A sunny and dry December

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The weather in Hong Kong was sunny and bright in December 2017, in contrast to that in November. Apart from December 13, 17 and 23, the territory enjoyed more than two hours of sunshine each day during the month. The monthly total bright sunshine duration amounted to 209.6 hours, 37.4 hours above the normal figure of 172.2 hours. The monthly mean temperature was 17.8 degrees, 0.1 degrees below the normal figure of 17.9 degrees. Only traces of rainfall were recorded in the month, with most occurring during a surge of strong easterly monsoon from December 12 to 14, making December 2017 one of the 10 Decembers with traces of rainfall since records began in 1884. The total rainfall accumulated in 2017 was 2,572.1 millimetres, exceeding the yearly normal figure of 2,398.5 millimetres by about 7 per cent.

     Under the prevalence of dry continental air brought by the northeast monsoon, the weather in Hong Kong was generally fine for the first 11 days of the month, with conditions becoming cooler and drier from December 8 to 10 as a replenishment of the northeast monsoon reached the south China coastal areas.

     A fresh to strong easterly airstream brought windy and cloudy weather with light rain patches to the territory from December 12 to 14. The easterly airstream was soon replaced by a strong northerly airstream as a cold front formed over Guangdong on December 15 and moved across the coastal areas during the night. Affected by an intense winter monsoon following the cold front, local weather became cold, windy and dry over the next six days. The temperature at the Hong Kong Observatory on December 18 dropped to a minimum of 9.8 degrees, the lowest of the month, and daytime relative humidity fell below 40 per cent from December 19 to 21. Apart from a cloudy day on December 17, the weather in Hong Kong remained mostly fine with plenty of sunshine during the period.

     The monsoon winds started to subside on December 22 and a moist maritime airstream returned to the south China coast, with cloud bands from the northern part of the South China Sea covering the territory the next day. A replenishment of dry continental air on December 24 soon cleared away the clouds, and weather conditions were generally fine towards the end of the month.

     Two tropical cyclones occurred over the South China Sea and the western North Pacific in December 2017.

     Details of issuance and cancellation of various warnings/signals in December are summarised in Table 1. Monthly meteorological figures and departures from normal for December are tabulated in Table 2.

Ends/Wednesday, January 3, 2018
Issued at HKT 17:00

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Table 1 (Warnings and Signals issued in December 2017)

Table 2 Figures and Departures from Normal - December 2017