Whenever a tropical cyclone barrels toward the Southeastern United States, forecasters assign it a name and a category based on a grading system that is widely used for storms in the Atlantic Ocean.
But if a similar storm sweeps west across the Pacific Ocean, there is no such uniform system.
There are variations in how different regions of the world define tropical systems, including storms, depressions and cyclones. In the Western Pacific, the process is especially complex because countries and territories have their own systems for measuring, grading and naming tropical cyclones, which they call typhoons instead of hurricanes.
“In Asia it is a bit complicated,” said Clarence Fong, a meteorologist in Macau, a Chinese territory, who works for an intergovernmental committee under the World Meteorological Organization that coordinates typhoon warnings across the region.
Let us explain.
What cyclones have in common.
The scientific definition of a tropical cyclone is straightforward: It is a storm, typically with a diameter of around 200 to 500 kilometers (124 to 311 miles), that begins over a tropical ocean and generates violent winds, torrential rain, high waves and other bad weather. Less powerful storms are called tropical depressions or disturbances.
Another clear fact: Tropical cyclones are destructive. Experts say that climate change has increased the frequency of major tropical cyclones, and the potential for destruction, because a warmer ocean provides more of the energy that fuels them.
But the terms and categories that forecasters use for a cyclone depend on its location and intensity. And they aren’t especially intuitive.
Numbers vs. words
The term hurricane derives from hurakan, an Arawak word for a storm god. It applies to tropical cyclones that have maximum sustained winds of at least 74 miles per hour and form in the North Atlantic, the northeastern Pacific, the Caribbean Sea or the Gulf of Mexico.
Major hurricanes — Category 3, 4 or 5 — have maximum sustained winds of 111 m.p.h. or higher on the five-tier Saffir-Simpson Hurricane Wind Scale, which was developed by American forecasters in the 1970s and has been modified over the years.
But other parts of the world have completely different systems, and the guidelines are set on a regional level by distinct typhoon committees.
Read More About Extreme Weather
In the Indian Ocean, for example, three separate grading systems classify tropical storms and depressions using adjectives that change depending on a system’s location. If the equivalent of a midrange Category 3 hurricane formed in the western Indian Ocean, off the east coast of Africa, it would be a “very intense tropical cyclone.” But if it formed in the Arabian Sea or the Bay of Bengal — both of which are in the northern Indian Ocean — it would be a “super cyclonic storm,” one notch up from an “extremely severe cyclonic storm.”
Why the western Pacific is different.
Then there are typhoons, the term for tropical cyclones that develop in the northwestern Pacific and affect Asia. The word was used as early as the 16th century by European travelers in the East Indies, and it may have etymological origins in Arabic, Chinese, Greek and Urdu.
The basic definition of a typhoon is the same as that of a hurricane: a tropical cyclone with maximum sustained winds of at least 74 m.p.h. But several Asian countries have their own typhoon grading systems.
China, for example, would call a midrange Category 3 hurricane a “super” typhoon. Japan would call it a “violent” one. And in South Korea, the storm would be “super strong” — a category that was created two years ago in response to a higher incidence of powerful typhoons in recent years, according to the Korea Meteorological Administration.
Some governments also have unique ways of describing typhoons to their citizens. Hong Kong, a Chinese territory, uses a numbered warning system that was introduced in 1917 by the city’s British colonial authorities. And since 1963, the Philippines has given typhoons local names, a parallel naming system to one used by other Pacific countries and the United States.
The local names must be Philippine proper nouns that should not exceed nine letters or three syllables, said Sheilla Reyes, a weather specialist at the country’s national meteorological service. Some people have complained that the system is confusing, she added, but others like it because they find Filipino names easier to remember.
Why isn’t there a standardized system?
The U.S. government has had weather observation sites in Florida since the 1870s, and the National Hurricane Center, established in 1966, has long been the dominant weather authority for countries in the Atlantic basin. But when the United States military established an agency based in Hawaii for tracking Pacific typhoons in 1959, many governments in Asia had already developed their own monitoring and measuring systems.
One result: There are lingering discrepancies in the “averaging period,” the length of time that forecasters measure a tropical cyclone’s wind speed to get a reading. In the United States, the interval is one minute. In China it is two minutes. And in many other Asian countries and territories, including Macau and Hong Kong, it is 10 minutes.
Those discrepancies affect how powerful a storm looks to civilians. For example, Ms. Reyes said, a typhoon with 150 m.p.h. winds on a one-minute interval — a “super” typhoon in the American definition — would have only 115 m.p.h. winds on a 10-minute interval. That is partly why the Philippines downgraded its threshold for a “super” typhoon last year, to 115 m.p.h. from 138 m.p.h.
Before that change, “there were times when we were questioned why J.T.W.C. was in super and we weren’t,” Ms. Reyes said, referring to the United States military’s Joint Typhoon Warning Center, which is based in Hawaii.
Taoyong Peng, a senior scientific officer for tropical cyclones at the World Meteorological Organization’s headquarters in Geneva, called the wind-measurement discrepancies “very strange.”
Dr. Peng, the former chief scientist at the forecasting office in Guangzhou, China, said that the W.M.O. had been talking about standardizing the world’s wind-averaging period for about 20 years. In 2010, the agency issued guidelines for converting wind measurements between disparate systems.
But many countries are already used to their own systems, he added, and standardizing weather gear around the world would be a large and expensive undertaking.
“It would be very, very costly, and I don’t think the W.M.O. is in a position to pay,” he said.
John Yoon contributed reporting.