For people in Hong Kong, a No. 8 typhoon signal immediately raises questions about transport, work and school. Yet the full sequence looks rather strange: 1, 3, 8, 9 and 10. Where did the missing numbers go? If rainstorm warnings can use amber, red and black, why not use colours for typhoons, or simply number the levels from 1 to 5? And why does Macao use almost the same system? These apparently untidy numbers reflect the needs of a port city and successive attempts to prevent misunderstanding. A warning must explain the weather threat and help people respond. The history of Hong Kong’s signals shows how those purposes have evolved together.
Hong Kong was already using storm signals in 1884, primarily to serve ships in the harbour. Long before mobile phones, television or widespread radio broadcasting, cylinders, balls and cones were hoisted at prominent locations to communicate information about tropical cyclones and their approximate positions. A gun was fired to warn residents when gales were expected. Explosive charges replaced the gun in 1907, and this practice continued until 1937. The familiar Cantonese expression for hoisting a typhoon signal describes something that once happened quite literally. A physical signal was raised where people could see it from a distance. Everyday language still carries traces of that maritime communications system.
On 1 July 1917, Hong Kong introduced a numbered local storm warning system, running from 1 to 7, to warn residents about the threat of local winds. No. 1 meant standby. Nos. 2 to 5 indicated gales expected from the north, south, east and west respectively. No. 6 meant increasing gales, while No. 7 indicated hurricane force winds. The numbers therefore encoded several kinds of information: readiness, direction and worsening conditions. The old No. 4 did not represent some intermediate wind strength between today’s Nos. 3 and 8. The old No. 7 had a role closer to today’s No. 10. Historical storm reports must be read against the system then in use.
In 1930, a conference of Far Eastern meteorological services promoted a common system of storm signals. Hong Kong adopted a sequence running from 1 to 10 in 1931. Nos. 2 and 3 indicated strong winds from the south-west and south-east. No. 4 was a non-local signal. Nos. 5 to 8 indicated gales from the north-west, south-west, north-east and south-east respectively. No. 9 meant increasing gales, and No. 10 indicated hurricane force winds. This established the framework behind today’s numbers. It combined wind strength, direction and other functions. Despite the sequence from 1 to 10, there were never 10 evenly spaced levels of increasing wind strength.
The use of Nos. 2, 3 and 4 subsequently changed, and they were discontinued by the late 1930s. In 1956, the Observatory introduced the modern No. 3 Strong Wind Signal between the No. 1 Standby Signal and the No. 5 Gale Signal. It also separated warnings for strong winds caused by tropical cyclones from those caused by the monsoon. Today’s No. 3 therefore has its own history. The gaps in the sequence reflect successive additions, removals and revisions. Existing numbers were retained or withdrawn as their functions changed. Arrangements from different periods accumulated, without the entire system being renumbered from 1 whenever a change was made.
The reform that established the modern numbering took effect on 1 January 1973. The old Nos. 5, 6, 7 and 8 all indicated gales, differing only in wind direction. People could nevertheless interpret a higher number as stronger winds. A change from the old No. 8 to the old No. 6 might therefore be mistaken for an easing of conditions, when it simply meant a change from south-easterly to south-westerly winds. The Observatory renamed all 4 signals No. 8, adding the directions north-west, south-west, north-east and south-east. This addressed a specific communication problem and produced the familiar sequence of 1, 3, 8, 9 and 10. Its gaps have a traceable history.
Why retain the direction alongside No. 8? Winds of the same strength can affect boats, windows and coastal locations differently depending on where they come from. A sheltered position may suddenly become exposed after the wind changes. No. 8 Northeast means winds blowing from the north-east; it does not describe the cyclone’s position relative to Hong Kong. No. 9 indicates that gale or storm force winds are increasing, or are expected to increase, significantly. No. 10 means sustained winds have reached, or are expected to reach, 118 kilometres per hour or more. No. 9 thus communicates worsening conditions, without having its own separate, fixed wind speed range between Nos. 8 and 10.
When Hong Kong later established rainstorm warnings, it chose colours. The system introduced in 1992 had 4 levels: green, amber, red and black. In 1998, it became today’s 3-level system of amber, red and black. The current criteria include rainfall recorded or expected generally over Hong Kong exceeding 30, 50 and 70 millimetres in an hour respectively, with the rain likely to continue. This demonstrates that numbers and colours are both ways of communicating warning levels. Colours can accompany clear meteorological thresholds too. Typhoon signals and rainstorm warnings emerged in different periods and followed their own paths, leaving Hong Kong with 2 different naming systems.
In the United Kingdom, yellow, amber and red weather warnings combine the severity of expected impacts with the likelihood of those impacts occurring. The Met Office moved to impact-based warnings in 2011. Similar weather in different places or circumstances can therefore receive different assessments. The choice of assessment method can be considered separately from the colours or numbers used to communicate it. Simply renaming Hong Kong’s No. 8 as a red warning would leave its issuing criteria unchanged. Adopting a different basis for assigning warning levels would involve forecasting, risk assessment and response arrangements. That would be a much broader reform than changing an icon.
Macao’s similar system also reflects regional coordination. Both cities face the Pearl River estuary, and their early warnings were closely connected with maritime needs. Macao used several arrangements, expanding its system to 12 signals in 1920 before simplifying it to Nos. 1 to 8 in 1923. Following coordination among Far Eastern meteorological services, it adopted a sequence from 1 to 10 in 1931. In 1973, Macao also brought its directional gale signals together under No. 8. The shared warning language reflects a long history of exchange and institutional development. Macao went through several reforms of its own, so its historical signal numbers cannot simply be read using today’s definitions.
The main wind speed thresholds are similar in both cities, but each sets its own detailed definitions and issues warnings according to local conditions. Macao’s current No. 9 definition includes a cyclone centre moving closer, sustained local winds already reaching 63 to 117 kilometres per hour, and winds increasing or expected to increase significantly. Its No. 10 definition also explicitly refers to the cyclone centre passing close to Macao. Hong Kong’s corresponding definitions are worded differently. The same cyclone can produce different winds and different timings of wind shifts in the 2 cities because of its track, wind field and local terrain. Shared numbers do not require identical signals at identical times.
The signal numbers preserve history and carry meanings learnt over generations. Hong Kong’s last physical signal station closed in 2002, while Macao’s regulations still provide for signals to be hoisted at the Mount Fortress and Guia Fortress. Communication methods can change, and the system has already been revised to prevent misunderstanding. Any further renaming should be judged against a practical trade-off: would it make the danger clearer, and would that benefit justify the cost of learning new labels? Numbers and colours both need information about specific threats and appropriate responses to help protect people. Should Hong Kong retain its familiar typhoon signal numbers, or switch to colours? Share your view in the comments.
