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)

     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.