Significant Wave Height, Explained
Significant wave height (Hs, reported as WVHT) is the average height from trough to crest of the highest one-third of waves measured in a sampling window. It is a statistical summary of a messy sea surface, not the height of any particular wave.
The term is odd until you know its history. It comes from mid-century oceanography, where researchers found that the wave height a trained observer reported by eye corresponded closely to the mean of the largest third of waves present — not the mean of all of them. So "significant" here means "the part a human notices", and the definition was reverse-engineered to match.
The number is an average, so bigger waves are guaranteed
Because Hs describes the top third, individual waves routinely exceed it. For a fully developed sea the rule of thumb is that the highest wave in roughly 1,000 is about 1.86 × Hs, and the highest in 100 is about 1.5 × Hs.
This is why a 3 ft reading can still produce a set that feels considerably bigger, and it is a safety point rather than a pedantic one: a buoy reading of 15 ft means waves near 28 ft are an expected part of that sea state, not a freak event.
Why it is not the height you ride
Hs is measured in deep water at the buoy. The wave you take off on has shoaled — slowed in shallow water, steepened and grown. That transformation depends on period and on the shape of the bottom at your break, and it can add substantially to the height.
This is the single most common source of confusion in surf forecasting, and it has its own page: wave height versus face height.
Reading it in context
Hs on its own is nearly meaningless for surf. Paired with period it becomes predictive: the same 3 ft at 6 seconds and 3 ft at 16 seconds are different oceans. Always read the two together.
See it on live data
The fastest way to make any of this concrete is to go and look at the numbers being described.
Common questions
Is significant wave height the average wave height?▾
Why is it abbreviated WVHT on NOAA reports?▾
Does significant wave height include wind chop?▾
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