Oceanography (2+1)

Significant Wave-Height

What do we mean by wave-height If we look at a wind-driven sea, one can waves of various heights. Some are much larger than most, others are much smaller. A practical definition that is often used is the height of the highest of the highest 1/3 of the waves, H _1/3. The height is computed as follows: measure wave-height for a few minutes, pick out say 120 wave crests and record their heights. Pick the 40 largest waves and calculate the average height of the 40 values. This is H _1/3 for the wave record.

 wave height estimated by observers corresponds to the average of the highest 20 to 40 per cent of waves…. Originally, the term significant wave-height was attached to the average of these observations, the highest 30 per cent of the waves, but has evolved to become the average of the highest one-third of the waves, (designated H_s to H _1/3)

More recently, significant wave-height is calculated from measured wave displacement. If the sea contains a narrow range of wave frequencies, H _1/3 is related to the standard deviation of sea-surface displacement

H _1/3= 4 < ?^{2  ½}

Where  <?^{2  ½} is the standard deviation of surface displacement. This relationship is much more useful, and it  is now the accepted way to calculate wave-height from wave measurements.