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, the business and industry sectors.

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.

The Observatory's lightning location system comprises six lightning sensor stations, which are located at Chung Hom Kok, Tsim Bei Tsui, Sha Tau Kok, Taipa in Macao, Sanshui and Huidong in Guangdong. Each sensor is equipped with special antenna and instrument (Figure 1), capable of receiving the electromagnetic waves generated by lightning. As the sensors are at different locations, the electromagnetic wave from the same lightning stroke arrives at each of the sensors in different times and directions (Figure 2). After collecting the data measured by each sensor, the lightning location system determines the location of the lightning stroke. The system is capable of distinguishing between cloud-to-ground and cloud-to-cloud lightning by analysing the waveform characteristics of the electromagnetic wave.

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/index.htm (Figure 3), updated every five minutes. Members of the public can also visit the Personal Digital Assistant (PDA) website at pda.hko.gov.hk/llise.htm to view the lightning location map.

As cloud-to-ground lightning strokes may affect people or facilities outdoor, a cloud-to-ground lightning alerting tool is available in the webpage. Users simply select their own location of interest and the webpage will automatically provide alerts when lightning strokes are detected within a 15 km range (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".

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 identify more easily the location of lightning. Users can also choose to overlay radar images on the lightning location map (Figure 4). The public can better grasp the adverse weather situation after appreciating the distribution and movement of lightning within a rain area.

Besides displaying a lightning information map on its website, the Observatory also provides a condensed version of the lightning location information in the hourly weather bulletin for radio and television broadcast. Regional lightning information, updated every 15 minutes, is also accessible from the Observatory's Dial-a-Weather system (1878 200, press 326).

Past lightning data : The number of lightning strokes recorded each hour in Hong Kong in the current and past three days is also available on the Observatory's website (Figure 5).

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 the webpage at www.weather.gov.hk/wservice/warning/thunder.htm provide details about the thunderstorm warning system operated by the Observatory. To know more about the science of thunderstorm, please visit the Observatory's website on education resource at www.weather.gov.hk/education/edue.htm.