Wind-generated sea waves
As we have seen in “Surface waves,” waves originate from different factors: tides, tsunamis, earthquakes, wind, etc. In this article, we will focus on those generated by the wind.
Wind-generated waves occur when the wind blows over the surface of the sea. They are usually classified in surface gravity waves and capillary waves. The restoring or damping force is the gravity force for the first type of waves, and the surface tension for the second.
Additionally, surface gravity waves are divided at their turn into wind sea and swell:
- Wind sea: they are generated by the local wind and are irregular and short-crested;
- Swell: they start as wind sea waves that eventually move out of the generation area. As they travel away, the original irregular system creates regular and long-crested waves that are not greatly affected by the local wind. The swell can reach areas very far away from the place the weather system first originated them.
Contrary to capillary waves, surface gravity waves contain more energy with increasing wavelength. Short wavelength gravity waves carry less energy and dissipate faster. Waves with a greater height also have more energy than smaller ones.
While tsunamis waves can have periods ranging from several minutes to hours, and tides can be considered as a single wave with a period of 24 hours, wind-generated waves display periods usually equal to or less than 30 seconds.
The following table shows the different types of wind-generated waves and some of their characteristics. We have classified them based on the period ranges that are usually assigned to each one and calculated, based on those periods, their frequency and wavelength:
Sea waves are observed and recorded by crew members, scientists, and engineers every day. They report wave height, period, and direction to meteorological organizations around the world. Visual estimations of the wave height are very reliable if experienced observers carry them out. On the contrary, it has been shown that estimations about the wave period made by a human observer are much less accurate.
But human observation is by no means the only method used in wave measurement and recording. Buoys, wave poles, echo-sounders, pressure transducers, current meters, and radars deployed either from the coast, fixed platforms, or even airplanes and satellites are also commonly used for this task. Each method has its own peculiarities regarding operational costs, accuracy, reliability, maintenance, and cost.
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