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The effect of a prolonged rainy period in the middle of July 2011 was more than compensated by two fine spells occurring before and after the episode, making the month drier than usual. The monthly total rainfall recorded at the Hong Kong Observatory was 226.8 millimetres, about 61 per cent of the normal figure. The accumulated rainfall since January 1 was only 934.7 millimetres, a deficit of 35 per cent compared with the normal figure of 1429.1 millimetres for the same period. On the other hand, the month had 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 July 2 to 5. 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 localised heavy rain and squally thunderstorms from July 10 to 12. 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 July 16 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 July 21. A ridge of high pressure established over the coastal areas of Guangdong on July 22, 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 July 27, 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 July 26. Nock-ten further intensified into a severe tropical storm in the morning of July 27 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 July 29. 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 July 28. Winds remained fresh to strong, occasionally reaching gale force on high ground on the first part of July 29.
As Nock-ten weakened and moved further away from Hong Kong, there were sunny intervals locally on July 30. 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 the issuance and cancellation of various warnings/signals in July 2011 are summarised in Table 1. Monthly meteorological figures and departures from normal of July are tabulated in Table 2.
Ends/Tuesday, August 2, 2011
Issued at HKT 20:33
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