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Helsinki - a city by the sea

Wave terminology

Wave properties
Significant wave height
Wavelength and direction
A regular ideal wave is described by a wave height (height between through and crest), a wavelength (distance between two crests) and a wave period (time between two crests). Real wind generated waves are irregular and can therefore only be described statistically. The so called wave field is made up by all the waves of different lengths that are generated by the wind. The energy of the waves determines their height, which can be measured by a wave buoy. The most important way to describe irregular waves is the significant wave height. The significant wave height is a certain average wave height that corresponds to the wave height that can be observed visually. The height of an individual wave can be almost twice as large as the significant wave height.

A wave buoy cannot measure the length of the waves, but the waves can instead be separated based on their period. The modal period (or dominant period) of the wave field describes the length of the waves that, on average, are the highest. If the water depth is known, one can deduce the wavelength. This wavelength is called the significant wavelength (or the dominant wavelength).

If a wind is blowing from a straight shoreline, the resulting waves of different length have a similar direction. An irregular shoreline or depth refraction can still results in waves of different length having different directions. These phenomena are typical especially in the archipelago. The swell that is left after the wind has calmed can have a totally different direction than the waves generated on top of the old swell by a new breeze. The dominant direction tells the direction of the waves containing the most energy. As with the wind, the wave direction is often given as the direction from.