Mainly attributing to a weaker than normal northeast monsoon over southern China in September, the month was much hotter than usual, the Hong Kong Observatory (HKO) said today.

The monthly mean temperature of 29.2 degrees Celsius, mean maximum temperature of 32 degrees Celsius and mean minimum temperature of 26.8 degrees Celsius were respectively the third, one of the fourth and one of the seventh highest for September on record.

With a stronger than normal troughing flow in the lower atmosphere over the coast of southern China and the northern part of the South China Sea, the month was also cloudier and wetter than usual.

The mean amount of cloud in the month was 74%, which is 8% above the normal and one of the ninth highest on record for September.

The monthly rainfall was 520.9 mm, about 62% above the norm. The accumulated rainfall in the first nine months of the year was 2104.3 mm, about 6% lower than the norm for the same period.

An area of low pressure over the seas east of the Philippines intensified into a tropical depression on September 1 and was named Yagi.

In Hong Kong, it was still very hot with sunny intervals during the day on September 5. With the approach of Yagi, Hong Kong’s weather deteriorated later on that day and the first No. 8 Gale or Storm Signal in the year was issued.

Yagi attained its peak intensity on September 6, making it the second strongest tropical cyclone in the South China Sea since the HKO’s records began in 1950.

The weather was generally fine and very hot during the days from September 17 to 19. It was extremely hot on September 17, with temperatures rising to a maximum of 35.7 degrees Celsius, the highest of the month and also the hottest Mid-Autumn Festival on record.

The showers were particularly heavy during the day on September 21 when an area of low pressure over the trough moved across the city, necessitating the issuance of the Red Rainstorm Warning Signal.

More than 100 mm of rainfall was generally recorded over the city and rainfall even exceeded 250 mm over most parts of Hong Kong Island from September 20 to 24.

Under the rain and affected by the northeast monsoon, the temperatures dropped to a minimum of 23.4 degrees Celsius on the morning of September 23, the lowest of the month.

A waterspout was also reported over the sea area off Hung Hom on September 28, the first occurrence in Victoria Harbour according to reports received by the HKO since 1959.

Eleven tropical cyclones occurred over the South China Sea and the western North Pacific in September, the HKO added.