The Physics of lightning
Lightning is a severe weather phenomenon commonly seen in Hong Kong and could result in loss of lives and damage in properties. Whenever lightning, and its associated thunderstorms, affects Hong Kong, the Observatory will issue a Thunderstorm Warning to alert people and relevant Government Departments to take suitable precautionary measures.
Lightning can occur when Hong Kong is affected by unstable weather systems, such as perturbations in airstreams, troughs of low pressure or tropical cyclones. Lightning is an atmospheric electric discharge phenomenon. The large electric current flowing through the atmosphere generates a huge amount of heat energy and strong flashes. The energy also heats up the air. Rapid air expansion results in strong shocks and generates loud bangs which are the thunders that often accompany lightning.
The electric discharge that results in lightning is generated by the enormous electric potential difference between clouds or clouds and the ground surface. Very often lightning is associated with the cumulonimbus (Cb) clouds. In situ measurement results indicated that potential differences within or between Cb clouds could reach 105V/m. But how such huge potential differences can be accumulated between clouds or clouds and the ground? The charge separation processes in the Cb is highly complicated with a number of mechanisms in action. One of the mechanism accepted by most scientists is that precipitating particles, e.g. water droplets, ice crystals and hail stones in the Cb are polarized by the prevailing nature electric field of the atmosphere with positive charges moving to the lower part of these particles and negative charges to the upper. The air movement inside a Cb is unstable and there are strong upward and downward drafts within the cloud. Neutral particles in the updraft will collide with the precipitating particles while moving up. Positive charges at the lower part of the latter, where impact will mostly happen, will be brought away and carried to the top of the cloud by the upward moving particles. Thus the upper part of the Cb will become positively charged and the lower negatively charged. Potential difference will build up between the cloud and the ground surface as negative charges accumulate at the bottom of a Cb, until it reaches a threshold at which electricity is discharged, resulting in lightning. (Figure 1)
In 2005, the Hong Kong Observatory set up a lightning location network in cooperation with the Guangdong Meteorological Bureau and the Macao Meteorological and Geophysical Bureau. The lightning location network monitors lightning activities over the Pearl River Estuary round the clock. After lightning has been detected, the location where lightning occurs is recorded and shown graphically on the Observatory's webpage for reference by the public.
How to detect lightning?
Apart from light and sound, lightning also generates a strong electromagnetic field. The situation can be visualized as a long electric cable extending from the thunderclouds to the earth surface through which a huge current passes through this cable transiently. From the basics of electromagnetism, this will generate a very large electromagnetic flux radiating outwards at the speed of light. Thus, we can confirm the occurrence of lightning through the detection of this electromagnetic flux, like the reception of radio broadcast. If we have several electromagnetic flux detectors in different locations, the detected direction and time of arrival of the electromagnetic flux generated from the same lightning stroke will be different for different detectors. Such information including the amplitude and polarity of the signals as well as the station code will be transmitted to the central computer for further calculation. At the database of the central computer, the precise geographical positions of all stations are already stored for detailed analysis. To trigger the analysis of lightning location, the minimum requirement is that there are at least valid data from two different stations received by the central computer within a few seconds. Hence, the more data from different stations the central computer receives, the more accurate the location and time of occurrence of lightning can be determined.
Stations closer to the location of lightning will receive the electromagnetic flux from the lightning earlier. The central computer will compute the location and time of occurrence of lightning according to the direction and time of reception of the signals detected at different stations. The computation process employs an iterative algorithm for calculating the precise location of the lightning. The computation is principally based on trigonometry (please see Figure 1). The distance of the lightning from each station is estimated from the time difference of reception of the electromagnetic flux at different stations. After that, we can then draw a circle for each station with the estimated distance as the radius. When all circles intersect closely at a point and the direction of that point from each station is consistent with the direction of the electromagnetic flux received at the station, the location of the lightning is then determined.
Hong Kong Observatory Lightning Location Information Service
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.
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 3) Each sensor is equipped with special antenna and instrument (Figure 4), 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.
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 http://www.weather.gov.hk/wxinfo/llis/alert_index.htm (Figure 5), updated every five minutes.
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 6). 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 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 6), 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 http://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 http://www.weather.gov.hk/wxinfo/llis/stat.htm (Figure 7).
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 http://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 http://www.weather.gov.hk/education/edue.htm