Waves are generated by wind. Offshore storms generate winds which blow on the surface of the sea and create ripples, much in the same way as the ripples in your post surf cuppa are made when you blow on it to cool it down. The wind can be seen on weather maps as low pressure areas. The more tightly packed the isobars are on the weather map the stronger the winds will be. Small waves (capillary waves) are initially generated in the direction that the wind is blowing.

Wind blowing on flat sea surface.
Wind blowing on the Flat Sea Surface

The stronger and longer the wind blows, the more effect it has on these ripples and the larger they become. Initially the waves will just be small chop, but these will soon increase in size.

Wind Creating a Larger Swell
Constant Wind Creates a Larger Swell

As the wind continues to blow and the waves generated remain under the influence of the wind, the smaller waves will increase in size. The wind will get hold of the small waves much more easily than the calm sea surface.

The wave size is dependent on the wind speed generating it. A certain wind speed will only be able to generate a wave of certain size. Once the largest waves that can be generated for a given wind speed have formed, the seas are “fully formed.”

Large Groundswell Generated
Large Groundswell

Waves being generated have differing speeds and wave periods. (See wave terms explained for more detail.) The longer period waves are faster and move farther ahead of the rest of the slower waves. As the waves travel farther away (propagate) from the wind source, they start to organise themselves into swell lines. “Wave trains” form and these inevitably hit the beach at the same time. You may have heard of sets already!

Waves that are no longer affected by the wind that generated them can be referred to as ground swell, gold dust for surfers!

What Affects the Size of Swell

There are three main factors that affect the size of a wave in open sea.

  • Wind speed – The greater the wind speed is, the larger the wave will be.
  • Wind duration – The longer the wind blows, the larger the wave will be.
  • Fetch – The greater the area the wind affects the wave, the larger the wave will be.

Once waves are no longer affected by wind, they’ll start to lose their energy. They’ll travel as far as they can while being decreased by friction on the sea bed and obstacles in the way. (A big island for example)

There are different factors affecting the wave size at a certain surf break. Examples include:

  • Swell direction – Will the break be “open” to the current swell direction?
  • Ocean floor – A swell coming straight from deep sea up onto a reef will generate big, barreling waves. A long, shallow ledge up to the shore will slow down the waves and they’ll lose their energy, causing the waves to have less oomph.
  • Tide – Some spots are totally tide dependant.