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Hurricane season, explained

Klaus Esterluss
September 7, 2017

Not every storm that occurs in the Atlantic becomes a hurricane. Nevertheless, there are always storms of the highest category - the latest, Irma, is one of them. But what does that mean?

Hurricane Irma, a record Category 5 storm, is seen approaching Puerto Rico in this NASA satellite image
Image: Reuters/NOAA

It's hurricane season in the Atlantic. From June 1 to November 30, there is a higher incidence of powerful storms. The latest is category 5 Irma, which hit several Caribbean islands on Wednesday, laying waste to homes, communities and claiming lives. It is now heading toward Florida.

With wind speeds of 250 kilometers per hour (185 miles per hour) and up, Irma is one of the most violent storms since records began. Its predecessor Harvey, which recently smashed into the US state of Texas, causing an estimated 150 to 180 billion dollars in damage, was category 4.

Strong, stronger, strongest

Members of the civil defense run as Hurricane Irma howls past Puerto Rico after thrashing several smaller Caribbean islands
Irma thrashed Puerto Rico and several other Caribbean islandsImage: Reuters/A. Baez

Irma's Category 5 classification is the highest the United States' National Hurricane Center issues, and implies "catastrophic consequences." In concrete terms, the authority says that means "a large number of houses will be destroyed, with total roof failure and wall collapse. Fallen trees and power poles will isolate residential areas. Power outages will last for weeks to possibly months."

The institution is also responsible for christening the storms. Altogether, there are six alphabetical lists, with a total of 21 male and female names each. Every year, one of the lists is chosen as the hurricanes' name-giver. After six years, the cycle starts over.

No end in sight?

Currently, about half this year's list has been used. Irma's successor, José is already on the horizon, and will be followed by Katia, which is brewing in the Gulf of Mexico.

 An E-3 Sentry flies over southeast Texas, U.S. after a mid-air refueling
Harvey inundated the Houston area with record rainfallImage: Reuters/Handout 2nd Lt. Faith Brodkorb/U.S. Air Force

Not all tropical storms develop into hurricanes - but José and Katia did intensify into hurricanes as of Wednesday. It's an unusually active hurricane season - and notably, one with particularly intensive storms.

This year is the first time three hurricanes are active in the Atlantic since 2010. In 1998, Hurricanes Georges, Ivan, Jeanne and Karl intensified simultaneously. The last time before that when four hurricanes were active at the same time was in August of 1893.

In the 1980s, there were only three category 5 storms, with another two the following year. From 2000 to 2010, that number rose to eight, including the most dramatic one so far: Katrina. It swept over New Orleans in 2005, and was responsible for the death of some 1,800 people. It is still regarded as the most expensive natural disaster in the history of the US.