Reference for the Issue of No.3 and No.8 Signals

Starting from 2007, the reference for the issue of No.3 and No.8 signals was expanded from the Victoria Harbour to a network of eight near-sea level reference anemometers covering the whole of Hong Kong as depicted in the following figure.

Network of reference anemometers in the tropical cyclone warning system

The reference anemometers were selected on account of their good exposure and geographical distribution, taking into account the natural separation by Hong Kong mountain ranges. Together, they provide a broad picture of the wind condition in Hong Kong.

The No.3 or No.8 signal, as the case may be, will be issued when half or more anemometers in the reference network register or are expected to register sustained strong winds or gale/storm force winds and the wind condition is expected to persist. The wind speed range of the No.3 signal is 41-62 km/h and that of the No.8 signal is 63-117 km/h.

When a tropical cyclone is located relatively far from Hong Kong, occasionally an outer rainband associated with the tropical cyclone might sweep across the territory, bringing squalls which cause transient winds at four or more of the reference anemometers exceeding the prescribed lower thresholds for the issue of No.3 or No.8 signals. In that case, as the wind condition is not expected to persist after the passage of the rainband, the Observatory would not issue the respective signals.