A cloudier July

     The weather in Hong Kong was cloudier than usual in July 2018. The mean cloud amount in the month was 77 per cent, 8 per cent above the normal figure of 69 per cent. The duration of bright sunshine in the month was only 181.1 hours, about 15 per cent below the normal figure of 212.0 hours. The month was also warmer than usual with mean temperature of 29.1 degrees, 0.3 degrees above the normal figure of 28.8 degrees. It was cloudy with rather showery conditions in Hong Kong for most of July. The monthly rainfall was 341.1 millimetres, about 9 per cent below the normal figure of 376.5 millimetres. The accumulated rainfall recorded in the first seven months of the year was 974.9 millimetres, a deficit of 34 per cent compared to the normal figure of 1 473.3 millimetres for the same period.

     Under the influence of the southwest monsoon, local weather was a mixture of sunshine and showers from July 1 to 7. There were occasional heavy showers and squally thunderstorms in isolated areas during these days. The isolated heavy showers on July 5 brought more than 200 millimetres of rainfall to Tai Po District. A broad trough of low pressure continued to bring showery weather to Hong Kong on July 8 and 9.

     Apart from one or two morning showers on July 10, the subsiding air associated with the outer circulation of tropical cyclone Maria brought generally fine weather to Hong Kong on July 10 and 11. While it was generally fine and very hot on July 12, with a broad trough of low pressure edging closer to the coast of Guangdong, the easterly winds strengthened and there were heavy showers and squally thunderstorms in Hong Kong from July 13 to 15. During the downpour, the temperature at the Hong Kong Observatory fell to a minimum of 25.0 degrees on July 15, the lowest in the month.

     A ridge of high pressure brought a strong easterly airstream over the coast of Guangdong on July 16 and the showery activities in Hong Kong subsided gradually. Meanwhile, an area of low pressure near Luzon developed into a tropical storm that was named Son-Tinh on July 17. Son-Tinh moved generally westward across the northern part of the South China Sea and entered Beibu Wan on July 18. It made landfall over the northern part of Vietnam and weakened into an area of low pressure inland on July 19. It was very hot with plenty of sunshine in Hong Kong, with temperature at the Hong Kong Observatory soaring to 34.3 degrees on the afternoon of July 17, the highest in the month. Under the influence of the outer rainbands of Son-Tinh, the weather in Hong Kong became mainly cloudy with occasional squally showers and thunderstorms that night and on the next day. Under the combined effect of the ridge of high pressure over the southeast coast of China and Son-Tinh, it was also windy on July 17 and 18 in Hong Kong.

     While the easterly airstream affecting the coast of Guangdong moderated gradually, a broad trough of low pressure continued to bring a few squally showers to Hong Kong on July 19 and 20. Under the influence of the subsiding air ahead of tropical cyclone Ampil, local weather became generally fine and very hot on July 21. After a fine and very hot morning, the weather became mainly cloudy with some squally showers over the western part of Hong Kong in the afternoon on July 22.

     Meanwhile, an area of low pressure associated with the remnant of Son-Tinh re-intensified into a tropical depression over Beibu Wan on July 22. It moved slowly over Beibu Wan and skirted past the northwestern part of Hainan Island. The tropical depression then made landfall over the Leizhou Peninsula on July 23 and dissipated over inland Guaugxi on the next day. With active showery activities over the northern part of the South China Sea, it was mainly cloudy with heavy showers and a few squally thunderstorms on July 23 in Hong Kong. Showers gradually subsided with more sunshine in the following two days.

     As an anticyclone aloft in southeastern China strengthened gradually, local weather remained generally fine and very hot apart from isolated showers towards the end of the month with the maximum temperature reaching the month's highest record of 34.3 degrees again on July 29.

     Seven tropical cyclones occurred over the South China Sea and the western North Pacific in July 2018.

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