An aerial view of the devastated coastline to the south of Banda Aceh city, Indonesia, some 8 months after the 2004 Indian Ocean Earthquake and Tsunami. The powerful waves striped bare the flat coastal plains causing immense loss of life and total devastation of coastal properties and farmlands.

The collision of warm and cold air often results in large rotating weather systems known as cyclones, low-pressure zones that can lead to days of cloudy weather and precipitation. When such systems develop over warm ocean water, typically between 5° and 25° above the Equator, they can produce the massive storms known as hurricanes, tropical cyclones, and typhoons. These three types of storm are the same defined by a

[wind speed in excess of 74 miles per hour. But they occur in different parts of the world: The storm is called a hurricane if it develops over the Atlantic or eastern Pacific Ocean, a cyclone if it develops over the Bay of Bengal or the Indian Ocean, and a typhoon if it develops over the western Pacific. Strong and deadly, these storms are dangerous not just for their winds, which can rage in excess of 150 miles per hour, but also for the massive

amounts of water they dump from the sky and push inland through a storm wall of ocean water. On September 8, 1900, between 6.000 and 8,000 people drowned in Galveston, Texas, for example, when a 20 foot storm surge flooded the town in the deadliest natural disaster in U.S. history. In Asia, individual typhoons have claimed hundreds of thousands of lives-including a storm in 1970 in Bangladesh that killed 300,000 with an estimated 30-foot surge of water. Hurricanes, cyclones, and typhoons form as winds from different directions meet over the ocean. Warmed by the ocean, moisture-laden, low- pressure air begins to rise.. As cooler air rushes in, it is also warmed and hydrated, and it begins moving upward. Typically, this air movement leads to little more than thunderstorms, but sometimes the system will organize it- self and begin to spin, gathering force as it moves across warm water. Now the system is classified as a tropical de- pression, because of its extreme low pressure. When its winds reach 39 miles per hour, it becomes classified as a tropical storm. At 74 miles per hour, it is reclassified as a hurricane or other serious storm. While these tropical storm systems may wander harmlessly at sea, they often do not fully dissipate until they reach land. The collision with land robs them of the warm ocean water that fuels them

KATRINA’S LEGACY Hurricane Katrina raked the U.S. Gulf coastline in August 2005. It left about 75 percent of New Orleans underwater when levees built to protect the city from Lake Pontchartrain and Lake Borgne failed. A study soon after found at least three different types of levee breaches, some of which might have been prevented with minor design adaptations. In several spots, the storm surge from the hurricane built up above the levee’s concrete wall, spilling over the top, eroding the supporting earthen embankment on the other side, and eventually collapsing the wall itself. Else- where pressure from the storm surge pushed through the underlying soil and eroded the levee from beneath. Where levees of different design met, weak spots in one led to larger breaches overall.