How Hurricane Irma became the second-strongest Atlantic hurricane on record; a question and answer

Hurricane Irma, a Category 5 storm, has already made history as one of the most powerful Atlantic hurricanes on record.

Unleashing 185-mph winds and torrential rainfall, Irma has already caused death and destruction on small islands in the Caribbean and was expected to strike Florida early Sunday. States of emergency have already been declared in Puerto Rico, the U.S. Virgin Islands, Florida and South Carolina.

Here is a breakdown on hurricane strengths and how Hurricane Irma gained power.

Question: What does a Category 5 storm mean?

A: When