Waves are caused by friction between the wind and water surface. The energy transferred from the wind to the ocean during strong wind events is immense, and this energy can travel quickly over long distances. Because of this, the sea state is a combination of waves that have travelled a long distance, known as swell, and waves that have been produced by local winds, known as wind waves.
There are three main components of the sea state; wave height, period, and direction.
Wave Height
Wave height is determined by wind speed, fetch, and the length of time that the wind blows. The maximum wave height can be limited by any one of these factors. Wave height is a continuous combination of different waves acting to cancel out or magnify each other, depending on the sequence. Rather than each wave being the same height, the combination of factors influencing the sea state results in a spectrum of wave heights. The most common measure used to describe wave height is the significant wave height. Significant wave height is the average wave height (trough to crest) of the top one-third of all waves.
In practical terms, this means that:
- Because the significant wave height is an average of the top one-third of waves only, most waves will be less than the significant wave height.
- Slightly less than 15 % of the waves will be higher than the significant wave height.
- Occasionally, the waves will be much higher. According to the Bureau of Meteorology in Australia (http://media.bom.gov.au/social/blog/870/ruling-the-waves-how-a-simple-wave-height-concept-can-help-you-judge-the-size-of-the-sea/), a wave will be double the significant wave height three times every 24 hours. Conversely NOAA states that a wave will be twice the significant wave height approximately once every hour (https://www.weather.gov/mfl/waves)
- If a wave is more than double the significant wave height, it can be considered a rogue wave.
Wave period
The period of a wave is determined by the length of time it takes for a complete wave cycle to pass a point; in other words, the time between two wave crests. In forecasts, the wave period is given by the dominant wave period, which refers to the period of the highest energy wave sequence. If the current sea state is dominated by locally generated wind waves, the wave period is likely to be short (4–6 seconds). On the other hand, if the sea state is dominated by swell that has travelled a long distance, then the wave period is likely to be long (10–20 seconds). If wind waves are the dominant wave period, short and steep seas can be expected. A preferred sea state is when swell dominates the sea state and the wave period is long.
Wave direction
Wave direction is determined by averaging the direction of propagation for all waves in the spectrum after weighting the waves based on their height to ensure that the direction of larger waves is more heavily represented when determining wave direction (https://confluence.ecmwf.int/display/FUG/Waves+and+Wave+Meteograms). While this method is usually very representative of the actual conditions, it can be misleading. For example, if you were crossing the Atlantic and had a northerly propagating swell and easterly propagating wind waves of the same magnitude, the forecast swell direction would be northeasterly. In reality, there would be two clear wave trains impacting your passage. It is difficult to represent this kind of information in a single wave forecast, so it is important to have an understanding of the larger synoptic influences impacting the wave forecast.
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