Hurricanes can get supercharged when they hit river mouths, researchers now find. Hurricanes keep alive by converting the warmth of tropical waters into motion. The strong winds they kick up in turn cause surface water heated by the sun to mix with deeper, cooler waters, and that drop in warmth causes hurricanes to weaken. Rivers and rainfall alter this pattern by adding freshwater, which is less dense than saltwater. Therefore, this freshwater sits on top of cold seawater for much the same reason oil sits on top of water. The resulting "barrier layer" of freshwater keeps winds from mixing the warm surface layer with cooler, deeper water, giving hurricanes more heat to intensify with. Scientists analyzed 587 tropical storms and cyclones between 1998 and 2007 in the western tropical Atlantic, the western Pacific, and the northern Indian Oceans. They found that tropical storms over thick barrier layers cooled off 36 percent less than storms over areas lacking barrier layers and drew in 7 percent more heat from the ocean.