The Weather of October 2012

     With the relatively dry northeast monsoon affecting southern China for most of the month, the weather of October 2012 was drier than usual.  The monthly total rainfall of 46.4 millimetres was only about 46 percent of the normal figure of 100.9 millimetres.  The accumulated rainfall since 1 January was 1804.8 millimetres, a deficit of 23 percent comparing to the normal figure of 2334.0 millimetres for the same period.  The mean temperature of the month was 25.6 degrees, near the normal figure of 25.5 degrees.

     Affected by the northeast monsoon, the weather in Hong Kong was mainly fine and dry for the first four days of the month.  A replenishment of the northeast monsoon reached the coast of Guangdong on the morning of 5 October, bringing fresh easterly winds.  Locally, apart from a few light rain patches in the morning, it was mainly fine and dry during the day from 5 to 7 October.  With the prevalence of a dry continental airstream, local weather remained generally fine and dry but with some haze for the ensuing eight days.  

     A weak cold front formed over central China and moved towards the coastal areas gradually on 16 October.  The weather in Hong Kong turned mainly cloudy with light rain patches on that day.  With the passage of the cold front across the coast of Guangdong, there was some haze locally on 17 October and the weather became slightly cooler on the morning of 18 October.  A ridge of high pressure established along southeastern China, bringing generally fine and dry weather to Hong Kong from 19 to 25 October.  

     Meanwhile, tropical cyclone Son-Tihn intensified into a tropical storm while crossing Luzon on 24 October.  It entered the South China Sea on 25 October and then tracked generally west-northwestward across the central part of the South China Sea in the next two days, intensifying to a severe typhoon on the evening of 27 October.  Over the south China coastal areas, the northeast monsoon continued to dominate.  Under the combined effect of the northeast monsoon and Son-Tihn, it was windy with a few rain patches in Hong Kong on 26 and 27 October.  The weather improved with sunny periods on 28 October, as Son-Tihn moved away, entering Bei Bu Wan as well as weakening gradually. 

     After making landfall over the coast of northern Vietnam, Son-Tihn turned to drift eastwards on 29 October, and finally dissipated over Bei Bu Wan the next day.  While the rain-bearing cloud band associated with Son-Tihn spread eastwards, affecting the South China coast on these two days, the northeast monsoon continued to prevail over the region.  In Hong Kong, it was cloudy and windy with rain and mist patches on 29 and 30 October.  The rain became persistent on 30 October with more than 40 millimetres of rainfall recorded over parts of the territory.  Meanwhile, a cold front formed over inland Guangdong and moved southwards to reach the coast on the night of 30 October.  Affected by the intense northeast monsoon behind the cold front, it became significantly cooler in Hong Kong with temperatures dropping generally to below 20 degrees that night.   The last day of the month remained cloudy and cool with rain in the morning.  The temperature at the Observatory fell to a minimum of 17.7 degrees on that day, the lowest of the month.  With the arrival of the dry continental airstream, local weather became fine and dry in the afternoon.

     Six 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 1.2.  Monthly meteorological figures and departures from normal for October are tabulated in Table 2.