Oct 16, 2020

What Wave Height Means

Because there is never only one wave height, oceanographers use a statistical analysis to forecast "significant wave height" (SWH). This is defined as the average of the largest one-third of all waves, and it is the wave height that an experienced observer will typically report. The actual wave height at a given time and place can be much higher, as much as twice the forecast SWH. With a forecast SWH of 10 feet, the mean wave height would be 6.4 feet; the highest 10 percent of waves would be 12.7 feet; the highest 1 percent of waves would be 16.7 feet; and the maximum wave height to be expected would be 20 feet.

What Influences Wave Shape, Height, And Direction?

Wind waves are independent of the swell and add to the wave height. The result is called "combined seas (CS)." The National Weather Service considers CS equal to SWH. Waves are defined by four components: height (trough to crest), length (distance between crests), period (time elapsed from the passage of one crest to the next), and steepness. Steepness is the ratio between height and length. When wave steepness exceeds 1:7, the wave will begin to break — resulting in whitecaps. This generally happens in 12 to 15 knots of wind.

Near-Shore Influences On Deep-Water Waves

There are a number of things that will change the shape, height, and direction of deep-water swell.

Reflection happens when waves bounce back from an obstruction and combine with still-approaching waves. Reflected waves have been seen as far as 15 miles off the California coast where the shore falls steeply into the ocean.

Refraction is a change in direction as a wave encounters shallower water near the shore. The shallower water slows the wave, causing a bending (or refraction) that favors parallel wave fronts to approach the shoreline. Near a headland, refraction focuses wave energy at the tip of the point. Near islands, waves often wrap all the way around the island. And when waves rejoin in the lee of the island, they can augment each other to form larger, sometimes breaking waves.

When current opposes the wind, waves can build quickly to steep and dangerous proportions. Common examples include the Gulf Stream, the Agulhas current, and places where prevailing winds oppose tidal currents (San Francisco Bay, or the Bay of Fundy during a strong ebb).

Wave illustration