What forms ocean waves
Giant waves on the sea
For a long time, researchers thought that sea waves whipped up by the wind could hardly exceed 15 meters. But they were wrong. Today we know that giant waves occur surprisingly often. The so-called freak waves tower above the surrounding waves many times over. They can tower up to 35 meters high. The origin of the oceanic giants is controversial. There are several possible mechanisms.
The numerous reports of giant waves that are said to have sunk even large ships in no time are nothing but spinning - that's what many scientists used to think. According to the statistics of the textbooks, the events could only be extreme outliers. But after January 1, 1995, the view changed: On that day, a 26.5 meter high wave was registered at the Draupner gas platform off the Norwegian coast, while the highest waves all around from the valley to the crest of the waves were only about 11 meters. From this measurement and other observations we know that giant waves occur much more frequently than previously assumed. Now researchers are trying to find out where and when to expect the wave monsters and how they arise.
In English, the gigantic waves are known as "Freak Waves". They have nothing to do with the tsunamis - while these initially very long and shallow waves, which are mostly generated by seaquakes, only pile up on the coasts, freak waves are breaking over ships in the open ocean.
Giant wave on the Bay of Biscay
There is no physical definition of freak waves. Because nobody knows exactly how the phenomenon arises. This is explained by Wolfgang Rosenthal from the Society for Applied Environmental Protection and Safety in Maritime Traffic in Bremen - the physicist has been dealing with giant waves for many years. In specialist circles it has become common practice to treat a wave that is more than twice as high as the "significant wave height" as a freak wave, says Rosenthal. The significant wave height is the average height of that third of the waves with the greatest wave height.
According to an estimate by a shipping consultancy, more than 22 larger freighters worldwide sank after confrontations with freak waves between 1969 and 1994 - 525 people were killed in the process. There have also been spectacular cases in recent years: the meeting of the MS Bremen with a huge wave in the South Atlantic in February 2001 caused a lot of sensation in Germany. The cruise ship, whose bridge was destroyed, narrowly escaped sinking. It can be dangerous even in the Mediterranean. On March 3, 2010, two passengers on the Louis Majesty cruise ship died when three waves, possibly freak waves, struck the windows of a saloon.
The Louis Majesty could have met the so-called "Three Sisters". This typical sequence of three freak waves has been observed many times. But giant waves also appear alone. Many eyewitnesses describe them as a single wall of water that had built up in front of them as if out of nowhere. In other cases the wave could be seen from afar. It is still unclear whether these different manifestations are the same physical phenomenon. In professional circles, several mechanisms for the development of freak waves are traded, as Rosenthal explains. In all cases, the starting point is the swell generated by the wind.
Giant wave on gas platform
Ocean currents can focus waves on a point like a magnifying glass. This happens, for example, in the Gulf Stream, in the Kuroshio off Japan and in the Agulhas Stream off South Africa. Especially when the waves move against the direction of the water flow, the risk of freak waves increases.
Bumps in the sea floor can also lead to the creation of giant waves, because wavelength and height are physically linked to the water depth. This principle causes the surf on beaches. Under certain circumstances, shallows can focus waves in a similar way to currents.
The third hypothesis is complicated and relies on non-linear interactions between the waves. On the high seas you don't have to deal with waves of a single frequency, height and speed, but with a whole spectrum of the most varied of shapes. On the one hand, there are linear effects such as constructive and destructive superposition, but on the other hand, the waves also react with one another in a non-linear way. Based on computer simulations and experiments in water tanks, marine researchers assume that freak waves can form, which are similar to so-called solitons. A soliton is a particularly stable, non-linear wave type.
The fourth hypothesis, according to which a special interaction of waves with the wind could produce giant waves - namely when small-scale high wind areas migrate at the group speed of a wave group, is still relatively little researched.
Damage to tankers
The mechanisms of formation must therefore be researched further. Meanwhile, in recent years, attempts have been made to issue tentative warnings about freak waves, reports Rosenthal. For example, the European Center for Medium-Range Weather Forecasts in Reading, England, produces global swell forecasts that calculate the likelihood of freak waves. There is also progress in the observation of the giant waves. Radar satellites such as the TerraSAR-X from the German Aerospace Center are able to scan the swell with a resolution of 1 to 6 meters. But for a detection that would be sufficient to provide useful warnings, such satellites would have to cover a larger area and fly over the same area more frequently than is currently the case. So far, seafarers have had no choice but to avoid areas with particularly high waves, where freak waves could appear, as a precaution.
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