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  • What You Need to Know about Thunderstorm Warning

  • Wednesday, 3rd September 2014

"The Observatory issued thunderstorm warning at one o'clock this afternoon......". During summer, you may often come across such warning messages on radio, the Observatory's website or mobile application "MyObservatory". However, on some occasions, you were out there in the sun without any sign of thunder or raindrop. You might ask doubtfully, "Where have all the thunderstorms gone?"

As a matter of fact, it is common to see sunshine and shower concurrently in Hong Kong, especially during the heat of the summer. In inland areas that heat up more quickly in the day, showers and thunderstorms are easily triggered due to high temperature. Sometimes, this kind of thunderstorms is limited in size, and thus weather at a distance may remain fine without showers or thunderstorms (Figure 1). When the Observatory issues the thunderstorm warning, it aims at alerting the public of the threat posed by thunderstorms within the territory. Where possible, the Observatory provides additional information on the specific areas affected by thunderstorms in the warning bulletin and the weather report (Figure 2). Under the situation as illustrated in Figure 1, people in the urban areas are not aware of the thunderstorms that are witnessed by people in Sai Kung. On some occasions, thunderstorms are affecting only part of the territory but they are forecast to evolve quickly so that there is a chance of them spreading across the territory or affecting more places in Hong Kong. For safety reason, the Observatory will not specify the areas affected by thunderstorms in the warning. This eliminates the need to update the warning frequently, which may cause confusion to the public.

Figure 1

Figure 1      Radar images showing the rapid growth and decay of thunderstorms. Lightning locations
are denoted by white squares.

Figure 2

Figure 2      Members of the public can check the thunderstorm warning and the affected
areas using "MyObservatory".

How should the public respond when thunderstorm warning is in force? If you are located within the affected area as mentioned in the warning, please take appropriate precautions such as staying indoors or stop engaging in water sports. For details, please visit the Observatory's webpage on thunderstorm warning: Whenever you want to know if there are any thunderstorms close by that may affect your outdoor activities, please take a look at the lightning location map in the Observatory's website ( or use the mobile application "MyObservatory" to check the location of lightning associated with thunderstorms. By keeping an eye on the news of thunderstorms and making good use of the Observatory's online information, members of the public can better plan and engage in outdoor activities.

The growth and decay of thunderstorms can be very rapid and sporadic. It is still a challenge to forecast accurately their variation several hours ahead owing to the limitation of the existing science and technology. Therefore, the validity period of Observatory's thunderstorm warning will not be too long to minimize false alarm and reduce disruption to the public. The Observatory will keep close watch on the evolution of thunderstorms and extend the warning period if needed, until the thunderstorms completely dissipate to ensure public safety.

YC Kong

Last revision date: <04 Mar 2015>