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  • The Weather of July 2017

  • 2 August 2017

    With a trough of low pressure lingering over the south China coastal region in the early part of the month and frequent tropical cyclone activities over the northern part of the South China Sea in the latter half, July 2017 was cloudier with more rain than usual. The monthly total rainfall was 570.0 millimetres, more than 50 percent above the normal figure of 376.5 millimetres. The accumulated rainfall recorded in the first seven months of the year was 1759.8 millimetres, nearly 20 percent above the normal figure of 1473.3 millimetres for the same period.

    An active southwest monsoon brought cloudy and showery weather to Hong Kong on the first three days of the month, with some isolated heavy downpour affecting mostly the New Territories. Under the influence of a broad trough of low pressure over the coast of Guangdong and the northern part of the South China Sea, the weather became even more unsettled with occasional heavy showers and squally thunderstorms that lasted till 8 July despite some interludes of sunshine. With the setting in of the subtropical ridge over southeastern China, showery activities gradually eased off after 9 July. While hot and mainly fine weather prevailed in Hong Kong over the next five days, some isolated heavy showers did affect the Sai Kung areas on 12 July.

    Meanwhile, easterly winds along the coastal areas of Guangdong gradually strengthened as an area of low pressure near Hainan Island eventually developed into a tropical cyclone named Talas on 15 July. Even though Talas moved away towards the coast of Vietnam, local weather turned cloudy and showery once again on 16 July. The weather deteriorated further over the next couple of days as enhanced easterly flow in the wake of Talas brought outbreaks of heavy rain and squally thunderstorms to Hong Kong that required the issuance of rainstorm warnings on 17 – 18 July, with more than 400 millimetres of rain falling over the eastern part of Hong Kong Island and the northeastern part of the New Territories during the 2-day stormy episode. The lowest temperature in the month at the Hong Kong Observatory, 24.4 degrees, was recorded in rain on 17 July.

    A mixture of sunshine and showers then prevailed on 19 – 22 July, culminating in a very hot day on 22 July as a couple of tropical cyclones, Roke and Sonca, hovered over the northern part of the South China Sea. With Roke making landfall over the eastern part of Hong Kong on the morning of 23 July, local weather deteriorated with outbreaks of heavy squally showers. Showery weather continued to affect the territory the next day as Roke soon dissipated inland and Sonca headed towards the coast of Vietnam.

    The establishment of an anticyclone over southeastern China brought fine and very hot conditions to Hong Kong on 25 and 26 July. With yet another tropical cyclone brewing over the northern part of the South China Sea, local weather turned cloudier with isolated showers on 27 July. The developing cyclone was eventually named Haitang and headed towards Taiwan in quick succession following the passage of another tropical cyclone Nesat that moved in from the western North Pacific. With both cyclones passing at a distance to the east of Hong Kong, subsiding air over the Guangdong region led to prolonged sunshine and very hot conditions in the territory towards the end of the month. Oppressive heat under a hazy sky saw temperature at the Hong Kong Observatory soaring to the month’s highest of 34.8 degrees on 30 July. The mean temperature that day was 31.8 degrees, one of the highest for July since record began in 1884.


Rainfall distribution map during 17 and 18 July 2017
Rainfall distribution map during 17 and 18 July 2017 (isohyets are in millimetres)

    Eight tropical cyclones occurred over the South China Sea and the western North Pacific 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 2017

Table 1.1   Tropical Cyclone Warning Signals
Name of
Tropical Cyclone
Signal
Number
Beginning Time Ending Time
Day/Month HKT Day/Month HKT
ROKE 1 22 / 7 1540 23 / 7 0340
3 23 / 7 0340 23 / 7 0920
8 NW 23 / 7 0920 23 / 7 1320
3 23 / 7 1320 23 / 7 1510
1 23 / 7 1510 23 / 7 1940


Table 1.2   Strong Monsoon Signal
Beginning Time Ending Time
Day/Month HKT Day/Month HKT
17 / 7 2355 18 / 7 0900
31 / 7 1430 31 / 7 2045


Table 1.3   Rainstorm Warning Signals
Colour Beginning Time Ending Time
Day/Month HKT Day/Month HKT
Amber 17 / 7 1515 17 / 7 1915
Amber 17 / 7 2015 17 / 7 2045
Red 17 / 7 2045 17 / 7 2230
Amber 17 / 7 2230 17 / 7 2325
Amber 18 / 7 0820 18 / 7 1130
Amber 18 / 7 1620 18 / 7 2000
Amber 18 / 7 2245 19 / 7 0120
Amber 23 / 7 2230 24 / 7 0045


Table 1.4   Landslip Warning
Beginning Time Ending Time
Day/Month HKT Day/Month HKT
17 / 7 2100 19 / 7 0740


Table 1.5   Thunderstorm Warning
Beginning Time Ending Time
Day/Month HKT Day/Month HKT
1 / 7 1920 1 / 7 2130
1 / 7 2335 2 / 7 0455
2 / 7 0815 2 / 7 1515
2 / 7 1735 2 / 7 1930
2 / 7 2103 3 / 7 0215
3 / 7 0315 3 / 7 1500
4 / 7 0235 4 / 7 1645
5 / 7 0025 5 / 7 0300
5 / 7 1115 5 / 7 1215
5 / 7 1610 5 / 7 2000
5 / 7 2145 6 / 7 0610
6 / 7 0800 6 / 7 1130
6 / 7 1305 6 / 7 1800
7 / 7 0140 7 / 7 0410
7 / 7 1320 7 / 7 1430
7 / 7 2200 8 / 7 0015
8 / 7 0050 8 / 7 1100
8 / 7 1430 8 / 7 2130
9 / 7 0435 9 / 7 0615
11 / 7 0435 11 / 7 0515
12 / 7 0545 12 / 7 1100
12 / 7 1355 12 / 7 1500
13 / 7 1000 13 / 7 1200
13 / 7 1910 13 / 7 2015
14 / 7 1135 14 / 7 1245
14 / 7 1350 14 / 7 1700
15 / 7 0115 15 / 7 0145
15 / 7 1310 15 / 7 1530
15 / 7 1740 15 / 7 1930
15 / 7 2005 15 / 7 2300
16 / 7 0125 16 / 7 1800
16 / 7 2030 17 / 7 0130
17 / 7 0440 18 / 7 0230
18 / 7 0610 18 / 7 1215
18 / 7 1420 19 / 7 0200
20 / 7 1320 20 / 7 1630
22 / 7 0215 22 / 7 0630
23 / 7 0725 23 / 7 1330
23 / 7 2135 24 / 7 0345
24 / 7 1535 24 / 7 1615
27 / 7 0800 27 / 7 1220
31 / 7 1940 31 / 7 2045


Table 1.6   Very Hot Weather Warning
Beginning Time Ending Time
Day/Month HKT Day/Month HKT
11 / 7 1100 11 / 7 1800
13 / 7 0645 14 / 7 1700
22 / 7 1330 22 / 7 1830
25 / 7 1245 27 / 7 0715
28 / 7 1145 30 / 7 2315
31 / 7 1215 31 / 7 1800


Table 1.7   Special Announcement on Flooding
in the Northern New Territories
Beginning Time Ending Time
Day/Month HKT Day/Month HKT
3 / 7 1100 3 / 7 1510
18 / 7 0840 18 / 7 1430
18 / 7 1700 19 / 7 0110


Table 2   Figures and Departures from Normal - July 2017
Meteorological Element Figure of the Month Departure from Normal*
  Mean Daily Maximum Air Temperature 31.4 degrees C Normal
  Mean Air Temperature 28.7 degrees C 0.1 degree below normal
  Mean Daily Minimum Air
Temperature
26.9 degrees C 0.1 degree above normal
  Mean Dew Point Temperature 25.5 degrees C 0.4 degree above normal
  Mean Relative Humidity 83 % 2 % above normal
  Mean Cloud Amount 79 % 10 % above normal
  Total Rainfall 570.0 mm 193.5 mm above normal
  Number of hours of Reduced VisibilityΔ 4 hours 10.7 hours below normal§
  Total Bright Sunshine Duration 162.9 hours 49.1 hours below normal
  Mean Daily Global Solar Radiation 16.55 Megajoule / square
metre
0.62 Megajoule below normal
  Total Evaporation 117.6 mm 28.6 mm below 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 2016

  &   Data incomplete
daily values of selected meteorological elements for HK and the percentile map of mean temperature for July 2017


The percentile map of mean temperature of July 2017
  Remarks : Extremely high: above 95th percentile
Above normal: between 75th and 95th percentile
Normal: between 25th and 75th percentile
Below normal: between 5th and 25th percentile
Extremely low: below 5th percentile
Percentile and 5-day running average values are
computed based on the data from 1981 to 2010


Extract of Meteorological Observations in Hong Kong for July 2017