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The Weather of July 2012

     Under the general prevalence of the subtropical ridge of high pressure, the weather of the first half of July 2012 was drier and warmer than usual.  However, the rainfall deficit was more than compensated by the heavy rain episodes in the latter part of the month respectively brought about by the southwest monsoon and Severe Typhoon Vicente.  Overall, the monthly total rainfall was 467.8 millimetres, about 24 percent above the normal.  The accumulated rainfall since 1 January was 1395.6 millimetres, about 5 percent below the normal figure of 1473.3 millimetres for the same period.  The mean temperature and relative humidity of the whole month are both near-normal.

     Under the influence of the subtropical ridge, the weather in Hong Kong was mainly fine and hot apart from a few showers for the first two days of the month.  With winds turning light, it was sunny and very hot during the day on 3 and 4 July.  The temperature at the Observatory rose to a maximum of 33.8 degrees on 4 July, the highest in the month. 

     Affected by the trough of low pressure over the northern part of the South China Sea, local weather became mainly cloudy with showers on 5 and 6 July.  With the return of the subtropical ridge extending westwards from the Pacific Ocean, it was sunny and hot with a few isolated showers for the ensuing six days.  

     Dominated by a southwesterly airstream, the weather remained hot with some showers from 13 to 17 July.  Affected by an active southwest monsoon, there were heavy showers and squally thunderstorms on the morning of 18 July.  More than 30 millimetres of rainfall were recorded over most parts of the territory.  Under the prevalence of the anticyclone aloft over southern China, local weather turned mainly fine and very hot for the next three days.  

     Meanwhile, Tropical Depression Vicente entered the South China Sea on 21 July and intensified into a tropical storm that night.  It stalled over the northeastern part of the South China Sea on 22 July.  Affected by the outer rainbands of Vicente, the weather became mainly cloudy with a few squally showers and thunderstorms on that day.  Vicente began to edge towards the south China coast to the west of the Pearl River Estuary on 23 July, undergoing rapid intensification to a typhoon in the afternoon and further to a severe typhoon.  With the approach of Vicente, local weather deteriorated significantly on that day with heavy squally showers.  Local winds also strengthened generally in the evening and reached gale to storm force in some places.  Hurricane force winds affected the southwestern part of the territory in the small hours on 24 July, necessitated the issuance of the Hurricane Signal No.10, the first time since 1999.  Vicente made landfall near the coastal areas of Taishan about 130 kilometers west-southwest of Hong Kong before dawn on 24 July and weakened gradually overland during the day.  As Vicente moved away, local wind and rain abated gradually in that afternoon.  Between 23 and 24 July, over 200 millimeters of rainfall were recorded at the Hong Kong Observatory. 

     Affected by a broad monsoon trough, local weather remained unsettled with heavy showers and squally thunderstorms from 25 to 27 July.  With the weakening of the trough, the weather improved gradually with sunny periods on 28 July.  A ridge of high pressure brought generally fine and very hot weather to the territory for the next two days.  Under the influence of the outer subsiding air mass of Typhoon Saola over the seas east of Taiwan, it was very hot in Hong Kong on 31 July.  However, thundery showers developed over inland Guangdong moved in to affect the territory towards that evening.

Trees blown down near Olympian City during the strike of Severe Typhoon Vicente (Photo courtesy of Ms. Carly Tse)
Trees blown down near Olympian City during the strike of Severe Typhoon Vicente
(Photo courtesy of Ms. Carly Tse)

     Four tropical cyclones occurred over the western North Pacific and the South China Sea in the month.

     Details of issuance and cancellation of various warnings/signals in the month are summarized in Tables 1.1 to 1.7.  Monthly meteorological figures and departures from normal for July are tabulated in Table 2.    

Warnings and Signals issued in July 2012

Table 1.1   Tropical Cyclone Warning Signals
Name of
Tropical Cyclone
Signal
Number
Beginning Time Ending Time
Day/Month HKT Day/Month HKT
VICENTE 1 21 / 7 1540 23 / 7 0520
3 23 / 7 0520 23 / 7 1740
8 NE 23 / 7 1740 23 / 7 2320
9 23 / 7 2320 24 / 7 0045
10 24 / 7 0045 24 / 7 0335
8 SE 24 / 7 0335 24 / 7 1010
3 24 / 7 1010 24 / 7 1440
1 24 / 7 1440 24 / 7 2315


Table 1.2   Strong Monsoon Signal
Beginning Time Ending Time
Day/Month HKT Day/Month HKT
24 / 7 2316 25 / 7 0520


Table 1.3   Rainstorm Warning Signals
Colour Beginning Time Ending Time
Day/Month HKT Day/Month HKT
Amber 5 / 7 0920 5 / 7 1020
Amber 18 / 7 0610 18 / 7 0820
Amber 21 / 7 1735 21 / 7 1840
Amber 24 / 7 0155 24 / 7 1040
Amber 25 / 7 1130 25 / 7 1420
Amber 27 / 7 1100 27 / 7 1230
Amber 31 / 7 1710 31 / 7 1835


Table 1.4   Landslip Warning
Beginning Time Ending Time
Day/Month HKT Day/Month HKT
24 / 7 0110 24 / 7 1500


Table 1.5   Thunderstorm Warning
Beginning Time Ending Time
Day/Month HKT Day/Month HKT
5 / 7 0250 5 / 7 0400
5 / 7 0425 5 / 7 0630
5 / 7 0725 5 / 7 1600
8 / 7 1232 8 / 7 1445
14 / 7 0455 14 / 7 0600
14 / 7 0720 14 / 7 0930
16 / 7 1140 16 / 7 1245
16 / 7 1900 16 / 7 2245
17 / 7 0625 17 / 7 0730
18 / 7 0130 18 / 7 1100
18 / 7 1245 18 / 7 1400
18 / 7 1945 18 / 7 2030
19 / 7 0225 19 / 7 0745
19 / 7 1355 19 / 7 1500
20 / 7 0705 20 / 7 0745
20 / 7 1430 20 / 7 1530
21 / 7 1445 21 / 7 1915
21 / 7 2125 21 / 7 2400
22 / 7 1135 22 / 7 1700
24 / 7 0015 24 / 7 0515
25 / 7 0420 25 / 7 0830
25 / 7 1030 25 / 7 1530
25 / 7 1755 25 / 7 2030
26 / 7 0845 26 / 7 1530
27 / 7 0400 27 / 7 1700
28 / 7 1100 28 / 7 1330
31 / 7 1605 31 / 7 1850


Table 1.6   Very Hot Weather Warning
Beginning Time Ending Time
Day/Month HKT Day/Month HKT
3 / 7 1345 4 / 7 1945
7 / 7 1455 12 / 7 1945
15 / 7 0915 15 / 7 1945
16 / 7 0745 16 / 7 1745
19 / 7 0745 21 / 7 1945
29 / 7 1505 31 / 7 1710


Table 1.7   Special Announcement on Flooding
in the Northern New Territories
Beginning Time Ending Time
Day/Month HKT Day/Month HKT
24 / 7 0155 24 / 7 1225
25 / 7 1300 25 / 7 1845


Table 2   Figures and Departures from Normal - July 2012
Meteorological Element Figure of the Month Departure from Normal*
Mean Daily Maximum Air Temperature 31.6 degrees C 0.2 degree above normal
Mean Air Temperature 28.8 degrees C normal
Mean Daily Minimum Air Temperature 26.8 degrees C normal
Mean Dew Point Temperature 25.2 degrees C 0.1 degree above normal
Mean Relative Humidity 81 % normal
Mean Cloud Amount 70 % 1 % above normal
Total Rainfall 467.8 mm 91.3 mm above normal
Number of hours of Reduced VisibilityΔ 7 hours 10.5 hours below normal§
Total Bright Sunshine Duration 197.6 hours 14.4 hours below normal
Mean Daily Global Solar Radiation 18.14 Megajoule / square metre 0.97 Megajoule above normal
Total Evaporation 154.7 mm 8.5 mm above normal


  Remarks : All measurements were made at the Hong Kong Observatory except sunshine, solar radiation and evaporation which were recorded at King's Park Meteorological Station and visibility which was observed at the Hong Kong International Airport.

  Δ

The visibility readings at the Hong Kong International Airport are based on hourly observations by professional meteorological observers in 2004 and before, and average readings over the 10-minute period before the clock hour of the visibility meter near the middle of the south runway from 2005 onwards. The change of the data source in 2005 is an improvement of the visibility assessment using instrumented observations following the international trend.
Before 10 October 2007, the number of hours of reduced visibility at the Hong Kong International Airport in 2005 and thereafter displayed in this web page was based on hourly visibility observations by professional meteorological observers. Since 10 October 2007, the data have been revised using the average visibility readings over the 10-minute period before the clock hour, as recorded by the visibility meter near the middle of the south runway.


  *   Departure from 1981 - 2010 climatological normal, except for number of hours of reduced visibility

  §   Departure from mean value between 1997 and 2011


daily values of selected meteorological elements for HK for July 2012


Rainfall Map For July 2012 (isohyets are in millimetres)

Last revision date: <31 Dec 2012>