Oceanographers use 6 labels to describe wave height but we only really need to know about 3 of them. H, Hsig and Hmax.
In the chart to the left, the green Hsig line shows the average of the highest one third of the waves recorded.
The suitably chosen red Hmax line shows the highest wave you should be alert for and that is two times the height of Hsig!!
Now you need to do a quick calculation as the figure that provides the average of all the waves recorded is H, which is not displayed in this style of chart.
H equals 2/3 of Hsig and is the wave height you are most likely to encounter during your session.
So this explains why the yellow line displaying the overall wave height in the 1st chart is 2 metres and not 3 metres.
Hsig = 3 metres
H (2/3 of Hsig) = 2 metres the most likely average wave height
Hmax ( 2 X Hsig) = A 6 metre wave is possible so watch out!