The mayor of New Orleans ordered mandatory evacuations over the weekend to protect residents from Hurricane Katrina. According to an emergency management expert, New Orleans is particularly vulnerable to flooding because parts of the city are 10 feet below sea level. How do you measure sea level?
With satellites and tide gauges. Satellites can record the level of oceans around the globe, whiletide gauges are used to measure the height of water with respect to a fixed, nearby point on land. Oceanographers combine data from satellites and tide gauges to study global effects, like the rise in average sea level that results from climate change. But specific information about local features, like the relative depth of certain parts of New Orleans, derives from specific tidal-gauge readings. (Land can also rise and fall over long time periods, so changes in land level must also be taken into account.)
The simplest kind of gauge measures the height of a float in still water. A submerged column surrounds the float and prevents choppy waves from creating sudden fluctuations in sea level. The gauge keeps track of the float as it drifts up and down throughout the day and records its height (with respect to a land-based bench mark) at regular intervals. More advanced instruments use sound waves or pressure to measure the same thing.
Stable features such as underwater mountains can affect the local sea level. So can systematic variations in water temperature, air temperature, and currents. Topographical features and predictable weather patterns cause parts of the Atlantic Ocean, for instance, to be 40 centimeters lower than parts of the Pacific Ocean. (The Panama Canal spans a sea level difference of 20 centimeters.)
Changing tides and weather conditions can also create dips and swells in local tide-gauge readings. Since local readings can fluctuate so rapidly, most descriptions of "sea level" refer to an average value measured across many years. First, readings are collected over an interval that takes into account regular tidal patterns. (A standard 19-year cycle covers a full cycle of Earth-Moon and Earth-Sun interactions.) Then researchers filter out some of the short-term influences on the data, like extreme weather patterns and storm surges. The readings are averaged together to produce a "mean sea level" reading for a specific station or group of stations in a region. As determined by nearby tide gauges, much of New Orleans sits below the mean local sea level.
Bonus Explainer: If New Orleans is below sea level, why isn't it underwater? Because it's protected by natural and artificial barriers. The city sits on the banks of the Mississippi, where sediment from the river had created areas of elevated land called "natural levees." New Orleans' earliest buildings sat on top of these levees, but as the population grew, houses were built farther inland at lower elevations. To create usable land, water had to be pumped out of the area, which in turn caused the ground to sink even lower. It's possible for part of New Orleans to exist below sea level because the levees that surround the city protect it (most of the time) from floods.
Explainer thanks Robert Dolan of the University of Virginia and Stephen Gill of NOAA's National Ocean Service.