If waves are caused by wind action on the sea surface, then why do they always appear to roll towards a beach?
Stephen Johnson, Eugene, Oregon, US
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Ocean waves are created by three forces: wind, seismic events and the gravitational pull of the moon and sun. However, the vast majority of the waves you see at the seashore are created by wind blowing against the ocean surface.
Their size is determined by the wind strength plus the length of time it blows for and the distance it travels over the water. There is a maximum wave height for any given wind strength. From this maximum point, waves will diminish with distance if the wind abates, but can travel for thousands of miles.
As waves are created, they move in the opposite direction to the wind source. If this was all that happened, they would hit beaches at almost any angle. However, as waves move into shallower water that is less than half as deep as their wavelength, they begin to be modified by the bottom of the ocean and are slowed. In addition, the wavefront aligns itself with the contours of the bottom, generally resulting in a turn towards the shore.
This process of wave refraction is why they mostly break in a line along the shore. Since ocean bottoms are all unique, there are many waves that hit the beach slightly off parallel, as can easily be seen in many locations.
Eric Kvaalen, Les Essarts-le-Roi, France
A sea can have waves going in different directions at the same time. If the wind is coming from the shore, it won鈥檛 create real waves until a long way out. There will just be very small waves going away from the shore, and they will grow bigger the further they go so long as the wind holds.
Waves created hundreds or thousands of miles away by a wind coming towards us will continue all the way to our shore, even if the wind near us is blowing in the other direction. So there are almost always waves coming ashore, no matter what the wind direction is.
Waves don鈥檛 necessarily come directly towards shore 鈥� they can arrive at an angle. But when the water gets shallow, they refract towards the shore, so they seem to be more straight on than they really are.
David Singer, Manchester, UK
Waves appear to roll towards the beach rather than away from it because those moving away have much less energy than those rolling in. A sloping shore absorbs much of the incoming wave energy, as well as scrambling the returning waves so they are hard to see.
Chris Daniel, Glan Conwy, UK
The phenomenon of waves breaking parallel or nearly parallel to the shore is due to refraction, and is analogous to the refraction of light as it passes from one medium to another.
As the wave travels, water molecules at the surface move in a circular motion with a diameter equal to the height of the wave. The size of this circular motion decreases the further the water molecules are below the surface, and is no longer felt at a depth called the wave base.
When the distance to the sea floor is less than the wave base, the speed of the wave will slow due to friction at the sea鈥檚 bottom and the wavelength will decrease. Parts of the wave still in deeper water will continue to travel faster, causing the wave as a whole to bend towards the shore, creating the refraction effect. In this way, waves tend to wrap around the contours of a typically indented coastline.
The shapes of coastlines and offshore sandbars can generate impressive waves through the refraction process, by focusing wave energy into breakers that occur reliably in certain weather conditions. These can be popular with surfers.
Richard Mellish, London, UK
In the open sea, waves can travel in any direction. There may even be a superposition of waves moving in different directions. Yet waves can only arrive at a beach if a part of their velocity is towards the beach.
Graham Cox, Hothfield, Kent, UK
Offshore winds that blow from the coast out to sea can never create waves that roll back towards the beach. Waves generated by wind or earthquakes can travel thousands of miles before they dissipate from the friction of water molecules and attenuation. So, on the majority of days, waves roll onto beaches because they consist of one or many waves from elsewhere, intermingled. Only in sheltered beaches after days of calm does this not happen and no waves occur. Where waves meet vertical walls of rock or concrete, they can bounce out to sea.
Waves have a direction despite being entirely created by water molecules rising and falling together. The physics of how wind can generate those vertical and forward motions should be explained by someone with more expertise than me!
Victor Stanwick, New York City, US
I can offer one partial answer: waves reaching beaches on the west coast of Africa appear to be moving away from beaches on the east coast of the Americas.
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