AbstractDuring World War II a group of Naval officers conducted visual measurements of ocean waves simultaneously with instrumental recordings. A comparison of the visual and instrumental values indicated ".... the natural tendency for the observer .... to record not the average wave height but a wave height based on some kind of average of the highest waves. The general experience is that an observer will give a value for the wave height which represents the average of the highest 20 to 40 per cent of the waves" (SIO, 1944). The average height of the highest one-third of the waves, H1/3, was therefore suggested as the characteristic (or significant) wave height. "Characteristic wave period" was given a corresponding definition as the average period of the highest one-third waves.
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