Lightning Location Information Service
Introduction : In 2005, the Hong Kong Observatory set up a lightning location system in cooperation with the Guangdong Meteorological Bureau and Macao Meteorological and Geophysical Bureau. The system monitors lightning activities over the Pearl River Delta round the clock and disseminates the lightning information through various channels, providing different types of lightning location information service to the public and various users.
Working principle of the lightning location system : Lightning is formed by severe turbulence inside a cumulonimbus cloud. The water droplets and ice crystals in the cloud break up and produce positive and negative electric charges. When the electric voltage is high enough, discharges take place between cloud and ground or between clouds. Besides lighting up the sky and producing a thunderous clap, the strong electric current in the discharge generates electromagnetic waves which spread in all directions.
Figure 1. Distribution of lightning sensor stations of the lightning location network
The Observatory's lightning location system comprises seven lightning sensor stations, located at Chung Hom Kok, Tsim Bei Tsui, Sha Tau Kok in Hong Kong, Taipa in Macao and Sanshui, Huidong and Yangjiang in Guangdong (Figure 1). Each sensor is equipped with special antenna and instrument (Figure 2), capable of receiving the electromagnetic waves generated by lightning. As the sensors are at different locations, the electromagnetic waves from the same lightning stroke reach the sensors in different times and directions, thereby allowing the originating location of the lightning to be computed by the system. The system is also capable of distinguishing between cloud-to-ground and cloud-to-cloud lightning by analysing the waveform characteristics of the electromagnetic waves.
Figure 2. Lightning sensor at Chung Hom Kok
Lightning location information service : The Observatory provides lightning location information service to the public through various channels. Lightning locations are displayed in graphical form on the Observatory's website www.weather.gov.hk/wxinfo/llis/alert_index.htm (Figure 3), updated every five minutes.
Figure 3. Lightning location map on the Observatory's website
As cloud-to-ground lightning strokes may affect people or facilities outdoor, a cloud-to-ground lightning alerting tool is available on the webpage. Users simply select their own location of interest and up to three alert ranges. The webpage will automatically provide alerts when lightning strokes are detected within the specified ranges (Figure 4). The public can also define the lightning alert region by choosing from one of the four preset regions, namely "New Territories West", "New Territories East", "Lantau" and "Hong Kong Island and Kowloon".
Figure 4. Lightning alerting service centred on a user-specified location (The map is overlaid with a radar image)
The webpage also provides a number of geographical references including positions of major landmarks, swimming pools and peaks, so as to enable the public to clearly identify the location of lightning, and to appreciate how the lightning development may affect their outdoor activities. Users can also choose to overlay radar images on the lightning location map (Figure 4), such that the adverse weather situation associated with the distribution and movement of lightning within a rain area can be better understood.
Besides, members of the public can obtain the latest lightning information through the mobile application "MyObservatory", the Observatory's mobile webpage at m.weather.gov.hk/wxradar/llis.htm, or the Observatory's Dial-a-Weather service (1878 200, press 326). The Observatory also delivers relevant lightning information to the public through radio and television.
Past lightning data : The numbers of lightning strokes recorded each hour in Hong Kong for the current day and in the past three days is also available on the Observatory's website at www.weather.gov.hk/wxinfo/llis/stat.htm(Figure 5).
Figure 5. Hourly lightning counts of the current and past three days
If members of the public need more detailed lightning data of the past, they can contact the Climatological Services division of the Observatory which provides the service on a cost-recovery basis.
Other relevant information : The "Thunderstorm Warning" pamphlet published by the Observatory and available on the webpage at www.weather.gov.hk/wservice/warning/thunder.htm provide details about thunderstorm warning. Some scientific facts about thunderstorms are available in the Observatory's website on education resources at www.weather.gov.hk/education/edue.htm.