The Weather of July 2011 - A relatively dry July

     The effect of a prolonged rainy period in the middle of the month was more than compensated by two fine spells occurring before and after the episode, making July 2011 drier than usual.  The monthly total rainfall recorded at the Hong Kong Observatory was 226.8 millimetres, about 61 percent of the normal figure.  The accumulated rainfall since 1 January was only 945.2 millimetres, a deficit of 34 percent compared to the normal figure of 1429.1 millimetres for the same period.  On the other hand, the month has a near-normal mean temperature of 28.8 degrees.  

     Affected by an unstable southerly airstream, the weather was mainly cloudy with a few showers and thunderstorms on the first day of the month.  A ridge of high pressure extended from the western North Pacific and brought generally fine conditions to the territory from 2 to 5 July.  Dominated by a warm southwesterly airstream, it was sunny and rather hot for the next four days.  

     Under the influence of a broad area of low pressure, local weather deteriorated with localized heavy rain and squally thunderstorms from 10 to 12 July.  A trough of low pressure lingered along the coast of southern China for most of the time in the ensuing eight days and continued to bring unsettled weather with occasional rain and thunderstorms to the territory.  The rain was particularly heavy on 16 July where more than 70 millimetres of rainfall was recorded on Hong Kong Island in the small hours.  

     With the weakening of the trough of low pressure, the rain eased off and there were sunny periods on 21 July.  A ridge of high pressure became established over the coastal areas of Guangdong on 22 July, bringing mainly fine weather apart from a few showers and thunderstorms to Hong Kong.  With the persistence of the ridge of high pressure, local weather remained fine and became very hot in the next five days.  The maximum temperature at the Hong Kong Observatory rose to 33.9 degrees on 27 July, the highest of the month.  

     Meanwhile, a tropical depression to east of the Philippines developed into a tropical storm and was named Nock-ten on 26 July.  Nock-ten further intensified into a severe tropical storm on the morning of 27 July and crossed Luzon.  It moved generally west to west-northwestwards across the northern part of the South China Sea subsequently and made landfall near Wenchang in the evening on 29 July.  Locally, affected by the outer circulation of Nock-ten, the fine and very hot weather gave way to thunderstorms with strengthening winds in the latter part of 28 July.  Winds remained fresh to strong, occasionally reaching gale force on high ground on the first part of 29 July.

     As Nock-ten weakened and moved further away from Hong Kong, there were sunny intervals locally on 30 July.  A ridge of high pressure brought fine and hot weather to the territory on the last day of the month.

     Four 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 1.4.  Monthly meteorological figures and departures from normal for July are tabulated in Table 2.