In fluid dynamics, wind waves or, more precisely, wind-generated waves are surface waves that occur on the free surface of oceans, seas, lakes, rivers, and canals or even on small puddles and ponds. They usually result from the wind blowing over a vast enough stretch of fluid surface. Some waves in the oceans can travel thousands of miles before reaching land. Wind waves range in size from small ripples to huge rogue waves. When directly being generated and affected by the local winds, a wind wave system is called a wind sea. After the wind ceases to blow, wind waves are called swell. Or, more generally, a swell consists of wind generated waves that are not â€” or hardly â€” affected by the local wind at that time. They have been generated elsewhere, or some time ago. Wind waves in the ocean are called ocean surface waves.
Tsunamis are a specific type of wave not caused by wind but by geological effects. In deep water, tsunamis are not visible because they are small in height and very long in wavelength. They may grow to devastating proportions at the coast due to reduced water depth.
The great majority of large breakers one observes on a beach result from distant winds. Five factors influence the formation of wind waves:
* Wind speed * Distance of open water that the wind has blown over (called the fetch) * Width of area affected by fetch * Time duration the wind has blown over a given area * Water depth
All of these factors work together to determine the size of wind waves. The greater each of the variables, the larger the waves. Waves are characterized by:
* Wave height (from trough to crest) * Wavelength (from crest to crest) * Period (time interval between arrival of consecutive crests at a stationary point) * Wave propagation direction
Waves in a given area typically have a range of heights. For weather reporting and for scientific analysis of wind wave statistics, their characteristic height over a period of time is usually expressed as significant wave height. This figure represents an average height of the highest one-third of the waves in a given time period (usually chosen somewhere in the range from 20 minutes to twelve hours), or in a specific wave or storm system. Given the variability of wave height, the largest individual waves are likely to be about twice the reported significant wave height for a particular day or storm.
Some waves undergo a phenomenon called â€œbreakingâ€. A breaking wave is one whose base can no longer support its top, causing it to collapse. A wave breaks when it runs into shallow water, or when two wave systems oppose and combine forces. When the slope, or steepness ratio, of a wave is too great, breaking is inevitable.
Individual waves in deep water break when the wave steepness â€” the ratio of the wave height H to the wavelength Î»â€” exceeds about 0.17, so for H > 0.17 Î». In shallow water, with the water depth small compared to the wavelength, the individual waves break when their wave height H is larger than 0.8 times the water depth h, that is H > 0.8 h.Waves can also break if the wind grows strong enough to blow the crest off the base of the wave.
Three main types of breaking waves are identified by surfers or surf lifesavers. Their varying characteristics make them more or less suitable for surfing, and present different dangers.
* Spilling, or rolling: these are the safest waves on which to surf. They can be found in most areas with relatively flat shorelines. They are the most common type of shorebreak * Plunging, or dumping: these break suddenly and can â€œdumpâ€ swimmersâ€”pushing them to the bottom with great force. These are the preferred waves for experienced surfers. Strong offshore winds and long wave periods can cause dumpers. They are often found where there is a sudden rise in the sea floor, such as a reef or sandbar. * Surging: these may never actually break as they approach the waterâ€™s edge, as the water below them is very deep. They tend to form on steep shorelines. These waves can knock swimmers over and drag them back into deeper water.