How is lightning counted?
On 8 September (Wednesday), Hong Kong experienced heavy thundershowers. From 9 p.m. to 3 a.m., more than 25,000 lightning strokes were recorded in the territory.
Lightning poses danger to people engaging outdoor activities. Hong Kong's lightning location network was established in 2005 to detect lightning in and around the territory. The network consists of 6 lightning detection stations, as shown in Figure 1: three in Hong Kong, two in Guangdong, and one in Macao, the latter three being implemented in cooperation with the Guangdong Meteorological Bureau and Macao Meteorological and Geophysical Bureau. The stations are spread out so that lightning from afar can be detected, and this enables earlier alerts to the users.
Figure 1 The Observatory's lightning location network
(Background map: courtesy of Google Maps)
First, what constitutes a lightning event?
Every time there is lightning, an electromagnetic wave is generated. When it reaches a lightning station the sensor there measures the wave and the voltage associated with it. To go through the necessary quality assurance, the wave has to meet certain characteristics and the voltage has to exceed a threshold for the lightning event to be treated as a genuine one.
Now the electromagnetic wave from lightning travels in all directions. It reaches the lightning stations at different times. As depicted in Figure 2, the different arrival times at the stations enable the lightning event to be located. The mathematical process, called triangulation, has been with us for a long time. Earthquakes are located in very much the same way, the only difference is that an earthquake sends out tremors instead of electromagnetic waves. These days, a car's position is determined by GPS (Global Positioning System) satellites using the same principle.
Figure 2 tells us that three stations are all it takes to locate a lightning event. The current 6-station network may give us more information than necessary, resulting in data redundancy. The surplus information enables double-checking and precision, and improves the reliability of the lightning location.
So how is lightning counted? A lightning is counted when it is successfully located in the manner described above.
The latest lightning information is made available to the public through various channels: mobile phones, the Observatory's website (http://www.weather.gov.hk/wxinfo/llis/index.htm), the dial-a-weather service (187 8200) and the hourly weather bulletins. One very popular application on mobile phones and on the Observatory's website has been the lightning alert service, where a user is automatically prompted whenever lightning is located within range, say 5, 15 or 30 km. The user can have up to three ranges of his/her choice. An example is given in Figure 3. The service can facilitate users to operate their swimming pools and decide on outdoor activities such as hiking and barbecuing.
Figure 3 Users will be alerted with a message (upper left) and an alarm when
lightning is detected within a specified range.
B.Y. Lee and W.M. Ma