Waves - Wind waves

The motion of wind waves
The waves at the surface of the ocean are called wind waves. Waves are set in motion by winds.

Where there is little wind, there are few waves and the sea surface may appear smooth like glass.

Where there are strong winds, many waves are generated as energy is transferred from the air to the water. This happens because of friction at the interface between air and underlying water.

But aren't there large waves on windless days too? Yes, because wind waves travel outward, beyond the places where they were generated by winds. The winds associated with storms create some of the largest waves.

Far away storms can make for excellent surfing as they deliver large waves to distant beaches that happen to have sunnier weather.

Under the surface - Although the surface of a wind wave appears to move forward in the general direction of the wind, the water inside a wind wave doesn't move forward very much.

For all the endless waves that appear to arrive at the beach, no water piles up there. The water underneath the wave moves in circles. This kind of internal motion is called oscillatory motion. Look closely at the motion of the red cylinder to see the oscillatory motion.

Horizontal extent - Oscillatory motion results in the development of a wave that moves forward at the water surface, even though water (the material or mass of water) doesn't actually move forward. Since the wave progresses forward, a wind wave is considered to be a kind of progressive wave.

Is anything else moving forward besides the wave form? Energy is moving forward and being trasmitted wherever the waves go.

Vertical extent - Althouh wind waves travel far across oceans, their motion dies out very rapidly in the vertical direction.. Even large waves that may travel hundreds of miles from their point of origin cause little disturbance to waters deeper than 150 meters.

The depth of water disturbed by a passing wave is called wave base.

The rule-of-thumb is that the thickness of water affected is equivalent to 1/2 of the wavelength of a wind wave. Since typical wind waves in the open ocean have wavelengths of about 100 meters, the upper 50 meters of water is affected by wind waves.

Where the sea floor is shallower than wave base, the motion of water begins to affect the bottom. This results in erosion and slows the motion of the wave. This always comes into play as waves approach shore.

Motion of water within a wind wave.
See also Wave - Seiche (standing wave).

* If you have any comments please let us know.
E-mail us


Updated: April 2, 2006

©2006 by Earthguide team members and the Regents of the University of California.
All rights reserved.