What Is A Swell In The Ocean?

Last updated on: December 3, 2023
What Is A Swell In The Ocean? - Ocean Swell Explained

Swells appear in the ocean when the wind transfers its energy from the air into the water. Every swell starts as small ripples on the ocean surface, and as they travel, the energy builds up and the swell will start to grow in size before it finally breaks into actual waves.

what causes ocean swell?

All swells are created by wind raging over the ocean’s surface. As a result of the wind blowing, waves start forming in sets. The power, duration, and fetch of the sea that the wind blows over will determine the size of the waves, how big the waves will be, how far they’ll travel, and how much force they will have once reaching the shore.

When winds blow very hard, over a long time, over massive distances, the distance between waves becomes more extended, and the waves become more powerful. The waves can therefore cover greater distances.

Swell Height

When a swell reaches the shoreline, the leading edge that drags along the seafloor will slow down more than the trailing edge of the swell. This gives the incoming water nowhere to go but up, and this makes the wave height increase in size before it finally breaks.

The length of the swell period will determine how much water gets pushed upward. A 4-foot wave with a 20-second swell period will grow significantly more than a 4-foot wave with a 10-second swell period – that’s why you need to understand the importance of swell period, before deciding whether the conditions suit your skill level.

The height of each wave in a swell will vary, but as surfers, we determine the swell height by the average size of the tallest one-third of the waves, that comes in during a given period. The height of a wave gets measured from trough to peak.

Swell period

Swell period defines the time it takes one wave crest to pass a fixed point before the next wave rolls in. The Swell period is given in seconds and will usually range from 4 to 22 seconds. The longer the swell period, the bigger the waves will become. Ideal waves for surfing have a swell period of 12 seconds or more, and such conditions are considered groundswell.

The actual individual waves will be traveling at three times the swell period, so a 12-second swell will have waves moving at up to 36 nautical mph.

Therefore, the swell period is just as important as swell height in terms of how powerful the waves will be.

Swell Direction

Where the swell comes from describes the swell direction. In other words, a north swell is heading south, but come comes from the north, and those north-facing beaches will receive the swell.

Swell direction is measured in compass degrees, with North at 0 (or 360) degrees, South at 180 degrees, East at 90 degrees, and West at 270 degrees.

The angle of a particular swell determines how the waves will be at a specific spot. Depending on the reefs, headlands, and sandbanks, the surf in one area could be totally different from another location in the immediate vicinity.

Seasonal Influences

The season also influences where storms are forming and how powerful they will become. Waves generated by storms will lose energy quickly as they move away from the storm. Sometimes they can combine with other swells and pick up more energy, and at other times they will dissipate.

Groundswell Vs. Windswell

As surfers, we categorize ocean swells as either groundswell or windswell. What separates the two types from each other, is the swell interval.

Groundswell consists of longer period swells (often 10-20 second+ intervals) with considerable energy that has been generated far away and has travelled long distances. This is what we want as it results in cleaner, organised, and powerful surf.

Windswell originates closer to shore and has a short swell period. (usually less than 10-second intervals). Windswell occurs when there is high wind pressure just off the coast. The waves are usually jumbled up and come in fast, so it’s not the best conditions for surfing.

Tropical storms

All waves occur by wind due to the temperature contrasts between air and ocean, but when these contrasts reach their maximum, tropical storms such as hurricanes, cyclones, or typhoons appear, and they will build up a significantly larger groundswell.

Tropical storms are less predictable and professional surfers tend to go on so-called strike missions to catch them. It basically means they will travel to the specific spot, where the swell is predicted to hit. If you’re not lucky enough to live in the specific country/area of impact, the timeframe for such a strike mission is often very tight.

Swell Shadows

Islands also block or shadow ocean swell and can radically diminish swell size. As a result, islands will slow down swell and sap energy from the waves, often leading to weaker waves within the island shadow.

It is a combination of ocean swell, ocean bottom and coastal characteristics, and currents and tide that will determine the look and feel of the surf we encounter and ride.

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