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Tsunamis and Rogue Waves: What Are They?

March 5, 2010

By WeatherBug Meteorologist, Andrew Rosenthal

Tsunamis and "rogue" waves have been in the news over the last several years, causing significant trauma.

A tsunami is a set of ocean waves that is caused by a large disturbance of the sea or sea floor. Often, this disturbance is caused by an earthquake, could also be the result of a massive object, such as a meteor, hitting the sea, or even a large submerged landslide. After the ocean is disrupted, the water and waves move away in all directions at equal speed of 400 to 500 mph, and with wave heights of just a few feet.

However, as the water approaches land, the waves slow down and begin to grow to heights often 10 to 30 feet high. When the waves smack into the shoreline, the force of the water wipes out everything in its path.

A rogue or "freak" wave is typically found over the open ocean, well away from land, although they have also been reported on the Great Lakes. These waves have grown as high as 100 feet and are formed by waves moving at differing speeds. As the faster waves overtake the slower ones, they start to join together, so that two three-foot waves grow into one six-foot wave. Given time, these waves can build upon each other to massive heights and will travel across the ocean in sets of several waves.

When these waves approach a ship, the ship often stands little chance of escape. Similar to a hurricane`s storm surge, the water`s force will pound against the side of the boat, causing damage or even causing the ship to capsize.

Rogue waves are often spawned during major storms that produce waves of 10 feet or higher. As these waves combine, their heights can grow to 30 or even 50 feet. The SS Edmund Fitzgerald was broken in two by a rogue wave during a 1975 Great Lakes winter storm. The German merchant ship Munchen was hit by a wave in the northern Atlantic in 1978, that caused damage 66 feet above the water surface, tipping the ship onto its side and capsizing it.

Just this week, a storm over southern Europe is believed to have had a hand in the 26-foot rogue waves that slammed the side of the Louis Majesty Wednesday off the southeastern French coast, breaking upper-level windows, causing two deaths and fourteen injuries.

By contrast, tsunamis have no meteorological origin. The 2004 Indian Ocean tsunami was spawned by a destructive 9.2-magnitude earthquake, which caused the sea floor to fold under itself and shift upward by more than 30 feet, creating a wave of equal height. The Chile quake, located about 60 miles offshore, only moved the land by about 7 feet, and waves of 3 to 7 feet were reported across the Pacific.

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