1. Typhoon Season
The Definition of Typhoon Season
Generally speaking, the term "typhoon season" is a period of time during which we have frequent typhoon effects, but within the recording of typhoons this term means the borders of one year in terms of the formation of typhoons. The starting point of counting for the number-based conventions of typhoon names is based on the typhoon season. The definition of typhoon season used in this site is as the following.
That is, a typhoon season in Western North Pacific begins in January, and the first typhoon of a typhoon season is named as Typhoon No. 1. The definition also indicates that the typhoon season is opposite between the northern hemisphere and the southern hemisphere.
The Earliest Typhoon and the Latest Typhoon
Then, in the Western North Pacific Basin, which typhoon is the earliest typhoon and the latest typhoon? To look for this information, we can use "Search by Month for the Birth of the Typhoon" in Search by Month, and search for typhoons that formed in January, the beginning of a season, and December, the end of a typhoon season.
These results show that the earliest typhoon and the latest typhoon is as follows.
In addition, Typhoon 200023 is a cross-season typhoon that exists across two years (in fact it exists across two centuries, or a cross-century typhoon). Searching for across-the-season typhoons, we can find 5 typhoons since 1951.
"True" Typhoon Season
However, the fact that we have several cross-season typhoons indicates that December 31 and January 1 is not the best day as the bounds of typhoon seasons. Probably the best day as bounds is a day such that the possibility of the existence of typhoons is minimum. Using Search by Day or Daily Typhoon Tracking Charts (Active Typhoon Maps), you can find typhoons present on each day.
Some time ago, the gap period of the typhoon was from February 8 to February 14, but Typhoon 201502 splitted this gap into two gaps, namely from February 7, 0:00 to February 8, 03:00 and from February 11, 21:00 to February 15, 18:00 in Japan Standard Time. In any case, the probability of the presence of typhoons is lowest in the middle of February, so we can regard "February 11" as the best day to define the bound of typhoon seasons.
In this case, which is the "truly earliest typhoon" and the "truly latest typhoon"? By analyzing Typhoons born in February, we can list them as follows.
In short, as is apparent in Typhoon Occurrence Statistics, and Monthly Typhoon Tracking Charts, February, not December, is the true bounds of the typhoon season. But in reality, this true typhoon season requires special treatment of the typhoon season, so it is more convenient to use a calendar year as a typhoon season.
2. The Average Birthday of the First Typhoon
Next we study the average birthday of the first typhoon. According to the statistics between 1951 and 2009,
On average, the first typhoon is born in the beginning of March on the calendar year-based typhoon season, but is born in the middle of April on the true typhoon season, In the latter case, the standard deviation is about one month, so we can say that the formation pattern is normal when we have the first typhoon between the mid of March and mid of May. The distribution of birthday, however, has a heavy tail toward late birthdays, so the median is a little earlier than the average.
Between two typhoon seasons, we usually have a long period without any typhoon formations. The ranking of those periods can be found in the following pages.