surface water temperature.
Potential Evapotranspiration : Measurements of potential evapotranspiration are made for three turfed plots at King's Park.
Global Solar Radiation : Hourly values of global solar radiation are measured at King's Park by a thermoelectric pyranometer (sealed thermo-pile dome solarimeter) together with an integrating counter. A bimetallic actinograph is used as a back-up.
Visibility : There are three sets of runway visual range transmissometers located at the runway of the Hong Kong International Airport.
Sea Surface Temperature : Sea surface temperature is measured at Waglan Island and North Point using a thermometer housed inside a standard rubber bucket.
Future Development : The present trend of automation in meteorological measurement, telemetry, real-time data processing and recording on computer compatible media will continue into the future. The existing network of automatic stations will be expanded to further improve spatial coverage. Within Hong Kong, a new automatic weather station will be set up at Tai Mo Shan. Over the Pearl River Estuary areas, another station will be established at Dangan Islands together with the Guangdong Meteorological Bureau. These stations will provide invaluable information on weather systems affecting Hong Kong from various directions.