Sunday, February 8, 2009
Ocean ‘Wind Waves’
The great majority of waves one sees on an ocean beach result from distant winds. Three factors influence the formation of "wind waves": wind speed; distance of open water that the wind has blown over; geometry of the area of wave generation by the wind; and length of time the wind has blown over a given area. All of these factors work together to determine the size and shape of ocean waves. The greater each of the variables, the larger the waves. There are three different types of wind waves that develop over time: ripples, seas, and swells. Ripples appear on smooth water when the wind is light, but die if the wind stops. Seas are created when the wind has blown for a while at a given velocity. They tend to last much longer, even after the wind has died. Swells are waves that have moved away from their area of origin and are unrelated to the local wind conditions. They may be thought of as seas that persist long after the wind that produced them has stopped. Some waves undergo a phenomenon called "breaking". A breaking wave is one whose base can no longer support its top, causing it to collapse. A very large breaking wave can impart a pressure of up to 50 to 100 kilopascals, enough force to crush the hull of a ship. A wave breaks when it runs into shallow water, or when two wave systems oppose and combine forces. Waves can also break if the wind grows strong enough to blow the crest off the base of the wave.