Difference between ripples, waves and swell
Below is a transcript of an article I wrote for the Outdoor Swimming Society newsletter in April 2010 about the relationship between wind and water.
Creating Ripples
A cup of tea can give us the energy to head out into the fresh air, but there is something else it can do for those of us who like to spend time on the water. It can give us an insight into a totally different kind of energy. Breath into your cuppa and you will see ripples form in the small hot pond in your hands. This is the start of the relationship between wind and water that gives the surface of all open water, from puddles to vast oceans, its character.
The harbour breakwater shows how the wind shapes the water.
Ripples are the instant effect of wind on water and they die down as quickly as they form, as the surface tension of the water dampens their efforts. If a wind blows steadily across a large enough patch of water for a few hours then the ripples become waves and these will not be dampened so easily. Waves always travel in the same direction as the wind is blowing: if the wind changes then the waves change with it. Wave size is determined by the strength of the wind, the length of time it has been blowing and the distance it has blown over or the ‘fetch’.
When strong winds blow for longer than a few hours, it gives the water sufficient energy that it then takes on a character of its own. This character and movement is known as swell, and it will march across open areas of water independent of the wind.
Waves and swell look subtly different. Swell marches in longer lines and appears less steep and more ‘stretched’ than waves – its wavelength does actually grow slowly as it grows older. The two also behave differently as swell will not normally break in open water, whereas waves will.
Swell continues to move under winds and waves that have long since changed direction, it can even head in the opposite direction as the wind and waves. Swell can cross an ocean of a thousand miles, something that is recognised and studied by surfers from Hawaii to Cornwall.
The fact that the water can store this wind energy explains why we often experience a rougher sea than the wind that was forecasted or felt before jumping in. It is also why we can often see big waves crashing onto a beach on a calm day.
This stored energy is also the reason that a change in direction when swimming outdoors can have such a dramatic effect on what we feel. If you have tuned into these effects then this change in sensation can actually be used to help determine direction. If the smoothest swimming is northwards and the roughest towards the south one minute, then it still will be half an hour later, even if your horizon has ceased to make sense. Inspiration for this technique of direction-finding can be found on the opposite side of the world.
The expert natural navigators of the Pacific have learned to read the water’s movement in a way that no other humans can, they are able to tell which way a canoe is heading while lying down on the deck with their eyes shut, just from the feel of the swell under them. They can even tell the difference between the swell of the open ocean and swell that has reflected back off an invisible island dozens of miles away, using this to find their way home.
Swimming outdoors is sometimes about reaching another island, as those who have embarked on a Greek SwimTrek will attest, but most swims have more modest goals. There is a still a huge amount to be gained from the fun detective work of trying to decipher the winds influence on the water.
In a lido you will probably find only ripples – if there any waves you are likely swimming on your own, as the sensible take shelter from the gale! But if you are swimming at sea, whether on a beach or off the back of a yacht in the middle of the Atlantic (highly recommended, but not when singlehanded!), then your swimming experience will be heavily influenced by the relationship between wind and water. Like all outdoor experiences, it can be made much more interesting and fun by observing and trying to understand the clues that nature offers.
