The Weather of September 1997

    September 1997 was cooler than normal and the mean temperature of 26.4 degrees was the eighth lowest for the month. The mean maximum temperature of 28.5 degrees was the fourth lowest for the month. The month was also cloudier than normal with the mean amount of cloud, 74 per cent, ranking the eighth highest for the month. As a result, the monthly total bright sunshine was only 117.1 hours, the second lowest for September.

    The total rainfall in the month was 232.9 millimetres, 66.8 millimetres below the normal figure. However, the accumulated rainfall since 1 January of 3 216.6 millimetres was 60 per cent above the normal figure and the highest ever recorded for the period.

    With no tropical cyclones in our vicinity, the monthly mean pressure of 1 012.1 hectopascals was the highest on record for September.

    The month started mainly fine and sunny with temperatures climbing to 31.5 degrees on 1 September, the highest in the month. With the approach of a trough of low pressure, the weather turned cloudy with thundery showers on 3 September. There were some mist patches the next day. Although sunny weather returned on 5 and 6 September as the trough moved into the South China Sea, the retreat of the trough northwards across the coast brought heavy showers to Hong Kong on 7 September and thundery showers the next day.

    It became fine again on 9 September and sunny conditions prevailed for the next few days. Temperatures reached 31.5 degrees again on 12 September. Isolated heavy showers affected the northeastern part of the New Territories on the afternoon of 13 September. A band of rain and thunderstorms swept across Hong Kong early the next morning. The weather remained cloudy with some showers the next few days, except on 17 September when it was fine and sunny. Heavy rainfall occurred over the southeastern part of Hong Kong Island on the early morning of 18 September.

    The northeast monsoon intensified over China and pushed southwards bringing cooler weather on 21 September and temperatures dropped to 19.3 degrees, the lowest in the month. This figure was the fifth lowest for the month of September. The combined effect of the monsoon and Tropical Storm Fritz over the South China Sea maintained strong easterly winds along the coastal areas for the next couple of days. The weather remained generally cloudy with some light rain patches.

    A drier airstream reached the coastal areas on 24 September bringing along fine and sunny conditions in the next three days. On 28 September, the weather became cloudy with periods of rain which persisted until the end of the month.

    Six tropical cyclones occurred in the western North Pacific and the South China Sea in the month.

    During the month, no aircraft was diverted due to adverse weather.