Number 8 Storm Signal Rare In November: Meteorologist

The Meteorological Society says it is rare for Hong Kong to have a No 8 typhoon signal in autumn, with Wednesday's warning the first in November for 50 years.

Storm Nalgae earlier killed more than 100 people in the Philippines. But it was a more lacklustre affair in Hong Kong, with few reports of damage and no known casualties.

The society's Leung Wing-mo said an anti-cyclone weather system occurs around this time of the year in East Asia, producing northerly winds that normally push away approaching storms from the south.

He said an anti-cyclone also explained why Nalgae moved slowly.

“Basically, you can imagine the movement of a tropical cyclone to be geared, or guided by an even larger weather system, and that is what we call a steering current," Leung said.

“Now the steering current is not pushing the tropical cyclone towards Hong Kong, therefore it’s moving very slowly, because on its own there’s no influence at all, because a tropical cyclone would normally move slowly northwards.”

Leung said moisture and rainfall are key factors when it comes to the intensity of tropical cyclones and a cold and dry environment in Hong Kong caused Nalgae to weaken.