Swell (English spelling)

Japanese: うねり（英語表記）swell

In addition to wind waves, which are visible when the wind blows, there are also waves that are not directly related to the wind at a given location. The latter are called swells, and include waves when there is no wind, the aftermath of a storm, and doyo waves. Swells have a longer period and wavelength than wind waves, and their backs are rounded, but they can be difficult to distinguish from wind waves. Swells can have wavelengths of several hundred meters and periods of more than 20 seconds. Wind waves occur in stormy areas, but even after the wind has stopped, waves with a wave height to wavelength ratio of around 0.02 remain and become swells, which can propagate far away. Swells move faster than typhoons and other storms, so when a typhoon occurs in the south, large swells wash up on the Pacific coast even though there is still no wind in the seas near Japan. The doyo waves in August and September are an example of this, and are a precursor to a typhoon. Swells appear on calm seas and can harass ships and cause damage to coastal structures, but they are more difficult to forecast than wind waves.

Source: Encyclopaedia Britannica Concise Encyclopedia About Encyclopaedia Britannica Concise Encyclopedia Information

Japanese:

海の波には風が吹いたときみえる風浪のほか，その場所の風に直接関係のない波がある。後者をうねりといい，風がないときの波や暴風の余波，土用波などがこれにあたる。うねりは風浪に比べて，周期も波長も長く，波の背が丸くなっているが，風浪と見分けにくいこともある。うねりでは波長が数百m，周期が 20秒以上のものがある。暴風域では風浪が起るが，風がやんだのちも波高と波長の比が 0.02内外の波が残ってうねりとなり，遠くまで伝播される。うねりの進行速度は台風などの暴風よりも速いので，南方に台風が発生すると，日本近海はまだ風がないのに大きなうねりが太平洋岸に押寄せる。8，9月の土用波がこの例で，台風の前ぶれとなる。うねりは無風の海面に現れ，船舶を悩ましたり，海岸構造物に被害を与えることがあるが，うねりの予報は風浪よりもむずかしい。

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