Not to be confused with a tsunami, a storm surge is a sudden upwelling of ocean water caused by winds and pressure changes affecting the water’s surface. Hurricanes generate large waves—swells—that radiate outwards in all directions as they travel over the ocean. The swells, which can move toward the shoreline about three or four times faster than the actual storm, arrive on land before hurricanes strike. Before advanced weather systems and the use of satellites, these swells warned people that a hurricane was approaching. The swells become storm surges by the time the main storm arrives, raising water levels as much as 25 feet (7.5 meters) and causing massive coastal flooding. By some estimates, the storm surge resulting from 2005’s Hurricane Katrina was 28 feet (8.5 meters).