A 60-foot rogue wave moves away after hitting a tanker off Charleston, S.C.

Photo courtesy National Weather Service

Introduction to How Rogue Waves Work

During the second season of "Deadliest Catch," a documentary television series about crab fishing in Alaska's Bering Sea, cameras recorded footage of a giant wave striking the ship "Aleutian Ballad." The 60-foot (18-meter) wave rolled the­ boat onto its side and caused significant damage, though fortunately none of the crew was seriously hurt. The Ballad limped back to port for repairs. The footage captures the suddenness of the massive wave, and just before the impact sends the camera operator tumbling, the "wall of water" breaking over the boat can be seen with frightening clarity.

What was this colossal wave that appeared seemingly out of nowhere? It was a rogue wave. Rogue waves sound like something straight out of a sailor's tall tale: ominous, mysterious, solitary waves of enormous height crashing down on ships at sea in seemingly calm waters. But as improbable as they might seem, recent st­udies suggest these rogues are more common than anyone previously guessed.

Imagine having an 80-foot wall of water barreling toward you. Actually, that might be too tall an order. It's easy to throw around heights like 50 feet or 90 feet without really grasping how huge a wave of such height would be. Here are some handy comparisons:

  • The average room in your house is probably about 8 feet high.
  • A typical two-story house is between 20 and 30 feet high.
  • The Statue of Liberty is 111 feet tall from her toes to the top of her head, not counting the pedestal or her arm and torch.

Understanding these giant waves is more than just a scientific curiosity -- being able to predict and avoid them could save dozens of lives and hundreds of millions of dollars in cargo every year.

In this article, you'll find out what separates rogue waves (also called freak waves) from other large waves and what causes them, and you'll learn about some of the better-known rogue wave incidents.­

Glacial calving can cause enormous waves, but they're not considered rogue waves.

© Photographer: Ironrodart | Agency: Dreamstime

A Rogue by Definition

There are many kinds of ocean waves, and some of them are definitely huge. However, not all large waves are rogue waves. Strong storms, such as hurricanes, can cause large waves, but these waves tend to be relatively regular and predictable, though certainly capable of causing serious harm to ships and coastal areas. Undersea earthquakes, coastal landslides and glacial calving (when a large chunk of a glacier breaks off and falls into the ocean) can also create enormous and catastrophic waves. Undersea earthquakes can produce tsunamis, and coastal landslides can produce tidal waves. These could be considered rogues, but, to a certain extent, they are predictable -- as long as someone noticed the event that caused them. So, that pretty much rules them out of rogue status.

A true rogue wave arises seemingly out of nowhere and is significantly higher than the other waves occurring in the area at the time. Exactly how much higher is open to interpretation -- some sources suggest anything twice as large as the current significant wave height is a rogue, while others think anything 33 percent larger counts. It is probably sufficient to say that any wave so large that it is unexpected based on current conditions can be counted as a rogue. A craft navigating 3-foot waves could encounter an 8-foot rogue wave -- while not a record-breaker, it would certainly cause problems for a small boat.

Rogue waves also tend to be steeper than most waves. The average ocean waves may take the form of massive swells, allowing vessels to maneuver up and down them even if they are many feet high. By contrast, consider this report of the Queen Elizabeth II's encounter with a freak wave:

At 0410 the rogue wave was sighted right ahead, looming out of the darkness from 220°, it looked as though the ship was heading straight for the white cliffs of Dover. The wave seemed to take ages to arrive but it was probably less than a minute before it broke with tremendous force over the bow [source: Science Frontiers].

The phrase "wall of water" is very common in rogue wave reports -- they are usually much steeper than other waves, and therefore slam into ships with tremendous force, often breaking over them.

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While scientists have gained a greater understanding of rogue waves in the last decade, they are still quite enigmatic. No one has ever ­filmed the formation of a rogue wave in the ocean or followed one through its entire life cycle. There are very few photographs of rogue waves. For centuries, the best evidence for their existence was anecdotal -- the countless stories told by sailors who had survived one.

Gallimore and another crewman were in the wheelhouse. The wind had been blowing fiercely at 100 knots for more than a day, and "Lady Alice" was struggling in rough seas with waves 16 to 23 feet high … At 8:00 A.M. Gallimore looked up and saw a huge wall of water bearing down on "Lady Alice." From his view in the wheelhouse, he could not see the top of the wave …The wave crashed down on top of the wheelhouse, driving the vessel underwater …The crewman in the wheelhouse with him was thrown down with such force that he suffered two fractured vertebrae. To top the radar antennas with enough force to rip them from the steel mast where they are bolted … the wave had to be 40 feet or higher [source: Smith, 195].
The Queen Elizabeth

During World War II, British cruise liners were converted to carry troops from the United States to Europe. One such vessel was the "RMS Queen Elizabeth." A rogue wave struck the ship near Greenland in 1942, shattering windows 90 feet above the waterline and nearly rolling the ship. It recovered and narrowly averted an unprecedented maritime disaster -- the ship was carrying more than 10,000 troops at the time [source: Sverre Haver].

What Causes Rogue Waves?

To understand what causes a rogue wave, first you must learn a little about regular waves. Think about waves you're familiar with -- such as the waves you bodysurf in at the beach or at the local water park's wave pools. A wave has several characteristics that can be used to define it.

  • The crest is the highest portion of the wave.
  • The trough is the lowest portion of the wave (the "dip" in between waves).
  • The distance from the trough to the crest represents a wave's height.
  • The distance between crests represents a wave's length.
  • The amount of time that passes between one crest and the next is the wave period or wave speed.
  • The amount of kinetic and potential energy carried by the wave is known as wave energy [source: Bryant, 156].

A huge number of variables influences these factors, including the depth of the water, tidal forces, wind blowing across the water, physical objects such as islands that reflect waves, and interaction with other waves and ocean currents. At any given moment, thousands of waves are passing and interacting through a specific area of ocean. The faster the wind is and the longer it blows, the stronger and larger the waves. Fetch is the unobstructed distance of ocean over which the wind can blow on the water -- it's how much ocean the wind is blowing on. More fetch means bigger waves.

Weather reports list the significant wave height, which is the height of the highest one-third of the waves. Why do rogue waves exceed the significant wave height by so much? Scientists aren't completely sure, but they have some good theories.

One possibility is that ocean currents cause waves to "pile up" when waves run into currents head on. Powerful storms can cause significant wave heights of 40 to 50 feet (12 to 15 meters). When such waves run into a strong current, the current can increase wave heights and cause the waves to break. This would explain monster waves 98 feet (30 meters) high or more, and account for the "wall of water" effect. Rogue waves frequently occur in areas known for strong ocean currents. For example, he Agulhas Current runs southward along the east coast of Africa. Storm waves moving up from the south crash into the current -- mathematical predictions suggest rogue waves there could reach 190 feet in height, and 20 ships have reported rogue wave strikes in that area since 1990 [source: Smith, 188]. The Gulf Stream, which runs up the east coast of the United States, is another potential rogue wave source. Rogues originating in the Gulf Stream could be responsible for much of the legend of the Bermuda Triangle.

Not all rogue waves occur in strong ocean currents, however. Scientists think some waves may be caused by randomly occurring wave reinforcement. Whenever two waves interact, their wave height is added together. If a 5-meter wave passes over a 10-meter wave, the result is a briefly occurring 15-meter wave. This can happen in the opposite manner as well. A 15-meter wave moving across a 10-meter trough results in a 5-meter wave. Dozens of waves could be interacting and reinforcing each other. Once in awhile, several waves may come together at just the right moment and create one huge wave in relatively calm seas. If 10 waves that are only 5 feet high come together, they will result in a 50-foot wave. This fits descriptions of rogue waves that seem to appear out of nowhere and disappear after just a few minutes.

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A recording of the rogue wave off the Draupner Platform in the North Sea on New Year's Day 1995

Photo courtesy Sverre Haver

Common Rogues

Most reports of rogue waves rely on size estimates by witnesses. These estimates are based on the height of the ship above the waterline and how far up the ship the wave reached when it hit. It was commonly assumed that tales of waves 100 feet tall or taller were exaggerations (and some of them certainly were). At best, such waves were incredibly rare.

Beginning in the 1990s, sailors and scientists began to suspect that rogue waves were responsible for many more losses at sea than they had previously guessed. The Queen Elizabeth II, Caledonian Star and Bremen cruise ships were all hit by monstrous waves in a span of six years. Previously, data collected by weather ships suggested that such waves would occur only every 50 years or more [source: Smith, 210]. In 2004, the European Space Agency (ESA) used data from two radar-equipped satellites to see how frequent rogue waves actually are. After analyzing radar images of worldwide oceans taken over a period of three weeks, the ESA's MaxWave Project found 10 waves 82 feet (25 meters) or higher. That was an astonishingly high number for such a relatively short time span; it forced scientists to seriously rethink their ideas on rogue waves [source: ESA]. The ESA is undertaking another project, WaveAtlas, to survey the oceans over a much longer period and develop the most accurate estimate possible for the frequency of rogue waves.

Other hard evidence of monster waves comes from instruments designed to measure wave heights. One such instrument was mounted on an offshore ­oil rig known as the Draupner Platform. On New Year's Day 1995, the platform was measuring waves no more than 16 to 23 feet (5 to 7 meters) high. Then it suddenly registered a single wave almost 66 feet (20 meters) high [source: Smith, 208]. Canadian weather buoys near Vancouver recorded waves 100 feet high and higher throughout the 1990s [source: Smith, 211].

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Rogue versus Tsunami

When you think of giant, frightening, destructive waves, tsunamis definitely come to mind. But don't confuse these giant waves with rogues -- while both can be catastrophic, they are quite different. The easiest way to remember the difference is by what causes the "wall of water" and where the destruction from it occurs.

 

Tsunamis are most often caused by undersea earthquakes, which send tons of rock shooting upward with tremendous force. The energy of that force is transferred to the water. So, unlike normal waves that are caused by wind forces, the driving energy of a tsunami moves through the water, not on top of it. Therefore, as the tsunami travels through deep water -- at up to 500 or 600 miles per hour -- it's barely evident above water. A tsunami is typically no more than 3 feet (1 meter) high. Of course, all that changes as the tsunami nears the coastline. It is then that it attains frightening height and achieves its more recognizable and disastrous form.

 

Rogue waves, as we've discussed in this article, arise seemingly out of nowhere, and they can attain their massive heights in deep water, not just along the shoreline.

Wave Defense

If the MaxWave study is correct, and rogue waves are much more common than previously thought, does that mean oceangoing vessels are far riskier than we thought? It might. Ships and offshore structures, such as oil rigs, are built to withstand a certain significant wave height, whatever is determined that the ship is likely to encounter in its lifetime. Few are built to handle 100-foot waves. Furthermore, a ship's ability to withstand a strike by a rogue wave depends in large part on the ballast, or stability. If a ship has the right amount of ballast and is floating at the proper level, it will be more likely to right itself after being pushed over by a wave [source: Smith, 233]. Today's international shipping laws don't necessarily take frequent rogue waves into account where ship construction and maintenance are concerned. But that's not to say all ships are unsafe -- perhaps it would be impossible to build a ship that could withstand any wave.

 

For more information on rogue waves and related topics, check out the links on the next page.

Lots More Information

Related HowStuffWorks ArticlesMore Great LinksSources
  • Amadio, Jordan Paul. "High Seas." Natural History, Oct. 2004. http://findarticles.com/p/articles/mi_m1134/is_8_113/ai_n6249039
  • Bryant, Edward. Natural Hazards. Cambridge University Press; Updated edition (February 21, 2005). 978-0521537438.
  • Cush, Cathie. Shipwrecks. MetroBooks (NY) (October 1997). 978-1567994759.
  • The Denver Channel. "Crippled Ship With CU Students Limping To Port." Jan. 27, 2005. http://www.thedenverchannel.com/news/4135033/detail.html
  • European Space Agency. "Ship-sinking monster waves revealed by ESA satellites." July 21, 2004. http://www.esa.int/esaCP/SEMOKQL26WD_index_0.html
  • Freeze, Ken. "Monster Waves Threaten Rescue Helicopters." http://www.check-six.com/Coast_Guard/Monster_Waves_Reprint-screen.pdf
  • Haver, Sverre. "Evidence for the Existence of Freak Waves." http://www.ifremer.fr/web-com/molagnon/bv/haver_w.pdf
  • Peterson, Ivars. "Rough math: focusing on rogue waves at sea." Science News, Nov. 23, 1996. http://findarticles.com/p/articles/mi_m1200/is_n21_v150/ai_18928307
  • Smith, Craig B. Extreme Waves. National Academy Press (Nov. 7, 2006). 978-0309100625.
  • Wheeler, Kyle, producer. "Finish Line." Deadliest Catch, Season 2.