Waypoint 3 of 4
Waypoint 3: Storm Scales and Signals
News reports love a number: force 10, Category 4, EF3, Signal No. 8. At this waypoint we unpack those numbers so you can hear one and instantly picture what it means on the ground.
The Beaufort scale: wind you can see
In 1805, Royal Navy officer Francis Beaufort devised a scale to describe wind at sea by its effects on sails and waves. It was later adapted for land, and today runs from force 0 (calm, smoke rises vertically) to force 12 (hurricane force). The beauty of Beaufort is that it's observable: at force 6 large branches move and umbrellas become hard to use; at force 8, a gale, twigs break off trees and walking is difficult.
You'll still hear Beaufort numbers in shipping forecasts, including the BBC's famous Shipping Forecast for the waters around the British Isles. If you sail, kayak or walk exposed coasts, it's worth learning.
Saffir-Simpson: hurricane categories
Developed in the early 1970s by engineer Herbert Saffir and meteorologist Robert Simpson, this scale rates Atlantic and eastern Pacific hurricanes by their maximum sustained wind speed. Category 3 and above are classed as major hurricanes.
| Category | mph | km/h |
|---|---|---|
| 1 | 74–95 | 119–153 |
| 2 | 96–110 | 154–177 |
| 3 | 111–129 | 178–208 |
| 4 | 130–156 | 209–251 |
| 5 | 157 or higher | 252 or higher |
One crucial note for walkers: the scale measures wind only. Storm surge and rainfall flooding often cause the most harm, and a "low" category storm can still be deadly. Always follow evacuation orders, whatever the category.
Enhanced Fujita: rating tornadoes after the fact
Tornadoes are too small and fast to measure reliably with instruments, so they're rated by the damage they leave. The original Fujita scale was introduced by Tetsuya Theodore Fujita in 1971. The United States replaced it with the Enhanced Fujita scale in 2007, which runs from EF0 to EF5 and uses a list of damage indicators — from barns to schools to trees — to estimate wind speed.
Because ratings come from damage surveys, an EF number is assigned after a tornado has passed. When a tornado warning is issued, there's no rating yet. The only safe response is to shelter immediately in an interior room on the lowest floor.
Hong Kong's numbered signals
Some places describe storms by the action they require rather than their strength. The Hong Kong Observatory's tropical cyclone warning signals are a well-known example. Signal No. 1 is a standby: a cyclone is within about 800 kilometres. No. 3 means strong winds are expected or blowing. No. 8 — issued with a direction, such as No. 8 Northeast — means gale or storm force winds, and businesses and schools typically close. No. 9 signals that winds are increasing significantly, and No. 10 means hurricane force winds are expected or blowing.
No. 10 is rare. It was raised for Super Typhoon Hato in August 2017 and Super Typhoon Mangkhut in September 2018, which brought record storm surge to parts of the city.
Numbers are a starting point
Scales help you picture a storm, but they're never the full story. Two storms with the same number can behave very differently depending on where they land, how fast they move and how much rain they carry. Let the scale tell you how serious things might be — then let the official warning text tell you what to do.
Trail habit: when you hear a number, ask "and what does the warning say I should do?"