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Friday, 24th May 2013

30 - 50 - 70, How long will the 'Black' last?

A couple of days ago (22 May 2013), the Hong Kong Observatory issued the Black Rainstorm Warning Signal (the so-called 'Black' rainstorm locally). This was the first 'Black' rainstorm since July 2010. It was also the first one in 13 years (13 years once again[1] ) that occurred during the morning rush hours[2] when people normally go to work or school. Naturally it drew much public attention, in particular on how long the 'Black' rainstorm would last and whether one would need to go to work or school.

The existing Rainstorm Warning System[3] by the Observatory has been in operation since 1998. The 'Amber', 'Red' and 'Black' rainstorm respectively represent heavy rain has fallen or is expected to fall generally over Hong Kong, exceeding 30, 50 and 70 millimetres in an hour, and is likely to continue. The Rainstorm Warning System operates on the basis of objective criteria and scientific approach. The rainfall values mentioned above are easily understood and effective, but they are not rigid mandates. When prolonged rain occurs, the warning signals may be issued even if those rainfall criteria are not reached for individual hours. By the same token, a temporary easing off of heavy rain while the weather remains unsettled would not warrant an instant change of the warning signal already in force. This avoids frequent changes in the warning status from 'up and down, down and up', which might cause more confusion. As for other weather warnings, public safety is always the primary consideration and the most important factor in the decision of issuance or cancellation of rainstorm warning signals

The formation, growth and decay of rainstorm can be very rapid and the variations rather substantial. All these changes can happen within just a few hours. Precise forecasting of rainstorms remains a major challenge and goal of the Observatory and the meteorological community worldwide. Since the state-of-the-art technology still cannot fully grasp the future evolution of rainstorms, it is yet hard to accurately predict the issuance or cancellation of a rainstorm warning signal.

Rainstorms can bring about severe traffic disruption, floods, landslips and even casualties. To safeguard the public, forecaster will continuously monitor the latest weather situation and provide updated information to the public. During heavy rain episodes, please stay tuned to the latest weather information and warning bulletins of the Observatory via radio, TV, Observatory's website (, 'MyObservatory' mobile app or 'Dial-a-weather 187 8200'. Remain on the alert against casualties and losses.

Rainstorms are inevitable. The prompt and orderly responses of the public and the society in taking the necessary precautions do not come easily. These were achieved through feedbacks from members of the public, dedicated efforts of various departments and organizations as well as educating the youths at primary/secondary schools over the years. The Observatory will continue to enhance our weather services, in particular the contents and dissemination of information, with a view to facilitating the public to clearly comprehend the latest weather situation.

K.C. Tsui


[1] The last two 'Typhoon Signal No.10' also separated by 13 years, i.e. 1999 and 2012.

[2] Refers to Monday - Friday, non-public holidays. The previous Black Rainstorm Warning Signal that happened during busy morning hours was on 12 June 2000 (Monday).

[3] Please refer to the link for more details on the Rainstorm Warning System.

Last revision date: <09 Aug 2013>