One of the commonest questions ocean watchers ask is:
What is the difference between waves and swell?
The concise answer is:
“Swell is best thought of as waves that have enough energy to travel well beyond the place of their origin. It marches in broader lines and with bigger gaps between each crest.”
(The fuller answer can be read on pages 165-7 of How to Read Water.)
In the picture above we can see waves moving from left to right and swell rolling away from us.
A similar effect can be seen in these images taken from varying ranges. In each case the waves are moving broadly from left to right and the swell is moving from bottom to top.
These pictures were taken from the right hand (starboard) side of an aircraft on final approach to the airport. The waves will usually be heading in the opposite direction to any aircraft on final approach, since it will be landing into wind.
At the coast, swell meets land and the shallower water means that swell starts to rear up and break, forming waves. A headland sometimes gives us a chance to look at the different appearance of each at the same location.
The two videos below show the effect more clearly, they are taken from the same location (Rhossili on The Gower, south Wales), at the same time.
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How to Read Water – The Book