Starting from 1884, a system of drum, ball and cone was employed to give information to the mariners in the harbour on the existence and approximate location of a tropical cyclone. For the local public a typhoon gun was used to warn imminent gale force winds brought about by tropical cyclones. In 1907, explosive bombs replaced the typhoon gun as they made louder sounds and were considered an improvement over the firing of a gun. The last typhoon boom was exploded in 1937.
In 1917, the first numbered signal system geared to the warning of wind conditions in Hong Kong was introduced. The numbers were from 1 to 7 with numbers 2 to 5 signifying gale force winds expected from the four quadrants, namely N, S, E and W.
In 1931, the signals were amended to 1 to 10 with signals 2 and 3 signifying strong winds from SW and SE respectively, signal 4 being a non-local signal, signals 5 to 8 signifying gales from the four quadrants, namely NW, SW, NE ad SE, signal 9 signifying increasing gales and signal 10 indicating the threat of hurricane force winds. Signals 2, 3 and 4 were used intermittently afterwards and were discontinued in the late 1930s.
In 1956, the No. 3 Strong Wind Signal was introduced between the No. 1 Stand-by Signal and the gale signals.
Starting from 1 January 1973, signals 5 to 8 were replaced by 8 NW, 8 SW, 8 NE and 8 SE respectively so as to avoid misunderstanding by the public. This system has been in use ever since.
Originally, the signals were intended mainly for the benefits of mariners but have over the years been also adopted for use by the public. Starting from 1987, the Observatory issued the Pre-No.8 Special Announcement to give an advance notice to the public when the No.8 signal is expected within two hours.
Evolution of the numbered typhoon signal system in Hong Kong over the past 100 years