Wave height, period and swell explained

What significant wave height means

Wave height is measured vertically from trough to crest. Significant wave height is roughly the average height of the highest third of the waves passing a point over a period of time. The definition looks odd, but it matches fairly well what an experienced observer would estimate by eye, since people tend to notice the larger waves.

The consequence is important. Most waves will be smaller than the forecast figure, but a fair number will be larger, and now and then one will be much larger. As a widely used rule of thumb, the highest single wave over a few hours can approach twice the significant height. A forecast of 1 metre (about 3 feet) therefore does not rule out an occasional wave nearer 2 metres (about 6 feet).

Wave period and why longer means more energy

Wave period is the time in seconds between successive crests passing a fixed point. In deep water, period fixes both the length and the speed of a wave: longer-period waves are longer from crest to crest and travel faster. The wavelength in metres is about 1.56 times the period squared, so a 5 second wave is around 39 metres (130 feet) long and a 10 second wave is around 156 metres (510 feet) long.

A longer wave of the same height carries more energy. The power in a wave rises with the square of its height and also in proportion to its period. In open deep water this hardly shows, because a long wave is such a gentle slope, but it shows at the coast. As a long-period wave moves into shallow water it slows, bunches up and grows taller before it breaks, so a 1 metre swell at 14 seconds can produce far heavier surf than 1 metre of 5 second chop.

Forecasts may quote the peak period (that of the most energetic waves) or a mean period, and the two are not interchangeable.

Wind waves and swell

Waves raised by the wind blowing where you are, or close by, are called wind waves, wind sea or simply sea. They are short, steep and irregular, with crests that are not well lined up, and they travel roughly downwind. Their size depends on three things: how strong the wind is, how long it has been blowing, and the fetch, which is the distance of open water it has blown across.

Swell is made of waves that have travelled out of the area where they were generated. As they travel, the longer and faster waves pull ahead and the short choppy ones die away, leaving smooth, regular, long-crested waves. Swell can cross an ocean and arrive on a day with no local wind at all. On open ocean coasts swell periods of ten seconds or more are common. In enclosed seas and on lakes there is rarely room for long swell to develop, and nearly all waves are wind sea.

Often both are present. Swell from one direction with wind waves from another gives a confused, lumpy sea that is tiring in any small craft.

Why steep chop can be worse than swell

What a small boat feels is steepness, the ratio of height to length. Take two 1 metre waves. At a 4 second period the wave is about 25 metres (80 feet) long. At 12 seconds it is about 225 metres (740 feet) long. The long swell raises and lowers a boat so gradually that it may barely be noticed offshore. The short chop comes at the boat every few seconds as a succession of steep faces: the bow slams, water comes aboard, and a kayak or small open boat can be turned side-on or swamped.

Waves also break when they become too steep, and a breaking wave is far more dangerous to a small boat than an unbroken one of the same height. Two things steepen waves sharply:

Wind against current. When a tidal stream or river flow runs against the wind, waves shorten, steepen and break. Tidal races and river mouths can become rough in a moderate wind for this reason and ease when the stream turns.

Shallow water. Over bars, shoals and reefs, waves slow down and heighten, and may break even when the open sea outside looks manageable.

None of this makes swell harmless. Long swell breaks heavily on bars, reefs and beaches, surges up rocks and boat ramps, and tends to arrive in sets, groups of larger waves separated by quieter spells.

What a wave forecast cannot see

Wave forecasts come from computer models of open water. They do not resolve the shelter behind a headland or the steepening over a particular bar. They are also not available everywhere: many lakes, rivers and enclosed harbours have no wave forecast at all. CatchWindow location pages show wave height and period where a marine forecast is available, and that figure should be read as a guide to the open water nearby, not a measurement at the shore.

Reading the numbers together

Treat the height as typical of the larger waves, and expect occasional waves well above it.

Always read the period alongside the height. A short period means steep and uncomfortable; a long period means powerful at the shore.

Watch a landing or launching place for several minutes before committing, so that you see a full set.

For any passage, bar crossing or exposed coast, use the official marine forecast and local knowledge.