Gregorian/Lunar Calendar
Traditional Chinese Simplified Chinese
  • The Weather of May 2017

  • 2 June 2017

    Due to the heavy rain on the morning of 24 May, the month was wetter than usual. The total rainfall recorded in the month was 399.3 millimetres, about 31 percent above of the normal figure of 304.7 millimetres. The accumulated rainfall recorded in the first five months of the year was 533.8 millimetres, a deficit of about 17 percent compared to the normal figure of 640.8 millimetres for the same period.

    Under the influence of a maritime airstream, the weather in Hong Kong was hot with sunny periods on the first three days of the month. There was also coastal fog on the morning of 2 May. Upon the passage of a trough of low pressure, local weather deteriorated with heavy showers and squally thunderstorms on 4 May. More than 30 millimetres of rainfall were recorded over widespread areas. With the weakening of the trough of low pressure, the weather became mainly fine and hot in the next couple of days apart from some mist and haze.

    A fresh easterly airstream brought cloudier weather and a few showers to the territory on 7-8 May. With the passage of a trough of low pressure, there were thundery showers on the early morning of 9 May. Under light wind condition, the visibility was rather low on the next day. With the setting up of a ridge of high pressure over the coast of Guangdong and the northern part of the South China Sea, the weather became mainly fine and hot on 11 May, with temperatures at the Hong Kong Observatory rising to a maximum of 31.6 degrees, the highest of the month.

    With a trough of low pressure lingering over the coastal areas of Guangdong and the northern part of the South China Sea, the weather became more showery in mid May. The showers were heavy at times on 15 May, bringing over 50 millimetres of rainfall to Kowloon and Shatin. With the trough of low pressure moving to the northern part of the South China Sea and the onset of a relatively dry easterly airstream, mainly fine weather returned on 17 May. Under the influence of a maritime airstream, the weather turned generally cloudy with a few showers over the next six days.

    With the passage of a trough of low pressure across the coast of Guangdong, local weather deteriorated with heavy showers and squally thunderstorms on 24 May. The heavy rain, which necessitated the issuance of the first Black Rainstorm Warning this year, brought more than 70 millimetres of rainfall to widespread areas with rainfall exceeding 300 millimetres in Kwai Tsing and Sham Shui Po. Serious flooding were reported in many places over the territory including Lai Chi Kok, Tseung Kwan O, Ho Man Tin, central and western parts of the Hong Kong Island.

    With the trough of low pressure moving towards the northern part of the South China Sea, local weather improved with sunny intervals on 25 May. An easterly airstream brought mainly cloudy weather with one or two isolated showers to the territory the next day. Affected by a dry continental airstream, it was mainly fine and dry on 27-29 May. With the continental airstream weakening and being gradually replaced by a southwesterly airstream, the weather was hot with a few showers towards the end of the month.

Radar echoes during black rainstorm episode affecting Hong Kong on the morning of 24 May 2017
Radar echoes during black rainstorm episode affecting Hong Kong on the morning of 24 May 2017


Rainfall distribution map For 24 May 2017
Rainfall distribution map for 24 May 2017 (isohyets are in millimetres)

    There was no tropical cyclone 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 1.5. Monthly meteorological figures and departures from normal for May are tabulated in Table 2.

Warnings and Signals issued in May 2017

Table 1.1   Rainstorm Warning Signals
Colour Beginning Time Ending Time
Day/Month HKT Day/Month HKT
Amber 4 / 5 0940 4 / 5 1210
Amber 24 / 5 0640 24 / 5 0915
Red 24 / 5 0915 24 / 5 1130
Black 24 / 5 1130 24 / 5 1230
Amber 24 / 5 1230 24 / 5 1500


Table 1.2   Landslip Warning
Beginning Time Ending Time
Day/Month HKT Day/Month HKT
24 / 5 1115 24 / 5 1715


Table 1.3   Thunderstorm Warning
Beginning Time Ending Time
Day/Month HKT Day/Month HKT
4 / 5 0850 4 / 5 1545
8 / 5 2200 9 / 5 0030
9 / 5 0145 9 / 5 0315
10 / 5 1413 10 / 5 1650
15 / 5 0440 15 / 5 1615
16 / 5 0215 16 / 5 0615
24 / 5 0440 24 / 5 1600


Table 1.4   Fire Danger Warnings
Colour Beginning Time Ending Time
Day/Month HKT Day/Month HKT
Yellow 1 / 5 0600 1 / 5 1800
Red 27 / 5 1145 27 / 5 2045


Table 1.5   Special Announcement on Flooding
in the Northern New Territories
Beginning Time Ending Time
Day/Month HKT Day/Month HKT
4 / 5 1050 4 / 5 1355
24 / 5 1120 24 / 5 1430


Table 2   Figures and Departures from Normal - May 2017
Meteorological Element Figure of the Month Departure from Normal*
  Mean Daily Maximum Air Temperature 28.6 degrees C 0.2 degree above normal
  Mean Air Temperature 26.0 degrees C 0.1 degree above normal
  Mean Daily Minimum Air
Temperature
24.2 degrees C 0.1 degree above normal
  Mean Dew Point Temperature 22.5 degrees C 0.1 degree below normal
  Mean Relative Humidity 82 % 1 % below normal
  Mean Cloud Amount 77 % 1 % above normal
  Total Rainfall 399.3 mm 94.6 mm above normal
  Number of hours of Reduced VisibilityΔ 46 hours 0.6 hours above normal§
  Total Bright Sunshine Duration 126.0 hours 14.4 hours below normal
  Mean Daily Global Solar Radiation 14.30 Megajoule / square
metre
0.11 Megajoule above normal
  Total Evaporation 96.2 mm 14.5 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
daily values of selected meteorological elements for HK and the percentile map of mean temperature for May 2017


The percentile map of mean temperature of May 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



Rainfall distribution map For May 2017 (isohyets are in millimetres)

Extract of Meteorological Observations in Hong Kong for May 2017