typhoon

Hong Kong's Number 8 tropical cyclone warning can be controversial; the signal means that gale force winds are expected or blowing in Victoria Harbour. Especially as this may mean hurricane force winds may soon follow - as the eye of a typhoon approaches - this signal can prompt a virtual shutdown of Hong Kong.

But, hurricane force winds rarely follow; and there have been occasions when the Number 8 has been issued, then little happened bar strong winds and rain - and business folk have complained about the city shutdown causing economic losses they believe are unnecessary.

Hearing a discussion about Hong Kong's tropical cyclone warning signals on RTHK Radio 3 recently, I emailed to suggest that there might be two signals when gales are due: a Number 8 if stronger winds don't seem imminent, and another signal if there appears to be a chance that the winds will reach and then surpass gale force. Dr Wong of the Hong Kong Observatory, who was on air at the time, thanked me for my suggestion but said we already have the second signal - the Number 9. But, the Number 9 is issued only when gales are blowing and increasing, not before gales have started; I emailed the Observatory to say so, and this led to the following correspondence.

Hong Kong lies in the tropical belt that's impacted by low pressure systems that can intensify into the world's most powerful storms: known as hurricanes in North America, cyclones in Australia and the Indian Ocean, and typhoons in the western Pacific. Happily, typhoons are scarce near Hong Kong, and rarely score direct hits (13 from 1946 to now) - but can be spectacular and deadly when they do impact the territory.

As they need warm sea water to spawn and grow, tropical storms are rare in winter, mainly occurring from around May to October - with Hong Kong mostly affected from mid-August to early October.