一発大波（ローグウェーブ）とは？

For centuries, sailors told stories of rogue waves, also called freak waves or killer waves. But scientists have only accepted that they are real in the past few decades. Scientists call them "extreme storm waves". They are waves that are more than twice the size of the waves around them. They are very hard to predict and often come suddenly from a different direction from the wind and the other waves. People who have seen rogue waves often describe them as "walls of water". They usually have steep sides, with unusually deep troughs in front of them. Because these waves are so rare, it is very difficult to measure and study them. Scientists are still investigating exactly how and when they form, but there are several known causes. The first is called constructive interference. Swells travel across the ocean at different speeds and in different directions. As they pass through one another, their crests and troughs sometimes line up and add together. This can create enormous, towering waves that disappear just as quickly. If the swells are traveling in the same direction, these mountainous waves may last for several minutes. The second is the focusing of wave energy. When storm waves run into a strong ocean current flowing the other way, the waves are squeezed closer together and can combine into very large rogue waves. This happens in strong currents such as the Gulf Stream in the Atlantic and the Agulhas Current off South Africa. Rogue waves that form this way tend to last longer.