When the Atlantic Hurricane Season Typically Ramps Up

Jon Erdman
Published: July 29,2016
Frequently in July, I am asked: "Why has the Atlantic hurricane season been so quiet?" or "When will the hurricane season ramp up?"
The Atlantic hurricane season runs from June through November, but most years, the first two months of the season are typically benign.
June averages only one named storm every other year, and July has averaged one named storm each year since 1950.

Monthly Number of Atlantic Named Storms to Form 1950-2015

Data: NOAA/National Hurricane Center

Then comes August and it's almost as if a switch is flipped.
August sees more than three times the number of named storms as July, and almost double the number of June and July storms combined.
(MORE: Five Tools Used to Monitor Hurricanes You've Probably Never Heard Of)

Origin points for Atlantic named storms since 1950 in 10-day increments from July 1 through August 31. You can see the increased number of named storms forming in August, particularly in the "main development region" between Africa and the Lesser Antilles.

Another way to visualize the "ramping up" of the hurricane season is through an index called accumulated cyclone energy, or ACE.
The ACE index is calculated by adding each tropical storm or hurricane's wind speed through its life cycle.
Long-lived, intense hurricanes have a high ACE index. Short-lived, weak tropical storms, a low ACE index.
The ACE of a hurricane season is the sum of each storm's ACE, thus taking into account the number, strength and duration of all the tropical storms or hurricanes in the season.
(MORE: Weather Underground's Seasonal ACE Tracker)
Colorado State University tropical scientist Dr. Phil Klotzbach constructed a graph of how the ACE index adds up during an avearge Atlantic hurricane season.

Average Seasonal North Atlantic Ocean ACE Index 

1981-2010 climatology (Credit: Dr. Phil Klotzbach/Colo. St. Univ.)

Where the slope of this graph is steepest, in other words, when the ACE index is increasing the fastest, generally marks out the most active stretch of the Atlantic hurricane season.
According to Klotzbach's plot, this is from the second week of August through September.
While landfalls don't necessarily correlate with numbers of named storms, there's a 17-day stretch from mid-August to early September during which the most intense U.S. hurricane landfalls all occurred.
Several factors contribute to the seasonal ramp-up in August:

• African easterly waves are most well-developed, often serving as a seed for tropical development. 
• Saharan air layers, surges of dry air into the central and eastern Atlantic basin, which normally squelch tropical development in those areas tend to give way by August, as the parade of African eastely waves gradually add moisture. This effectively opens up more favorable real-estate for tropical cyclone development.
• Wind shear, the change in wind speed and/or direction with height which can rip apart a tropical cyclone wanna-be, tends to be low.
• Sea-surface temperatures rise toward a peak in early fall.
• Instability, namely, the atmosphere's ability to generate convection (t-storms) to help initiate tropical cyclones, also rises toward an early fall peak.

Typical origins and tracks of tropical cyclones in July and August in the Atlantic Basin. The orange and red contours show where named storms are more likely, in this case, in August compared to a fewer number in July.

Of course, averages and climatology are no guarantee of an outcome in any individual hurricane season.
However, if you have plans for, say, a Caribbean cruise, and you're concerned about hurricane season, the long-term data would suggest a lower chance of a hurricane interrupting your vacation in June or July, compared to August or September.
Then again, you may be able to nab a great discount on Caribbean travel in August or September. Just make sure to buy travel insurance, just in case.
Jonathan Erdman is a senior meteorologist at weather.com and has been an incurable weather geek since a tornado narrowly missed his childhood home in Wisconsin at age 7.