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Wind, Swell and Rouge Waves
The wind not only produces currents, it creates waves. As wind blows across the smooth water surface, the friction or drag between the air and the water tends to stretch the surface. As waves form, the surface becomes rougher and it is easier for the wind to grip the water surface and intensify the waves.
Take it to the MAX! Anatomy of a Wave
In Oceanography and Seamanship, William G. Van Dorn provided an example of what the wave heights would be if a steady 33 mph (30 knot) wind blew for 24 hours over a fetch of 340 miles.
- 10% of all waves will be less than 3.6 ft (1 m).
- The most frequent wave height will be 8½ ft (2½ m).
- The average wave height will be 11 ft (3 m).
- The significant wave height will be 17 ft (5 m).
- 10% of all waves will be higher than 18 ft (5 m).
- The average wave height of the highest 10% of all waves will be 22 ft (7 m).
- A 5% chance of encountering a single wave higher than 35 ft (11 m) among every 200 waves that pass in about 30 minutes.
- A 5% chance of encountering a single wave higher than 40 ft (12 m) among every 2,600 waves that pass in about five hours.
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How big wind waves get depends on three things:
- Wind strength. The wind must be moving faster than the wave crests for energy to be transferred.
- Wind duration. Strong wind that does not blow for a long period will not generate large waves.
- Fetch. This is the uninterrupted distance over which the wind blows without significant change in direction.
Storms of equal size can generate much larger waves in the open Pacific Ocean as compared to the other oceans due to the long open distance of water.
After the wind begins to blow for a while, the waves get higher from trough to crest, and both the wave length and period become longer. As the wind continues or strengthens, the water first forms whitecaps and eventually the waves start to break. This is referred to as a fully developed sea.
Take it to the MAX! Beaufort Wind Scale
The waves in a fully developed sea outrun the storm that creates them, lengthening and reducing in height in the process. There are called swell waves. Swells organize into groups smooth and regular in appearance. They are able to travel thousands of miles unchanged in height and period.
The longer the wave, the faster it travels. As waves leave a storm area, they tend to sort themselves out with the long ones ahead of the short ones, and the energy is simultaneously spread out over an increasingly larger area. As the waves close in on the coast, they begin to feel the bottom and their direction of travel might change due to the contour of the land. Eventually, the waves run ashore, increasing in height up to 1.5 times their height in deep water, finally breaking up as surf.
There are many sailor tales of "rogue waves", "freak waves", "three sisters" and other "killer waves". Properly called "extreme storm waves" these tales were ridiculed and mariners were accused of using them as an excuse to cover their own mistakes in wrecks. Rogue waves are simply unusually large waves appearing in a set of smaller waves.
Some of the characteristics of rogue waves are:
- their height is greater than twice the size of surrounding waves,
- they often come unexpectedly from directions other than prevailing wind and waves, and
- they are unpredictable.
Most reports of extreme storm waves say they look like "walls of water," and are seen as steep-sided with unusually deep troughs. The USS Ramapo reported one such wave with a height of 112 feet in the Pacific in 1933. Another report of a freak wave occurred with it struck the Queen Mary amidships, south of Newfoundland, at the end of World War II, rolling her to within a degree or two of capsizing.
Just recently, a 70 foot wave crashed down on the Norwegian Dawn cruise ship in April 2005. The average waves that day were 25 to 30 feet high before this monster wave struck. The wave even damaged the hull of the Norwegian Dawn.
What causes these enormous waves? Generally they form because of swells, while traveling across the ocean, do so at different speeds and directions. As these swells pass through one another their crests, troughs, and lengths happen to coincide and reinforce each other, combining to form unusually large waves that tower then disappear. If the swell are travelling in the same direction, these mountainous waves may last for several minutes before subsiding.
It is very seldom that huge waves over 65 feet (20 meters) are developed and normally sailors do not even see them, because ships nowadays will try to avoid such conditions by altering course before the storm hits. But they do occur. |
| Next: Tides |
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National Weather Service
Southern Region Headquarters
819 Taylor Street
Fort Worth, Texas 76102 |
www.srh.weather.gov
Updated: March 29, 2006 |
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