Swell and waves, two movements, two stories
At first glance, swell and waves seem interchangeable. Yet their origin and nature differ. Waves are created directly by the wind blowing across the surface of the water . Their height, direction and frequency vary according to wind strength, duration and extent. It's a local and often temporary phenomenon.
The swell for her part, is the result of wave movement long after the initial wind has died down . It's a wave that travels long distances, sometimes across an entire ocean. It often has a more ordered surface appearance, with regular, continuous undulations.
Waves are created by the breath of the wind
Waves are created when the wind rubs against the surface of the water. This action transmits part of its energy to the water, creating small ripples which, under the action of a persistent wind, grow to become clear waves.
Wind speed plays a key role: the faster the wind, the bigger the waves. The length of time the wind lasts is also crucial. The longer it lasts, the more time the waves have to develop. Finally, the open expanse of water over which the wind blows, known as the "fetch", has a direct influence on wave formation.
When these conditions are met, the surface of the sea is covered with choppy, disordered waves. This is known as the "wind sea".
The swell created by the imprint of the disappearing wind
When the wind dies down, the waves created continue on their way, driven by their own energy. Under the combined effect of gravity and the cohesive force of water, they become more organized, spaced out and take on a more regular shape. This is the birth of the swell.
The swell is characterized by long, spaced, harmonious undulations. Their direction is stable and corresponds to the origin of the wind that generated them. As a result, swell can travel hundreds or even thousands of kilometers without losing much of its energy.
For example, a yachtsman may encounter a sea with an impressive swell, even though the sky is blue and the wind is almost non-existent.
How to recognize swell and waves when sailing?
For yachtsmen, distinguishing between swell and waves is essential to anticipate their behavior at sea. When you see a choppy, disordered water surface with numerous, short, confused crests, you're dealing with a windy sea, dominated by local waves.
On the other hand, when you're sailing on a sea with long, evenly-spaced ripples, this is called a swell.
Navigation in swell-formed seas is generally more comfortable, although the boat's movements can be extensive and cause pronounced rolling. Close to the coast, swell turns into breaking waves when it encounters shoals, which can make for a tricky approach.
Swell, a key factor in marine weather
In marine weather reports, swell information is invaluable. Swell height, period (i.e. the time between two successive crests) and direction are all elements that can be used to assess future sea conditions.
A long swell, with a period of more than ten seconds, indicates powerful, widely-spaced undulations, capable of generating a strong roll even in light winds. Conversely, a short, cross swell is a sign of confused seas that are difficult to navigate.
Although invisible on the weather map, swell remains a fundamental parameter in the preparation of any navigation on the high seas or along the coast.
