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Interesting statistics on rainstorm warning signals


   Written by: LI Kin-wai    June 2011


    The rain season in Hong Kong normally lies between April and September. Troughs of low pressure would affect the south China coastal areas in May and June and bring heavy downpours to Hong Kong.  These rainstorms will cause traffic disruption, floodings, landslips and even casualties.  In 1992, the Observatory first operated a colour rainstorm warning system for different intensity of rainstorms to alert the public.  The rainstorm warning signals were classified as "Green", "Amber", "Red" and "Black" colours.  In 1998, the rainstorm warning system was revised to adopt a 3-level warnings classified as "Amber", "Red" and "Black" rainstorm warning signals.  According to the warning signal definitions, "Amber" rainstorm warning signal represents heavy rain has fallen or is expected to fall generally over Hong Kong exceeding 30 millimetres an hour; "Red" rainstorm warning signal represents heavy rain has fallen or is expected to fall generally over Hong Kong exceeding 50 millimetres an hour and "Black" rainstorm warning signal represents heavy rain has fallen or is expected to fall generally over Hong Kong exceeding 70 millimetres an hour. 

    According to the statistics from 1992 to 2010, the average numbers of "Amber", "Red" and "Black" rainstorm warning signals issued each year are 21.3, 4.6 and 1.2 respecively.  June is the month with the highest number of rainstorm warning signals issued.  The average numbers of "Amber", "Red" and "Black" rainstorm warning signals issued in June are 5.3, 1.5 and 0.3 respectively.  No rainstorm warning signals has ever been issued in January, February and December.  The earliest issuance for "Amber", "Red" and "Black" rainstorm warning signals in a year were 5 March in 2009, 3 April in 2000 and 19 April in 2008 respectively while the latest issuance were 21 November in 2006, 18 October in 2002 and 2 September in 2001 respectively.  The highest number of "Black" rainstorm warning signals issued in a year was 3 times, happened in 2000 and 2006.  The shortest break between two "Black" rainstorm warning signals was recorded on 23 and 24 August 1999 in which the separation of these two rainstorm warning signals was less than 17 hours.  The longest durations for "Amber", "Red" and "Black" rainstorm warning signals being in effect were 17.4, 13.2 and 5.8 hours respectively.

    There is a tendency for "Red" and "Black" rainstorm warning signals to occur at late night or in the early morning.  In fact, the effective time for "Red" rainstorm warning signal during the period from midnight to noon is about 1.5 times of those from noon to midnight and that for "Black" rainstorm warning signal is even about 2.5 times higher.  One of the possible reasons is that the low-level advection of warm moist air from the South China Sea towards the coastal region tends to occur at late night or early morning during the summer time which reduces the atmospheric stability and favours the formation of heavy rain.  Furthermore, the rainstorms affecting Hong Kong are usually caused by troughs of low pressure, tropical cyclones, southwest monsoon and cold fronts.  By analysing the rainstorm cases of "Red" and "Black" rainstorm warning signals, most of them were attributed to troughs of low pressure, about 59%.  About 24% of the cases were attributed to tropical cyclones.  Southwest monsoon and cold fronts contributed 6% and 4% respectively.

    In recent years, the heaviest rainstorm in Hong Kong was the one on 7 June 2008.  Under the influence of an active trough of low pressure, a rain band swept across Hong Kong from west to east in that morning.  Due to the rapid development of the rain band, the Observatory first issued "Amber" rainstorm warning signal at 5:15 am, followed by "Red" rainstorm warning signal in 40 minutes later and finally "Black" rainstorm warning signal at 6:40 am.  The "Black" rainstorm warning signal last for 4 hours and 20 minutes till 11:00 am, making it the 4th longest duration of "Black" rainstorm warning signal being in force.  During the period of the "Black" rainstorm warning signal, the total rainfall recorded at the Observatory for one hour between 8 and 9 am was 145.5 millimetres, which was the highest record of one hour rainfall since 1884.  

     

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Last revision date: <21 Dec 2012>