Why One Storm Has Three Different Numbers: Category 5, Violent, and Signal 8 Explained

The same typhoon can be a Category 5 in one headline, a "violent typhoon" in another, and a Signal 8 somewhere else on the same afternoon.

None of them are wrong. They are measuring different things, on different clocks, for different purposes.

If you are travelling in Asia during typhoon season, knowing which number applies to you is more useful than knowing which one is highest.

The one difference that explains most of it

Wind speed is an average, and agencies do not average over the same amount of time.

The United States agencies, including the Joint Typhoon Warning Center, use a one-minute sustained wind. The Japan Meteorological Agency and most of the region use a ten-minute sustained wind.

A ten-minute average smooths out the peaks that a one-minute average captures. The same storm therefore produces a lower number under the ten-minute standard, by roughly ten to fifteen percent.

So a storm reported at 260 km/h by an American source and 215 km/h by a Japanese one is not a disagreement. It is the same wind, measured with a different stopwatch.

This single fact resolves most of the confusion people run into.

Category 5 is not an official category in the western Pacific

The Saffir-Simpson scale, the one that produces Categories 1 through 5, was built for Atlantic hurricanes.

No official agency in the western Pacific issues warnings in Saffir-Simpson categories. When you see "Category 5 super typhoon" in international coverage, that number is usually a conversion applied by media or derived from the JTWC's one-minute winds.

It is a useful shorthand for readers who know the hurricane scale. It is not what local authorities are acting on.

It also has a known limitation: the scale describes wind only. It says nothing about rainfall, storm surge, or the storm's size, and those cause a large share of actual deaths and disruption.

What Japan uses

The JMA classifies by ten-minute sustained wind, with names rather than numbers.

A system becomes a typhoon at 34 knots. Above that it is described as strong, then very strong, then violent, which is the top tier.

The JMA also publishes storm and gale warning areas, and those are the ones that matter operationally. Local governments issue evacuation information separately, on their own scale of levels.

For a traveller in Japan, the useful thing to watch is not the classification but the warning issued for the specific prefecture or city you are in.

What China uses

The China Meteorological Administration runs its own ladder, from tropical depression up through typhoon, severe typhoon, and super typhoon.

China also operates a coloured warning system, running blue, yellow, orange and red, with red the most serious. That colour system covers many hazards, not just typhoons, and it is what triggers local action.

Hong Kong's signals are the most practical for travellers

Hong Kong uses numbered signals, and they are unusually clear about what actually happens to your day.

Signal 1 is a standby. Life continues normally.

Signal 3 means strong winds. Some ferries stop, some outdoor activities are cancelled.

Signal 8 is the one that changes everything. Most businesses close, schools close, public transport reduces, and flights are widely disrupted. When people talk about a typhoon shutting Hong Kong down, they mean Signal 8.

Signals 9 and 10 are more severe still, with 10 indicating hurricane-force winds over the territory.

The signals are not a measure of the storm's strength in the open ocean. They describe expected conditions in Hong Kong itself, which is precisely why they are more useful to a person standing there than any category number.

The Philippines has its own numbers and its own names

PAGASA issues Tropical Cyclone Wind Signals, numbered 1 through 5, describing expected wind in a given area.

More confusingly for travellers, the Philippines assigns its own local name to any storm entering its area of responsibility. The same system can be called by an international name in Japanese coverage and a completely different Filipino name in local coverage, at the same moment.

If you are following a storm across the region, this is the single most common reason people think there are two storms when there is one.

So which number should you actually follow

The one issued by the agency responsible for the place you are standing in.

In Japan, the JMA warning for your prefecture. In Hong Kong, the Observatory's signal. In the Philippines, the PAGASA signal for your province. In Korea, the KMA advisory for your region.

International category numbers are for comparing storms to each other across years and basins. They are a journalism tool. They are not an instruction.

A storm can weaken from Category 5 to Category 2 and still close your airport, because it grew larger while it weakened, or because it slowed down and the rain totals went up. Following the category alone will mislead you in both directions.

Intensity is not the same as impact

The categories describe maximum sustained wind near the centre. That is one variable among several.

Size determines how long the bad conditions last and how wide the disruption is. Forward speed determines rainfall totals; a slow storm dumps far more water. Timing relative to high tide determines surge damage. Terrain determines whether rain becomes a landslide.

Some of the deadliest tropical systems in Asian history were not especially strong by wind category. They were slow, wet, and over the wrong terrain.

Where this site has been imprecise

On this site we have used Category 5 as shorthand in headlines about western Pacific storms.

That is defensible for an international audience, since the term communicates severity quickly. But it is worth being explicit that it is a translation, not the official classification any regional agency is issuing, and that we are converting from one-minute wind data when we use it.

We will keep using it for clarity, and we will keep saying so.

Where to check this yourself

Japan Meteorological Agency: https://www.jma.go.jp/en/typh/

Korea Meteorological Administration: https://www.kma.go.kr/eng/weather/typoon/typhoon_05.jsp

Joint Typhoon Warning Center: https://www.metoc.navy.mil/jtwc/jtwc.html

Hong Kong Observatory: https://www.hko.gov.hk/en/index.html

China National Meteorological Center: http://www.nmc.cn/publish/typhoon/typhoon.html

The short version

One storm, several numbers, because of averaging periods and different national systems.

Category numbers in the western Pacific are usually a conversion, not an official warning.

Hong Kong's signals and Japan's prefecture-level warnings describe what happens to you, which is what you need.

The Philippines names storms separately, so the same storm can appear to be two.

And the category tells you about wind near the centre, which is often not what causes the trouble.

Photo credits

Photographs in this post come from Wikimedia Commons and are used under their respective Creative Commons licences.