Thanks to news coverage of hurricanes and other major storms, almost everyone has heard the term "storm surge," and most people have at least some notion of what a storm surge is, at least to the extent that the words "storm" and "surge" together in the same sentence mean something bad. Finding a definition that makes sense to anyone who hasn't taken a course in basic meteorology, however, is a little challenging.
A storm surge, in the simplest possible terms, is water that is driven onto land by an approaching storm. Storm surges are most often associated with hurricanes and typhoons, and most often occur in coastal areas, but they can occur on any large body of water; one of the most destructive storm surges in U.S. history occurred in September 1928, when a passing hurricane created a six- to nine-foot surge on Lake Okeechobee in Florida, killing over 1,800 people. A surge is caused by two things, the air pressure in a storm and its winds. Air pressure, which is the weight of the atmosphere pressing down on the surface of the Earth, is much lower in a large storm such as a hurricane than it is in calm weather. When the air pressure is lower, the surface of the sea or other body of water is not as compressed, and bulges upward. High winds passing over the surface of the water enhance this effect, because air pressure decreases as the speed of moving air increases. The raised water is then physically moved forward by the high winds, which can have devastating effects once it reaches land.
Storm surge is measured in terms of feet or meters above the "mean sea level," which is the average normal level of the sea between its high and low tides. The highest recorded storm surges in the U.S. occurred during Hurricane Katrina in 2005, with heights of 25 to 28 feet reported in some locations of the Mississippi coast. The storm surge was made slightly worse because it arrived at the same time as the normal high tide for the area, which increased its height by about two feet. A storm surge combined with a high tide is called a "storm tide," an interesting but largely academic distinction; it is hard to imagine much obvious practical difference between a surge of 26 feet and one of 28 feet.
There is no consistent "average" height of a storm surge for a typical hurricane, because many factors affect the surge. In general, areas that are in the "right-front quadrant" of an approaching storm are most at risk of dangerous surges. In the northern hemisphere, hurricanes and typhoons rotate in a counter-clockwise direction. As the storm approaches the coast, then, the winds on the right side of the storm relative to its forward direction of travel are blowing straight towards the shore. Other factors such as the forward speed of the storm and the slope of the land near the shore also affect how strong a surge will be. Slower-moving storms have higher surges than fast ones, because their wind and pressure effects remain over areas longer. And areas where the land slopes gently into the sea rather than having a steep drop-off suffer worse damage from surges, because the water can travel much farther inland before its energy is dissipated.
In the U.S., much of the heavily-populated Atlantic and Gulf coasts, some of the most desirable real estate in the country, lies less than 10 feet above sea level. With historic storm surges of 12 to 15 feet being common, and massive surges of 20 feet or more happening more than once, it doesn't take a math professor to figure out that those numbers mean trouble. Hurricanes and storm surges will always occur; as long as people keep building in risky coastal locations, they will be at the mercy of the weather. It is surprising, perhaps, that so many consider it a risk worth taking.
National Hurricane Center information about storm surges and historic hurricanes: