For centuries sailors have told tales of giant waves in the middle of ocean that materialize from beyond the horizon on clear calm days to sink ships. For a long time scientists disregarded these accounts as superstition. Recently, when confronted with satellite evidence, scientists have come around and now have accepted the idea of these "rogue waves".
Measurements by offshore oil platforms and satellites put the waves at heights up to 30 meters, but most ships are built to withstand just 15-meter waves. Despite the need, researchers still don't understand exactly how rogue waves form. To make a rogue wave, you need to combine the energy of several ordinary waves. This is unlikely to happen by mere chance, so theorists have devised several ways to focus ocean waves into one spot. One way is to have an ocean current oppose the direction of wind-swept waves, causing a traffic jam out of which a giant could arise. But rogues often occur without significant currents, so one popular alternative is to assume a so-called non-linear property of the waves, in which the wave velocity depends on the wave height. In a group of waves, smaller ones travel slower and can pile up on a faster-moving large wave if it comes from behind. The small additions make the large wave still larger, and therefore able to speed up and "swallow" even more small waves. "Because of this positive feedback, the energy is self-focused," explains Mattias Marklund of Umea University in Sweden.
http://focus.aps.org/story/v18/st7
Waves of Destruction
