The Science Of Why Hurricane Season Ends After Thanksgiving

You probably know that hurricane season officially “ends” on November 30th, and it was certainly one to remember. As predicted, the Atlantic hurricane season was quite active and record-breaking too. There were eighteen named storms. Eleven of them became hurricanes, and five of those were categorized as major (category three or higher). Devastating storms like Beryl, Helene, and Milton causes hundreds of billions of dollars in damage and changed lives forever. You know all of that, but I suspect that many readers may wonder why the season “ends” shortly after Thanksgiving. Here’s why.

Like many things in meteorology, physics is at the root of why hurricane season “ends” in November. By the way, I will explain later the use of quotations around the word “ends.” The Atlantic hurricane season starts on June 1st, which is also the first day of meteorological summer. Climatologically, ocean heat content and associated sea surface temperatures become supportive of tropical development around that time. Ocean heat content is the integrated temperature from the sea surface down to where the 26°C temperature can be found.

There are six basic conditions required to support hurricane development according to the National Oceanic and Atmospheric Administration website: warm water, distance from the equator, an atmosphere conducive for air to rise, ample moisture, weak vertical wind shear, and some type of seedling wave or system. Sea surface temperature is often used as an indicator of hurricane potential. My colleague Scott Lindstrom’s CIMSS Satellite blog illustrates many of these concepts with Hurricane Milton (2024).

According to NOAA, “The first condition is that ocean waters must be above 26 degrees Celsius (79 degrees Fahrenheit). Below this threshold temperature, hurricanes will not form or will weaken rapidly once they move over water below this threshold.” This condition is strongly tied to the seasonal cycle. While air temperature starts to increase in June, water temperature typically lags because of the higher specific heat of water. What’s specific heat? It is the amount of heat for a given unit mass needed to raise temperature by 1 degree Celsius.

Because the specific heat of water is high, peak hurricane activity happens in the August to September time frame. If you have ever gone to a Florida beach on a hot June day, you probably have experienced the power of specific heat. The sand (lower specific heat so heats up faster) felt hot to your feet while the water was likely cool.

The graphic above shows the 2024 SST cycle for the Atlantic Main Development Region in 2024 and how it compares to previous years. Historically, the threshold SST is reached around the May to June timeframe. Temperatures start to cool down substantially after November. Additionally, stronger wind shear in the basin starts to enter the equation as more jet stream activity meanders equatorward as winter approaches.

In recent years, climate warming has shifted the entire SST cycle to higher values. The orange and red lines represent 2023 and 2024, respectively. In 2024, the mean temperature has remained above 26 degrees Celsius all year. There are many other years in which temperatures remained warm enough for tropical cyclone activity outside of the June to November window and that is reflected in the graphic below.

I remember being impacted by a “preseason” tropical storm during a cruise with my family out of Charleston, South Carolina. It is evident that storms can form in May or December too. This brings me back to why I wrote “ends” earlier in this article. In 2016, I wrote a piece at Forbes.com after witnessing Hurricane Alex in the middle of January. I opined, “At this point I could be provocative and ask if the concept of a hurricane season has become obsolete, but I won’t take the click-bait route.” The 2005 season brought Hurricane Katrina, but it was also the first season in which we ran out of names and had to use the Greek alphabet. The last named storm of that season was in January of 2006.

Over the past decade, scholars have started to question whether the hurricane season needs to be lengthened. In a 2016 American Meteorological Society blog entry, the author wrote, “In an e-mail exchange with James Franklin, branch chief of the Hurricane Specialist Unit of forecasters at the National Hurricane Center, he noted that the current 6-month Atlantic hurricane season was established in 1965 and was based on the formation dates of roughly 97% of the total annual tropical cyclone activity in the Atlantic basin, which includes the northern hemisphere Atlantic Ocean, Caribbean Sea, and Gulf of Mexico.” In the same blog, analysis by Bob Henson and Jeff Masters of Weather Underground revealed, “41 preseason tropical or subtropical systems have formed in 33 separate years since record-keeping began in 1851. Since the satellite era began in 1960, which improved detection of tropical systems basin wide, they find that there has been on average about one such system in the Atlantic every 2-3 years.” A 2022 study published in Nature attributed trends in preseason storms to a thermodynamic environment (warming) more conducive for formation earlier in the year.

Time and further analysis will determine whether an extension of the season is warranted. For now, it ends a few days after Turkey Day.

This post was originally published on here