Hurricane Irma was a Cape Verde hurricane. These types of Atlantic hurricanes originate from tropical waves, in the deep tropics, that pass over or near the Cape Verde Islands. Cape Verde hurricanes are typically the largest and most intense storms.
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The path of a Cape Verde hurricane is mainly westward, due to the placement of the subtropical ridge. Because of the region they form and this westward movement of the storm, they avoid any interaction with land, or cool water, which would weaken the storm. The placement and strength of the ridge help determine how long the hurricane remains on a westward track and when it will turn toward the north. Most Cape Verde storms develop in August and September, the peak of hurricane season.