All tropical storms begin as numbered, unnamed tropical waves. The National Hurricane Center designates such tropical waves with a letter-number combination such as L97, where the “L” stands for Atlantic. While the number seems high, numbering in this region actually only cycles between 90 and 99. Other possibilities are “E” (eastern Pacific), “P” (south Pacific), “C” (central Pacific), “W” (western Pacific), “A” (Arabian Sea), “B” (Bay of Bengal), and S (southern Indian Ocean). Each region has its own separate number cycle.
The wave is watched until it either dies out or develops a closed convection. When the convection closes, the tropical wave designation is replaced with a tropical depression designation consisting of TD plus a number, such as TD9. Thus a tropical depression is actually a tropical storm with winds under the tropical storm threshold. As soon as the winds circling the tropical depression reach 39 mph (62 kph), the tropical depression becomes a tropical storm and is given a name.
Until 1951, tropical storms had been officially referred to by their latitude and longitude; but it was quickly discovered that using short, distinctive names for storms led to far less confusion in communications. The practice may have indirectly derived from the centuries-old Caribbean custom of naming storms for the saint’s day on which they occurred, or perhaps from the much more recent World War II practice of naming storms after wives and girlfriends. For two years, the United States experimented with trying to name storms for the radio phonetic alphabet (Able, Baker, Charlie). It never really caught on. Finally, in 1953, the United States introduced the modern practice of giving all tropical storms familiar female names, though it was not until 1979 that male names were included as well.
For the purpose of naming tropical storms, the waters of the world are divided into twelve zones:
* Atlantic * Eastern North Pacific * Central North Pacific * Western North Pacific * Western Australian * Northern Australian * Eastern Australian * Fiji * Papua New Guinea * Philippines [PAGASA] * Northern Indian * Southwest Indian
Each region names those storms which originate within their region. The name remains thereafter, even if the storm moves into a new region. (However, both the United States and Japan also track storms within the entire Pacific region, which can result in a Pacific storm having two names, especially within the Philippines [PAGASA] region.) Each region uses its own pre-established lists of names and has its own conventions to establish lists and change between them.
Both the American (Atlantic, Eastern North Pacific) and the Australian regions use alternating male and female lists of 21 names in alphabetical order. The American and most Australian lists omit the letters Q, U, X, Y and Z, ending the list at W, while the Western Australian list alternates its ending names between W, Y, and Z.
Uniquely, each country which is part of the Western North Pacific region provides one name drawn from the natural world (such as Nari, ‘Lily’) to the list. These names are then arranged alphabetically by the providing country.
The American lists start anew each year, using Greek letters if a given list runs out of names (as happened in 2005). Every other region uses its lists sequentially, beginning each new year with the next name in the previous year’s list.
One convention all meteorological services hold in common is to retire the name of a tropical storm, cyclone, or hurricane which has resulted in particularly high destruction or loss of life, to be replaced on the rotational list by another pre-determined name starting with the same letter.
