1. Unit of Pressure and Wind

This page summarizes the unit of pressure and wind used in this web site.

Hecto Pascal (hPa)

Hecto Pascal is a unit for pressure, and, in this web site, used mainly for representing the central pressure of a typhoon. In Japan, the unit of "millibar" (mb) was used through November 1992, but since December 1992, the unit of "hectopascal" (hPa) has been used to comply with the International System of Units. Conversion between those two units is 1hPa = 1mb, however, so the value itself is the same as before.

Knot (kt)

Knot is a unit for speed. One knot means a speed of moving one nautical mile (nm) in one hour. Knot is used for representing the maximum wind speed at the center of a typhoon, or a movement speed of a typhoon on this website. Because 1nm = 1.852km, 1kt = 1.852km/h = 0.5144m/s. Roughly speaking, halving the knot makes the speed in meter per second, while doubling it makes the speed in kilometer per hour.

By the way, Japan Meteorological Agency publishes conversion tables of motion speed, wind speed and distance. This site refers to these conversion tables.

2. Classification of Typhoons

The Classification of Intensity of Typhoons

The intensity of a tropical cyclone is classified by the maximum sustained wind (10-min mean) according to World Meteorological Organization (WMO). The following table summarizes categories for tropical cyclones. Here "Tropical Depression" is a tropical cyclone weaker than a typhoon, and a tropical cyclone stronger than a typhoon, Japan Meteorological Agency (JMA) classification has four levels (previously five) and international classification has three levels.

Note that the definition of "typhoon" is different between the Japanese standard and the international standard. A tropical storm with the wind speed of more than 34 kt is called a "typhoon" in Japan, while in the international standard, that with the wind speed of more than 64 kt is called a "typhoon." Tropical cyclones in the world are called by different names in each basin, such as a "typhoon" and a "hurricane," but the standard to be called by such names is the same : more than 64 kt of wind.

The Classification of Intensity of Typhoons and Hurricanes (USA Standard)

Joint Typhoon Warning Center (JTWC) and other US meteorological organizations use Saffir-Simpson Scale to classify tropical storms stronger than the hurricane (or typhoon) intensity based on the maximum sustained wind (1-min mean).

A typhoon with maximum sustained surface winds greather than or equal to 130 knots (approximately Category 5) is called a "super typhoon," and a hurricane of Category 3 and above is called a "major hurricane." A tropical cyclone weaker than Category 1 is not a "typhoon" in the international standard, but may be classified as a "typhoon" in the Japanese standard.

In mainland China and Hong Kong, a typhoon with maximum sustained surface winds greather than or equal to 100 knots are called Super Typhoon. However, maximum sustained winds are measured differently in mainland China and Hong Kong, where the former uses 2-min mean and the latter uses 10-min mean.

In addition to maximum wind, maximum gust is also used to represent the strength of winds in a shorter time scale. Japan Meteorological Agency defines maximum gust as the average of 3 seconds (the average of 12 measurements by the frequency of 0.25 seconds), in comparison to the average of 10 minutes for maximum wind. The ration of maximum gust divided by maximum wind is called "gust factor" and it is known to take some values between 1.5 and 2. This means that 50m/s maximum gust could happen within the storm-wind circle of 25m/s maximum wind.

For other regions than Japan and the United States, please refer to the classifiction of tropical cyclones in the world.

The Classification of Size of Typhoons

The size of a typhoon is classified by the radius of the area in which the wind speed exceeds 15 m/s. The intensity and size represents different aspect of a typhoon. That is, we could have a strong but not large typhoon, and also a large but not strong typhoon.

Effect of Typhoons and Typhoon Classes

Before 2000, JMA has been using additional typhoon classes for intensity and size. The intensity class had "weak" (which corresponds to Tropical Storm) and "middle" (which corresponds to Severe Tropical Storm), and the size class had "very small," "small," and "middle." These classes, however, might have given unreasonable relief to the people's attitude, such as "this typhoon is OK because it's very small and weak."

In addition, these typhoon classes were regarded as one reason for a big accident at a river on 1999, when a weak tropical depression caused heavy rain resulted in more than 10 people died of increased water level. Difference between typhoons and tropical depressions is only in terms of winds, and it has nothing to do with rains, but the expression of "weak" might have given different impression for preparedness. Based on this reflection, these classes are removed after 2000.