Why the Area We Watch in the Atlantic Expands in July to the Cape Verde Islands

Hurricanes begin to develop further east as we head deeper into summer in a period called Cape Verde hurricane season. 

This part of the Atlantic hurricane season gets this name because many tropical storms and hurricanes begin to develop and grow near the Cabo Verde Islands in the far east Atlantic just off the coast of Africa. 

A large percentage of tropical storm formations in late July, August and into September occurs in the Main Development Region, or MDR, which stretches from the coast of Africa westward to the Lesser Antilles and into the Caribbean. 

Roughly 85 percent of all Atlantic major hurricanes – Category 3 or stronger – have origins traceable to tropical waves.  About 60 percent of all tropical storms and Category 1 and 2 hurricanes are spawned by these waves.

On the other hand, some tropical waves cross the Atlantic Ocean and the Caribbean Sea without developing into a cyclone.  

Hurricanes that develop there this time of the year often have the longest time period to strengthen and organize into monsters. 

This area is typically favorable for development for a few reasons: 

• Tropical waves move westward off of the African continent every few days, especially in July, August and into September.
• The warmest and deepest waters of the Atlantic are often held in this area. 
• During the peak of overall hurricane season, wind shear wanes and storms are able to grow taller. 
• The air from northern Africa moistens and allows storms to persist and grow as they transit the Atlantic instead of drying out and collapsing as they do from May into June or July. 

Tropical waves are driven by the contrast of dry heat over the Sahara over far northern Africa and the cooler, more humid air over forested areas of the Gulf of Guinea and central Africa. 

(MORE: Why Tropical Waves are Important During Hurricane Season)

Once tropical waves move westward into the Atlantic, they are partially driven by the warm waters of the ocean. Waters in the eastern Atlantic are the warmest in late summer and early fall. Without this warm water, tropical waves are unable to grow and will eventually fall apart. 

Atmospheric variables such as humidity and wind shear are also the most favorable during this time of the year.  

This allows the success rate of tropical waves crossing the Atlantic higher than any other time of the year. 

Storms that form in the MDR usually track west-northwestward or northwestward into the Gulf of Mexico or western Atlantic. 

(MORE: Strange Atlantic Tropics in 2017: Early Start and an Unusual Location for Formation)

Notable Cape Verde Hurricanes

Although a few Cape Verde type hurricanes are common each year, here are a few notable ones: 

Ivan, 2004

Ivan strengthened as it reached the western MDR and became a formidable Category 5 hurricane with winds of 165 mph as it passed through the northwestern Caribbean Sea. Ivan is one of the most intense hurricanes to have roamed the Atlantic basin. 

Matthew, 2016

Although not the most typical Cape Verde system, this major hurricane did have its origins as a tropical wave in Africa.

It formed very late in the Cape Verde hurricane season by reaching the Caribbean in late September but still managed to become a powerful and low latitude Category 5 hurricane before grinding through Haiti and Cuba as a slightly weaker hurricane. 

Katia, 2011

Hurricane Katia trekked across the Atlantic at a more northerly latitude than most Cape Verde hurricanes. Katia gained strength as it rode the Bermuda High northwestward. The system became a Category 4 hurricane halfway between Bermuda and Hispaniola with winds of 140 mph. 

After becoming non-tropical, Katia crossed through Europe where it did damage in the United Kingdom and Russia. 

(MORE: Top 10 Most Extreme Hurricane Seasons in the Satellite Era) 

Gloria, 1985

Hurricane Gloria was another high latitude hurricane, but this one came much closer to the United States. Gloria became a Category 4 hurricane over the open Atlantic between Bermuda and the Bahamas with measured sustained winds of 145 to 155 mph.

As a Category 2 hurricane, Gloria dropped six plus inches of rainfall from North Carolina to New England. The system weakened slightly to a Category 1 hurricane before rushing through New York City. 

Emily, 2005

Hurricane Emily was a further south hurricane that crossed the entire Caribbean Sea and then crossed Mexico. At one point in its lifespan, Emily's sustained winds of 160 mph in the Caribbean as one the earliest Category 5 hurricanes. 

Note: Cape Verde officially changed its name to Cabo Verde in 2013, and the two are used interchangeably to fit both geography and meteorological naming systems.

MORE FROM WEATHER.COM: Life and Landfall of Hurricane Hermine: Africa to North Atlantic