Temperature Measurement Stations in Hong Kong
Spring has always been a season dear to people's heart. The famous Chinese poet Dufu (Tang Dynasty, some 1300 years ago) wrote a poem to celebrate the arrival of Spring : "The good rain knows its season. When spring arrives, so does the rain. In the dark of the night, it comes quietly with the gentle breeze, nourishing all life on Earth, without a fanfare." At a time when spring has just started and when we are blessed with the auspicious rain, I wish all of you a happy and prosperous New Year.
The Year of the Tiger commenced with a cold surge
Hong Kong experienced a rather cold Chinese New Year. The daily minimum temperature at the Observatory headquarters at Tsimshatsui dropped gradually from 14.6 oC on the first day of the New Year (14 February) to 7.7 oC on the 6th day of the New Year (19 February). In Tsuen Wan and Kowloon City, the lowest temperatures recorded were 5.3 oC and 5.9 oC respectively, both colder than the 6.0 oC at Ta Kwu Ling.
Is the urban area colder than the New Territories ? Are there problems with the sites of weather stations ?
People in Hong Kong have in mind the fact that in winter, the New Territories is colder than the urban areas. Do the temperature records during the Chinese New Year suggest that this is no longer valid ?
Let's take a look at the average minimum temperature at Ta Kwu King, Shek Kong (rural areas), Tsuen Wan and Kowloon City (urban areas) during the past 2 winters (December, January and February). They are respectively 12.7 oC, 13.5 oC, 13.9 oC and 14.7 oC. The fact remains that the New Territories is on average colder than the urban areas. During fine winter nights when there are little clouds and the winds are light, Ta Kwu King and Shek Kong have lower temperatures. This is because the places are less built up and more exposed. Like having a blanket pulled away, any heat at the ground is easily lost (through a process known as radiation cooling in meteorology). However, the cold surge that began before the New Year's eve (on 12 February to be exact) brought long periods of overcast weather and occasional rain to Hong Kong. The mechanism of radiation cooling did not come into play. Rather, under persistent northerlies, the atmosphere was quite well-mixed, making the temperature difference between the rural and urban areas insignificant.
One factor that was highlighted during this cold spell was the height of the weather stations. Tsuen Wan and Kowloon City, at 142 and 92 metres above mean sea level respectively, are the highest stations in the urban area. Based on the an average temperature fall of about 0.6 oC for a rise of 100 metres, Tsuen Wan and Kowloon City are cooler by 0.9 oC and 0.6 oC respectively compared with the sea level, i.e. cooler by within 1 oC. Temperatures at these two stations are thus representative of the respective areas.
Because of the two factors described above, the Tsuen Wan and Kowloon City stations registered slightly lower temperature than other stations in the urban area as well as other stations near the mean sea level. Nonetheless, their readings are still useful references to the public.
The temperature station at Tsuen Wan is sited at
Ho Koon Education Center near the foot of Tai Mo Shan
As a matter of fact, choosing a site for a weather station is a complicated issue. A balance has to be struck between complying with the strict requirements of meteorological science and the perspective of providing weather services to the public. From the scientific point of view, an ideal temperature station should fulfill a number of conditions. For example, the temperature sensor should not be exposed to direct sunlight, the ground should be level and covered with short grass, the adjacent environment is one that is not affected by nearby buildings or artificial heat sources. The temperatures measured this way are considered to be representative of the district.
In Hong Kong, because of rapid development and urbanization, sites ideal for temperature measurement are hard to come by. To meet the information needs of the public, stations have to be established in sites which are less than perfect, but nonetheless meet the basic requirements. These basic requirements include station exposure, and other non-meteorological factors such as site security and easy access for maintenance.
Meeting public needs for regional weather information
Because of the complex terrain in Hong Kong, and the different population and degree of urbanization, the temperatures at the districts are bound to vary. For this reason, we embarked upon a "One District One Station" project in 2007 to provide at least one temperature station in each of the eighteen districts in Hong Kong. This project was completed in March this year. We have also partnered with the Hong Kong Polytechnic University and the Joint School Meteorological Network to set up a Community Weather Information Network, or Co-WIN for short. We help schools and organizations to set up weather stations of their own, and the weather data collected are displayed on the internet for the benefit of the public. A few clicks (http://weather.ap.polyu.edu.hk/index.php) will take you to real-time weather information from dozens of Co-WIN members in various parts of Hong Kong.
Some real-time temperature readings from automatic weather stations
of the Community Weather Information Network