Here's When the Atlantic Hurricane Season Typically Ramps 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 per year since 1950.

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. As August progresses, the number of named storms steadily increases.

Another way to look at the "ramping up" of the hurricane season is through an index called accumulated cyclone energy, or ACE. The ACE index is calculated by taking the square of each tropical storm or hurricane's wind speed (the wind speed multiplied by itself) and then adding those numbers from each point in 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.

According to Klotzbach's plot, the second week of August through September is the most active stretch of the Atlantic hurricane season, or when the ACE index increases fastest, shown where the graph is steepest.

In 2018, two storms were named in August, Tropical Storm Debby and Tropical Storm Ernesto. They both developed in the northern Atlantic and started off as subtropical storms. Early on September 1, just a few hours after August ended, destructive Hurricane Florence was named as a tropical storm.

The year prior, in 2017, four named storms developed in August and two major hurricanes, Harvey and Irma, formed at the end of August.

Several factors contribute to the seasonal ramp-up in August:

• African easterly waves are most developed, often serving as a seed for tropical development.
• Saharan air layers, surges of dry air into the central and eastern Atlantic Basin that normally squelch tropical development in those areas, tend to give way by August as the parade of African easterly 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 wannabe, tends to be low.
• Sea-surface temperatures rise toward a peak in early fall.
• Instability – the atmosphere's ability to generate convection (thunderstorms) to help initiate tropical cyclones – also rises toward an early fall peak.

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 might be able to nab a great discount on Caribbean travel in August or September. But make sure to buy travel insurance, just in case.