Winds aren't the most dangerous part of a hurricane

It has already touched down in the Bahamas and is causing heavy flooding there.

The storm is rapidly increasing in intensity, having grown from a tropical storm to a Category 3 hurricane in just 24 hours. It is expected to reach at least Category 4 status by the end of the day Thursday.

Yet while a hurricane's strength is defined by the speed of its wind, the water is really what can kill you.

In fact, the National Hurricane Center estimates that 75% of all deaths from hurricanes in the US between 1963 and 2012 were due to storm surges and rainfall flooding.

Storm surge

Heavy rainfall and flooding

The second most deadly effect of a tropical cyclone is flooding, which is especially dangerous for people living inland. Storms can produce torrential rains of more than 6 inches, and can cause a rapid rise in water levels quickly during a flash flood. 

Rainfall is not directly related to the strength of a hurricane, but to the speed and size of the storm, as well as an area's geography. "Slower moving and larger storms produce more rainfall," the National Hurricane Center reports. "In addition, mountainous terrain enhances rainfall from a tropical cyclone."

In 2011, Hurricane Irene caused a surge of flooding in New York state and New England. Of the 41 deaths in the US resulting from the storm, Weather.com reports, more than half were from rainfall flooding.

As Marshall Shepherd, director of the atmospheric-sciences program at the University of Georgia, told Business Insider: "The Bahamas are experiencing the hurricane at its strongest. But it might be just as treacherous because of all the rain."