Storm surges—when the ocean rises rapidly due to changes in pressure around it—are, in part, driven by wind speed, but they're more directly determined by how large the cyclone is. Some storms may have 135 mile per hour winds, enough to be a Category 4 system, but the area in which the wind reaches that speed may be small. Conversely, a hurricane with slower winds that covers a much larger area is likely to do more damage. Take Hurricanes Ivan and Dennis. These Category 3 storms hit roughly the same part of the Gulf Coast one year apart. Dennis, which struck in 2005, caused approximately $2.2 billion in damage. One year before, Ivan had caused $14.2 billion. When meteorologists analyzed the differences between these two storms, they found that Ivan caused more than six times the damage because it covered five times the area, and thus could last much longer over land. This means a greater swatch of the U.S. experienced hurricane-force winds and torrential rain for longer periods of time.