The 2017 hurricane season is on track to break the record for the most named storms

Every year at the end of the summer in the northern hemisphere, we start hearing reports of horrible people in the ocean poised to destroy the coasts of North America.

So far this year, Arlene, Bret, Cindy, Don, Emily, Franklin, Gert, Harvey, Irma, Jose, Katia, Lee, and now Maria have all spent time raging in the Atlantic.

The onslaught of tropical storm names during hurricane season can feel almost like American public is getting to know villainous characters from a movie, complete with personalities and pathologies. Harvey “ripped apart Houston” and Irma “lingered in the Atlantic.” This has been especially true in 2017, which has seen been introduced to more named tropical storms than usual.

How are hurricane names determined?

The practice of naming tropical cyclones (storm systems that occur over tropical waters, such as tropical storms, hurricanes, and typhoons) grew popular during World War II, when US Army and Navy meteorologists had to track storm movements across the Pacific Ocean.

The US National Hurricane Center (NHC) began officially started naming storms in 1953, a responsibility that has since shifted to the Geneva-based World Meteorological Organization (WMO).

Prior to World War II, hurricanes were identified by everything from the day they formed to their location coordinates to distinctive features of the storm. Human names are just easier to remember than complicated and varied weather terms. Using names allows for quicker exchange of storm information between weather stations, coastal bases, and ships, making it easier to distinguish between multiple storms occurring at the same time.

Storm systems get named when they achieve tropical storm strength (maximum sustained winds of 39 mph or greater); each tropical storm in a season is given a name starting with a unique letter, starting with “A” and going down an alphabetical list, alternating between traditionally male and female designations.

The NHC doesn’t choose the storm names; it follows a strict WMO procedure, which involves a pre-selected six-year list of Atlantic and Pacific storm names (in other words, they repeat every seventh year). The names on the list only change if a hurricane is so deadly or so costly that using the same name in the future would be inappropriate (Katrina and Sandy have both been retired, for example). If a storm name is dropped from the list, an international committee at the WMO chooses a new one at their annual meeting.

Why are there two hurricanes in the same year with the same letter?

Hurricane Max dissipated over Mexico, and the next day Hurricane Maria formed in the Atlantic basin.

It’s not a mistake—there are separate alphabetical lists for the Atlantic and Pacific basins, so you can have a Max and a Maria spinning at the same time.

What happens when hurricane names reach the end of the alphabet?

The WMO doesn’t use names starting with Q, U, X, Y, or Z, so each season actually starts with 21 possible names for potential hurricanes.

If more than 21 tropical cyclones are named during a season, additional storm names are taken from the Greek alphabet (Alpha, Beta, Gamma, etc). Only one Atlantic hurricane season in the last 20 years has resulted in Greek letter storms: 2005 went all the way to Tropical Storm Zeta, and included the devastating Hurricane Katrina. It was the busiest storm season in recent meteorological memory.

If a hurricane season doesn’t reach the end of the alphabet, the following year will reset at the “A” name of a new list. A tropical cyclone does not have to form during the defined hurricane season in order to get a name—If a hurricane formed on Dec. 31 this year, it would receive the next name on the list, but if it formed on Jan. 1, it would be called Alberto.

Is this hurricane season busier than usual?

Weather experts predicted the 2017 Atlantic hurricane season would have more named storms than typical because of a combination of a weak El Niño, above-average ocean-surface temperatures, and weaker vertical wind shear.

So far that prediction has been accurate; this year’s season is shaping up to be one of the most active on record. As of today (Sept. 19), the Atlantic has seen 13 named tropical cyclones, seven of which became hurricanes. According to Colorado State University tropical meteorologist Phil Klotzbach, only eight other years in recorded history had generated seven or more Atlantic hurricanes by the same point in the season:

The Atlantic season runs June 1 – Nov. 30 and the Pacific season is May 15 – Nov. 30. Hurricane activity ramps up when ocean waters get warm in late August into early September, and on average, Sept. 10 is the peak of the season.

Maria became a named storm on Sept. 16, 2017. This chart shows how many named storms had occurred by that date during the previous 21 Atlantic hurricane seasons:

The pace for 2017 is tracking very close to 2005’s record hurricane year, which saw 15 named storms at this point in the Atlantic season. North America has already experienced the same number of major hurricanes (category 3 or stronger) this year as it did during the entirety of the 2005 season.

This year is also on pace to meet or surpass the busiest Pacific hurricane seasons of the past two decades.

If you look the Atlantic and Pacific combined, the 2017 hurricane season on pace to break records. This chart shows the total number of named storms in both basins by Sept. 16:

With 28 named systems as of Sept. 16, 2017 is pacing ahead of the previous busiest hurricane seasons—2012 and 2005—at this point. By the end of 2012, there were 36 total named storms; 2005 saw a record 42. There’s still nearly 40% of the hurricane season left, which means 2017 is on pace to surpass 2005.

As seen in the above charts, seasons are cyclical, with the number of named systems rising and falling over time depending on meteorological conditions. Weather experts think we could be entering a less-active period, but caution that this doesn’t mean catastrophic and deadly hurricanes won’t form.