What is a Swell?

A swell is a series of mechanical or surface gravity waves generated by distant weather systems that propagate thousands of miles across oceans and seas.

It is a succession of crestless, massive wind waves characterized by a narrow range of long wavelengths.

Ocean storm surges are often generated in mid-latitude trough regions between 30 and 60 degrees, in deep-water regions, especially in the North Atlantic and North Pacific, but also in the South Pacific.

Most of the high-energy storm surges produced on the planet peak during the winter in the northern and southern hemispheres.

They may be visible reaching the horizon in evenly spaced parallel wave lines.

The Formation Of A Swell

Swells are not generated by local winds blowing, for example, near the coast.

Instead, they are the result of the interaction of severe storms with large amounts of water that takes place in the open ocean, thousands of miles away from land masses.

A storm acts like a stone dropped into a pond, sending out waves of energy in concentric circles.

The longer and stronger the wind blows, the bigger the ocean swell.

As a swell moves away from the storm, wave trains with longer wavelengths will travel faster and overtake shorter wavelength swells.

The ripples, or capillary waves, will continue to grow until gravity does its part.

As waves propagate in shallow water, they begin to slow down, the wavelength shortens, and the height of the waves increases.

On a gently sloping beach, waves will come in soft and crumbly, breaking in water deeper than a depth of 1.3 times wave height.

A steep slope or reef will form hollow, nodding waves in shallower water.

A 10-foot wave should break in 13 feet of water, but a sudden change in depth, offshore winds, and a fast-moving swell can greatly reduce the breaking depth before the tide is accounted for.

A swell often refers to bottom swells and not wind swells.

Swells And Wind Surges

A  swell  is a group of traveling waves that leave the area of ​​generation and propagate by themselves over long distances.

By the time they reach shore, it has already accumulated a lot of energy and period (in seconds) between consistent and powerful wave trains.

They are the perfect swell formula for surfers and surfers.

On the contrary, the wind swell -also called sea wind- is a short duration swell generated by local winds.

These short-period, disorganized swells are common in the North Sea, the Baltic Sea, and the Mediterranean Sea, although they can be seen anywhere in the world.

Period, Height And Direction Of The Swell

The swell period is the time it takes two successive wave crests to pass a given point.

Long period swells harness energy, travel faster, and easily cope with local winds and currents, resulting in larger beach waves.

The first few waves of a long-period swell are called precursors, and they generally move faster than the remaining cars in the wave train.

The height of the swell corresponds to the average height of the highest third of the waves in a given period.

It is measured from the valley to the peak. The seconds between one peak and the next can be determined using ocean buoys to calculate the height of the swell.

The stronger the wind, the longer the distance it will blow in the search, and the longer the period, the bigger the wave, the longer the wavelength, and the longer the period between crests.

A steady, sustained 50-knot wind blowing for almost three days for a minimum distance of 1,600 miles (2,600 kilometers) could generate wave heights of 50 feet (15 meters).

The direction of the swell is the direction from which it travels and is measured in degrees or cardinal points.

Propagation, Dispersal And Grouping

As the waves propagate away from the source, they begin to gather in swell lines and travel and propagate in circumferential dispersion.

And for every doubling of travel distance, the significant height of the wave is reduced by about a third.

In terms of wavelength, the greater the distance between two crests, the faster waves will travel across the open ocean, which means that faster waves with longer wavelengths will progressively outpace slower waves with higher wavelengths. shorter wavelength.

Surfers refer to the groups of waves that come ashore as “sets.”

Scientifically speaking, wave clustering is the result of different swells traveling in the same direction and merging.

When the peaks of two different wave trains coincide, a bigger wave will result.

However, when the peak of one wave train coincides with the trough of another, a cancellation effect occurs, resulting in calms on the beach.

Ripples Vs Waves Vs Swells

There is widespread and understandable confusion about the difference between swell and waves.

A swell is a group of waves that have obtained enough energy from the wind to reach another state of maturity that allows them to travel far beyond their place of origin.

It can pass through and under another swell or even more than one and absorb them in its path, resulting in more organized, well-spaced and well-defined swell lines.

A single wave (pictured below) will not be able to travel thousands of miles without strong winds blowing for a long period.

Unlike a swell, waves do not build up on top of each other over a long period of time.

From a technical and oceanographic perspective, the ocean observes:

  1. Ripples  – Waves with a period between crests of up to one second;
  2. Waves  – Ripples with a period between crests of between one and ten seconds;
  3. Swells  – groups of waves with a period between crests of more than ten seconds;

Micronesian and Polynesian ocean explorers were some of the first to spot the difference between ripples, waves, and swell.

As experienced sailors, they learned to identify waves by their shape, period, and rhythm or movement to navigate smoothly through calm seas and avoid rogue waves.

These Pacific islanders were able to read the interference patterns of tidal surges driven by the prevailing northeasterly trade winds.

The swell tends to curve around the islands and spread out in the channels between them.

The overlap of multiple waves from different directions produces a distinctive interference pattern that can help pinpoint your location.

Tropical Storm Surges

Hurricane, cyclone, or typhoon surges are tidal surges generated by depressions that often travel from east to west at subtropical latitudes (between 10 and 30 degrees).

As a result, when these storms occur, the ocean will host E-surges, mostly.

Tropical storms only form when the temperature contrasts between the air and the ocean are maximum.

These unpredictable and slow seasonal swells produce a significant number of large waves.

Walter Munk And The Weber Brothers

Wilhelm Eduard Weber and his brother Ernst Heinrich Weber were the first scientists to make great strides in the experimental study of waves.

In a laboratory, they discovered a close connection between the revolution of water droplets in stationary orbits and the passage of a surface wave.

Both have the same period, and the entire wave train advances one wavelength in one period of oscillation.

Physicists call this a traveling wave train. Common people may call it a swell.

Walter Heinrich Munk was one of the most famous physical oceanographers of all time.

In the 1940s, he and Harald Sverdrup “invented” the  science of forecasting waves  and wave heights.

Later, in 1963, he showed that the swell decays very little after traveling a distance from the edge of a storm equal to the diameter of the storm.

The swell also decays, extending laterally, not due to the resistance of the air on it, but due to the fact that it is absorbed at the limits of the ocean, that is, in the zone of surf breaks.

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