For centuries, sailors told stories of freak waves appearing out of nowhere. For centuries, those stories were folklore. Only in the past few decades have scientists accepted that rogue waves are real — large, unexpected and dangerous.
A rogue wave estimated at 18.3 meters (60 feet) in the Gulf Stream off Charleston, South Carolina, moving away from the ship it had just struck. Surface winds at the time were light, at 15 knots. Credit: NOAA.
What makes a wave "rogue"
Scientists call them extreme storm waves. They are more than twice the size of the waves around them, very unpredictable, and often come from directions other than the prevailing wind and waves. Most reports describe them as "walls of water," often steep-sided, with unusually deep troughs.
Because they are rare, measurements of them are rare too, and exactly how and when they form is still being studied. But several causes are known.
Cause one: waves adding up
Swells cross the ocean at different speeds and in different directions. As they pass through one another, their crests, troughs and lengths sometimes line up and reinforce each other — constructive interference. The result can be an unusually large, towering wave that quickly disappears. If the swells travel in the same direction, such mountainous waves can last several minutes before subsiding.
Cause two: waves against a current
When storm waves develop in a current flowing against the normal wave direction, the interaction can shorten the wave frequency, and waves can combine into very big rogue waves. This happens in currents such as the Gulf Stream and the Agulhas current, and rogue waves formed this way tend to last longer.
Sources
Based on "What is a rogue wave?" from NOAA's National Ocean Service, a work of the United States government in the public domain.
许可协议： CC0 1.0（公有领域） · 改编自 oceanservice.noaa.gov
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