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Historically, the Atlantic Hurricane Season has shifted, as close as June 15 to October 31, but since 1965, it has been between June 1 and November 30(1).

Why is the Atlantic Hurricane Season defined as being between June 1 and November 30? I know most hurricanes happen within this timeframe, but it seems, to the civilian eye, to be the strongest during August and September.

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    $\begingroup$ Hi, welcome to the nascent Earth Science site! A clarification question please: Are you asking why most hurricanes occur at a particular time of year, or are you asking why the Atlantic Hurricane Season is defined as being between those dates? Thanks. $\endgroup$
    – Semidiurnal Simon
    May 6, 2014 at 17:39
  • $\begingroup$ Hi @SimonW. See my question in bold...which would be "defined as being between those dates." I wouldn't think the hurricane season would take place when hurricanes aren't happening, so perhaps one question would answer the other? $\endgroup$
    – user402
    May 6, 2014 at 17:40
  • $\begingroup$ Just to refine things, are you asking "Why June 1 rather than June 15th?" or are you asking "Why do they all cluster during that timeframe?" It's a question of statistics/politics versus meteorology. $\endgroup$
    – Richard
    May 6, 2014 at 18:16
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    $\begingroup$ I assume the question is only referring to tropical cyclones in the North Atlantic, and not the South Atlantic. $\endgroup$
    – Siv
    May 6, 2014 at 18:17
  • $\begingroup$ @Richard Why between June 1 and November 30. A meteorological perspective would be nice to support, but as stated above, "defined as being between those dates." $\endgroup$
    – user402
    May 6, 2014 at 18:23

2 Answers 2

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The hurricane season is set by the National Hurricane Center. They use statistics to determine the dates. Specifically:

The Atlantic hurricane season is officially from 1 June to 30 November. There is nothing magical in these dates, and hurricanes have occurred outside of these six months, but these dates were selected to encompass over 97% of tropical activity. June 1st has been the traditional start of the Atlantic hurricane season for decades. However, the end date has been slowly shifted outward, from October 31st to November 15th until its current date of November 30th

NHC

As for why the dates are spreading out (from October to November), it appears that the dates of the hurricanes themselves are spreading out. (per James P. Kossin [1])

So, the dates are getting wider because the hurricanes are spreading out throughout the season. So, the NHC widens the dates throughout the years to correctly cover the majority of the hurricanes.

[1] James P. Kossin - Is the North Atlantic hurricane season getting longer?
GEOPHYSICAL RESEARCH LETTERS, VOL. 35, L23705, doi:10.1029/2008GL036012, 2008 James P. Kossin

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Hurricanes form over tropical waters (between 8 and 20 degrees latitude) in areas of high humidity, light winds, and warm sea surface temperatures [typically 26.5 degrees Celsius (80 Fahrenheit) or greater]. These conditions usually prevail in the summer and early fall months of the tropical North Atlantic and North Pacific Oceans, and for this reason, hurricane “season” in the northern hemisphere runs from June through November.

source: NASA

Global map of hurricane formation timing based on warm sea surface temperatures

CAPTION: Global map of hurricane formation timing based on warm sea surface temperatures.

Mosaic of all tropical storms that formed in the world between 1985 and 2005. As can be seen the majority occur in the Pacific Basin.

CAPTION: Mosaic of all tropical storms that formed in the world between 1985 and 2005. As can be seen the majority occur in the Pacific Basin.

As for the stronger storms, the logical answer is that the frequency of storms during a given period is directly related to the likelihood that the storm will have more energy.

Chart illustrating the worldwide frequency of tropical storms (blue area) and storms of hurricane/typhoon intensity (pink area) by month of occurrence.

CAPTION: Chart illustrating the worldwide frequency of tropical storms (blue area) and storms of hurricane/typhoon intensity (pink area) by month of occurrence.

Frequency graph of Atlantic hurricanes and tropical storms by month with peak in September and 97% between of all activity taking place between June 1 to November 30.

CAPTION: Frequency graph of Atlantic hurricanes and tropical storms by month with peak in September and 97% between of all activity taking place between June 1 to November 30.

Is the hurricane season getting longer per year?

Yes, it is getting longer according to research based on the first and last hurricanes of the year - (which I found as a result of this answer) - though my analysis that this research just reflects that we see more hurricanes now as a result of satellite imagery, not as a result of there being more storms.

Time series of the first and last tropical storm formation events each year in the Atlantic

CAPTION: Time series of the first and last tropical storm formation events each year in the Atlantic; red-lines (non-aircraft, non-satellite records); blue-lines (aircraft-based records included); green (saterllite based).

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