During summer, typhoons, or tropical cyclones as they're known in Hong Kong regularly skirt the city. These can cause varying degrees of damage and on rare occasions injury and deaths.
Typhoon season runs from May through to late September, with September particularly susceptible to typhoons. Although the danger of these massive storms shouldn’t be underplayed, Hong Kong is adept in dealing with them.
Unless the city suffers a direct hit (which is rare) your holiday plans won't be blown too far off course.
Hong Kong's Warning System
Luckily, Hong Kong has an easy warning system which lets you know what intensity of storm is coming your way. The warning system is posted on all TV Stations (look for the box in the top right-hand corner), and most buildings will also have signs with the warnings on. See below for an explanation of the various signs.
T1. This simply means that a Typhoon has been spotted within 800km of Hong Kong. In practical terms, that means that the typhoon is still a day or two away and there is a good chance it will still change course and miss Hong Kong completely. Typhoon signal one is only intended as a notice to watch for further developments.
T3. Now things are taking a turn for the worse. Winds of up to 110km are expected in Victoria Harbor. You should tie down any objects on balconies and rooftops, and stay away from coastal areas.
Depending on the severity of the winds you may want to stay indoors. However, for the most part, Hong Kong will carry on as usual during a T3 warning—public transport will run and museums and shops will be open. It is worth checking your flights or ferries to Macau as these can be delayed. Hong Kong will usually issue a T3 signal about a dozen times each year.
T8. Time to baton down the hatches. Winds in Victoria Harbor may now be in excess of 180km. Most of Hong Kong will shut up shop and workers will be sent home. The Hong Kong Observatory will give a warning of a T8 signal at least two hours ahead of time to allow people time to get indoors. Public transport will operate during the warning period but not once the T8 signal is hoisted. You should stay indoors and away from exposed windows. If you’re staying in an older building, you may want to fix adhesive tape to the windows as this will reduce the possibility of injury if the window should shatter. Most restaurants will be closed and most, if not all flights will be canceled or diverted. T8 signals can last anywhere from an hour or two to all day, but the city gets back to business almost immediately after the signal is canceled. You'll find transport running and shops open almost immediately. The T8 signal is rarely raised more than once or twice each year.
T10. Known locally as a direct hit, a T10 means the eye of the storm will be parking itself directly over Hong Kong. Direct hits are rare. However, when one does hit, the damage can be immense, and sadly a number of people are usually killed.
You should follow the directions for a T8 and tune into local news for more information. There will always be a number 8 signal before a number 10 signal, which allows you plenty of time to seek refuge indoors. Remember, there may be a lull in the storm when the eye is directly over Hong Kong but you should remain indoors as the wind will return. Even with a direct hit Hong Kong does get itself back up and running pretty quickly. Expect some localized disruption but for the most part, everything should return to normal in just a few hours.
Both of these pages are from the Hong Kong Observatory.