Definition
Defined as the mean height, trough to crest, of the highest one third of waves in a record, chosen because it approximates what an experienced observer reports by eye. It is the quantity given in forecasts, GRIB files and buoy data. In a given sea state the highest individual wave in a three hour period is commonly around 1.8 to 2 times Hs, so a 3 metre forecast implies occasional waves near 6 metres.
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