The Weather of July 2017

    With a trough of low pressure lingering over the south China coastal region in the early part of the month and frequent tropical cyclone activities over the northern part of the South China Sea in the latter half, July 2017 was cloudier with more rain than usual. The monthly total rainfall was 570.0 millimetres, more than 50 percent above the normal figure of 376.5 millimetres. The accumulated rainfall recorded in the first seven months of the year was 1759.8 millimetres, nearly 20 percent above the normal figure of 1473.3 millimetres for the same period.

    An active southwest monsoon brought cloudy and showery weather to Hong Kong on the first three days of the month, with some isolated heavy downpour affecting mostly the New Territories. Under the influence of a broad trough of low pressure over the coast of Guangdong and the northern part of the South China Sea, the weather became even more unsettled with occasional heavy showers and squally thunderstorms that lasted till 8 July despite some interludes of sunshine. With the setting in of the subtropical ridge over southeastern China, showery activities gradually eased off after 9 July. While hot and mainly fine weather prevailed in Hong Kong over the next five days, some isolated heavy showers did affect the Sai Kung areas on 12 July.

    Meanwhile, easterly winds along the coastal areas of Guangdong gradually strengthened as an area of low pressure near Hainan Island eventually developed into a tropical cyclone named Talas on 15 July. Even though Talas moved away towards the coast of Vietnam, local weather turned cloudy and showery once again on 16 July. The weather deteriorated further over the next couple of days as enhanced easterly flow in the wake of Talas brought outbreaks of heavy rain and squally thunderstorms to Hong Kong that required the issuance of rainstorm warnings on 17 – 18 July, with more than 400 millimetres of rain falling over the eastern part of Hong Kong Island and the northeastern part of the New Territories during the 2-day stormy episode. The lowest temperature in the month at the Hong Kong Observatory, 24.4 degrees, was recorded in rain on 17 July.

    A mixture of sunshine and showers then prevailed on 19 – 22 July, culminating in a very hot day on 22 July as a couple of tropical cyclones, Roke and Sonca, hovered over the northern part of the South China Sea. With Roke making landfall over the eastern part of Hong Kong on the morning of 23 July, local weather deteriorated with outbreaks of heavy squally showers. Showery weather continued to affect the territory the next day as Roke soon dissipated inland and Sonca headed towards the coast of Vietnam.

    The establishment of an anticyclone over southeastern China brought fine and very hot conditions to Hong Kong on 25 and 26 July. With yet another tropical cyclone brewing over the northern part of the South China Sea, local weather turned cloudier with isolated showers on 27 July. The developing cyclone was eventually named Haitang and headed towards Taiwan in quick succession following the passage of another tropical cyclone Nesat that moved in from the western North Pacific. With both cyclones passing at a distance to the east of Hong Kong, subsiding air over the Guangdong region led to prolonged sunshine and very hot conditions in the territory towards the end of the month. Oppressive heat under a hazy sky saw temperature at the Hong Kong Observatory soaring to the month’s highest of 34.8 degrees on 30 July. The mean temperature that day was 31.8 degrees, one of the highest for July since record began in 1884.