Understanding Swells – What Causes Ocean Waves?

understanding swell

All swells are created by wind blowing over the surface of the ocean.

As wind blows, waves begin to form. The strength, duration, and area of ocean that the wind blows determines how big the waves will be, how far they’ll travel, and how much power they’ll still have once they reach shore.

When winds blows very strong, for a long time, over vast distances (i.e. storms), the distance between waves becomes longer and the energy driving the waves becomes greater. This allows the waves to cover more distance.

The time of year is going to have a big influence on where the storms are forming and how strong they’ll be.

The waves generated by a storm will lose energy as they travel away from the source of the wind.

From there, they can combine with other swell, pick up more energy, or dissipate.

Ocean swells can be described as either:

  • Groundswell, which are generated by winds far out at sea that have covered longer distances. This results in longer swell periods with more energy. This can typically result in cleaner, more organized surf.
  • Windswell describes swell that has originated closer to shore. The swell period is much shorter, the waves are less organized, and come in faster.

Where the swell originates describes the swell direction. A North Swell is heading South, and North facing coastlines will receive the swell.

Swell direction is typically described in degrees from North 0° to South 180°. Swell direction breaks down as follows:

  • 0°/360° – North (N)
  • 45° – Northeast (NE)
  • 90° – East (E)
  • 135° – Southeast (SE)
  • 180° – South (S)
  • 225° – Southwest (SW)
  • 270° – West (W)
  • 315° – Northwest (NW)

The angle of the swell can be used to determine how the surf will be at a particular spot. Depending on the coastline, points, reefs, and sandbars, the surf in one spot could be totally different from somewhere nearby.

Island blockage or shadowing can have an impact on the resulting surf on the coastline too. If there are islands offshore, or even far out at sea, some of the particular swell’s energy will be reduced before it gets a chance to reach the shore as surf.

All of the characteristics of the swell along with the characteristics of the local spot will combine with current weather conditions and tides to result in surf.

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