Published: 13 June, 2007

Wind's speed, duation, and 'fetch' all affect waves on water

How does wind affect waves?

This perceptive, perennial question has led researchers to uncover a variety of beautiful and intertwined natural phenomena over the centuries. Waves on the surface of water may be generated through a single event, such as a stone thrown into a pond, or through an extended episode, such as a windy storm.

Energy from the wind is transferred to a quiescent water surface through friction and pressure changes, generating small ripples on the surface. These ripples are very short, both in height and length, less than about a centimeter. Once these tiny waves have formed, they begin to interact and, through a wonderful but complex process, energy is transferred to form longer and taller waves. As the waves grow, the wind begins to push directly on the windward side of the wave, transferring even more energy. If the wind blows for long enough these taller waves may gain enough energy to "white-cap" or break, spilling energy back into the wave system. For conditions of sustained high winds over a large area of water, enough energy will be transferred to generate "swells." Swells escape from the wind that generated them and travel great distances, since longer waves have higher speed.

In studying how wind affects waves scientists have identified three important factors: the speed of the wind itself, the length of time the wind blows, and the "fetch," which is the size of the area over which the wind blows. Surfers relish waves created by distant windy storms because the shorter wavelengths (called "chop") are left behind. And so, on a windless day at the beach, surfers can enjoy a perfect wave created by winds that were blowing thousands of kilometers away.