Waves demystified

urfers need more than a surfboard and the ocean. Their ride is better if they know the physical properties of waves and what to look for in the weather.

Waves form in the ocean when wind blows across the surface of the water and transfers energy to the water, says James H. Duncan, professor of mechanical engineering at the University of Maryland’s Clark School of Engineering in College Park.

The surface of the water rises and falls because of the ocean’s restoring force of gravity, says Mr. Duncan, who holds a doctorate in geophysical fluid dynamics.

“A wave is not moving water,” says Jeffrey Hanson, research oceanographer for the U.S. Army Corps of Engineers’ Field Research Facility in Duck, N.C. “The water moves in an up-and-down motion, which is the primary motion.”

The water’s movement is difficult to see because the water is clear, but placing a piece of styrofoam on the water’s surface can help mark the movement, Mr. Duncan says. As the elevation of the water surface increases, the styrofoam can be seen moving forward, and as it decreases, it moves back, he says. The vertical and horizontal movements combine to form a circular motion, he says.

“The particles of water go in a circle when a wave goes by,” Mr. Duncan says.

Surfers ride wind-driven gravity waves, which are generated by the wind and propagate from the disturbance, says Mr. Hanson, who holds a doctorate in ocean-wave physics. He is an amateur surfer of two years.

This type of wave constitutes the majority of the ocean’s waves, Mr. Hanson says. Other types of waves include tsunamis, or seismic waves caused by an underwater earthquake or volcanic eruption, and much smaller waves that result from boats on the water, he says.

“Waves are disorganized as the wind transfers its energy to the ocean,” says Michael Forte, hydrographer with the Army Corps of Engineers. He is a member of Surfrider Foundation USA, a nonprofit environmental organization based in San Clemente, Calif., that protects oceans, waves and beaches. He has been surfing for 15 years.

Waves near a storm range in size and wavelength, which is measured from crest to crest or trough to trough, Mr. Hanson says. Wind speed and wind duration influence the size of the waves, along with wind fetch, or the distance over the ocean that the wind blows, and the speed of the storm’s movement across the ocean, he says.

“As the waves travel away from the generating region or storm, they sort themselves out,” he says.

The longer waves travel faster and move in front of the shorter waves, Mr. Hanson says. The waves become what are called “swells” when they leave the generating area and no longer receive their energy from the wind, he says.

“When they reach the distant shore, they’re quite uniform. They are well organized in these beautiful surfing waves,” Mr. Hanson says.

Swells are similar to icebergs, with most of their energy below the surface, says Kevin Wallis, forecast manager of surfline.com, a Web site in Huntington Beach, Calif., that provides surfing forecasts and reports.

“The energy of swells can extend hundreds of feet below the surface of the water,” says Mr. Wallis, who has been surfing for 14 years.

Story Continues →