One of the summer’s hottest movies has been The Perfect Storm, based on the best-selling book by Sebastian Junger about a massive storm off the Grand Banks in 1991 that resulted in the loss of the swordfishing boat Andrea Gail with all hands. The book is better than the movie, but the movie does let you actually see what the 30-metre waves generated by the storm looked like.
Waves are caused by the friction of wind on water. The wind pushes the water into little hills. As the hills sink again (under the influence of surface tension when the waves are tiny, under the influence of gravity as they become larger), they overshoot their starting point, creating a series of crests and troughs. Once small waves form, the rough surface increases the friction between the wind and the water, making it easier for bigger waves to form.
How big the waves grow depends on the force of the wind, how long the wind blows, and the distance the waves move under the wind (called the fetch). A long hard blow over a wide storm area can create Perfect Storm-sized waves of over 30 metres, but that’s unusual. Most waves are under 3.3 metres in height, and even in hurricanes, usually peak at around 10 metres.
Waves can travel enormous distances. In summer, California surfers may enjoy waves created by winter storms in Antarctica two weeks earlier. When waves first form, the sea is choppy, but once the waves move out of the area of wind, they smooth out, creating a swell which loses roughly half its height for every 300 to 500 nautical miles it travels.
Waves in deep water don’t affect the water below them at a depth greater than half their wavelength. Since wavelengths of wind-generated waves at sea are rarely any longer than 150 metres, water below 75 metres isn’t stirred by it at all.
When the wave nears the shore, friction between it and the land slows down the wave. The wave literally piles up, becoming so high and steep that gravity causes it to fall over, or break, crashing into the shore.
In any particular group of waves, most are of approximately the same height. Every so often, however, an unusually large, “rogue” wave will form. At Daytona Beach in 1992, a rogue wave measuring between four and six metres ran up the beach, smashing into cars and causing 75 minor injuries. The ordinary waves that day were only half a metre high.
In 1984, a 117-foot three-masted tall ship was hit by a rogue wave 75 miles north of Bermuda, filled with water and sank in less than a minute, killing 19 crew members. And on September 15, 1995, Captain Ronald Warwick, commanding the Queen Elizabeth 2 ocean liner, thought for in an instant he had spotted the white cliffs of Dover–except he was south of Newfoundland. A 30-metre high wall of water was bearing down on the ship. He turned the QE2 into the wave, and although the ship was damaged (water weighs a tonne per cubic metre) no one was injured.
Some rogue waves form when several waves meet crest to crest, combining their forces. As well, a wave traveling one direction that meets a current traveling the other will pile up against it, gaining height. That’s why rogue waves are most common in fast-moving currents such as the Gulf Stream. Islands and underwater shoals also bend and break apart waves, which can recombine at the far end to reach new heights.
When most people think of giant waves, they think of tsunamis (often called “tidal waves”), which are caused by anything that displaces a large amount of the ocean’s water. Usually, that’s an undersea earthquake.
Tsunamis seem harmless at sea. As ship might rise and fall a metre or less as the wave passes. But the energy contained in them is enormous, because they have wavelengths in the hundreds of kilometres, which means the depth of water being disturbed goes all the way to the ocean floor–and because they move at around 720 kilometres per hour.
The most destructive tsunami in history killed more than 100,000 people in Japan, in 1703. Today scientists constantly monitor seismic readings to try to predict when tsunamis might strike. (Among the vulnerable areas is the coast of British Columbia.)
Saskatchewan is only one of two landlocked provinces in Canada; maybe it’s not such a bad thing to be.
