Points to Note:

• This page is best viewed by Microsoft Internet Explorer 6.0, Netscape Communicator 7.0, or above. You may need to download Java Runtime Environment (J2SE JRE v 1.4.2) here to view this page properly.

• The image shows cloud-to-ground or cloud-to-cloud lightning strokes as detected by the Lightning Location Network over the Pearl River Estuary. The network comprises six stations jointly established by the Hong Kong Observatory, the Guangdong Meteorological Bureau and the Macao Meteorological and Geophysical Bureau. The stations are located at Chung Hom Kok, Tsim Bei Tsui, Sha Tau Kok, Taipa in Macao, Sanshui and Huidong in Guangdong. (Click here to see the location of these stations)

• The lightning records are updated at 5-minute intervals. Animation of the images for the past 30 minutes and 2 hours are available to enable an assessment of lightning movement and development.

• An alert message is given on the top left only for cloud-to-ground lightning. The message will pop up on the display whenever lightning is detected in the past 5 minutes in region(s) selected under "Alert options". (Click here to view the alert regions)

• A number of geographical reference overlays can be added to the lightning location map. These overlays include highways, railways, peaks, well-known places, beaches of Hong Kong, and municipal names of the Guangdong province (Location of the cities is provided by the Guangdong Meteorological Bureau). These references will be displayed only when the scale of the map is suitably chosen. For example, "HK highway" and "HK railway" will be displayed only when the proportion of the Hong Kong territory in the map is sufficiently large. Similarly, "Guangdong municipal name" will be shown only when the map is selected to cover part of the Guangdong province.

• The user can select one or more of the overlays. However, it should be noted that if too many overlays are selected, the names of the geographical references will crowd the map and they might become indiscernible.

• Cloud-to-cloud lightning strokes are those detected strokes that did not reach the ground. As such, they might not affect people or facilities on the ground. However, it should be noted that they may sometimes precede cloud-to-ground strokes. The location as shown depicts roughly the starting point of the discharge from one cloud to the other.

• As it takes time for communication and computation, there is normally a delay of a few minutes in the display of data. Other factors, including interruption to telecommunications services and power supply, can also affect the reliability and availability of any lightning information.

• Not all cloud-to-cloud and cloud-to-ground lightning strokes detected by the network are sensed by human beings. Also, there is no guarantee that all occurred lightning strokes can be detected and communicated to users. False detection of lightning strokes cannot be ruled out completely.

• The performance of the lightning location network is continuously being optimized since the start of data collection in February 2005. It would take at least one year for the process to be satisfactory.

• After the above optimization, the accuracy in determining the location of lightning is 500 m within the network when all stations are operative. The lightning detection efficiency, i.e. the probability that a stroke with peak current greater than a certain level can be detected by the network, is estimated to be around 90 %. The location accuracy and detection efficiency will be reduced when the lightning stroke occurs outside of the network or when not all of the stations are operative.

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