Wind makes Waves

Waves are created when the wind blows. Even the gentlest wind causes ripples on the sea surface. Inside the waves the water mass moves in a circular motion backwards and forwards, up and down. There is no forward movement inside the wave – it is the wave energy that moves.

When the wind blows across water, energy is transferred to the water, creating both waves and surface currents. These two occurrences develop independently.

Just as for all other wave types, water waves involve the transport of energy, and not the transport of the water mass itself. There is therefore very little net water transport in the direction of the wave.

In deep water particles move in a circular motion when the wave passes. The wave movement decreases with depth and becomes very small at depths greater than half the wave length.

How long does it take for the sea to calm down?

When the wind dies down, the waves continue for long distances until they reach the coast. They are a very efficient means of transporting energy. Short waves dissipate fairly quickly, while longer waves can continue for thousands of kilometres, since the viscosity of water is very low. These long waves, decoupled from the local wind field, are called swell.

You may experience a fully developed sea for example if you are in deep water off the coast of Gotland’s southern tip, and there has been a south westerly wind blowing 15 m/s for at least 10 hours across the entire southern Baltic.

In this example the wind fetch from the Polish-German coast is about 200 nm (about 400 km), and only 150 km is needed to allow a fully developed sea to develop. The biggest waves will be 9 m high, 300 m long travelling with a velocity of 27 m/s. The significant wave height would be 5 m with an associated wavelength of 80 m and a velocity of 14 m/s.

Since the group velocity is half the wave velocity, the group’s respective velocities are about 14 m/s and 7 m/s. When the wind stops blowing it would take 5 hours until the last big wave has passed, and 10 hours before the waves as high as the significant wave height have passed.

The longest wind fetch in the Baltic is 800 km. This gives 16 hours until the biggest waves reach the coast and 32 hours before the waves with the significant wave height reach the coast - so you can expect a long wait before you can comfortably go out on the northern Baltic after the wind has blown at 15 m/s for at least 10 hours over the southern Baltic.

If you observe waves starting to increase in size, it can be an early warning that windier weather is on the way, since the waves frequently move faster than the wind itself.

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