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Waves are mostly caused by Friction of wind on surface of water.

Wind blows from sea to land in day and land to sea in night due to pressure and temperature difference.

So it is intuitive why there are waves towards shore in day, but even in night there are waves towards shore. Sometimes even stronger than waves during the day.

Why is that?

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  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ – gerrit Feb 17 at 8:27
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Ocean waves (and also in mediterranean type seas and larger lakes, but on a smaller scale) are generated by two processes:

  • locally generated waves ("wind waves"), which follow the direction of the wind;
  • waves generated further out in the sea (i.e. "swell waves"), which do not necessarily follow the direction of the wind.

During the night, you are probably seeing swell waves. Of course, some wave energy is generated also in the opposite direction by the wind blowing offshore, but one can only "see" these waves further out in the sea if the wind is strong enough (see the definition of fetch).

Also check the Figure [1] below for clarification on how swell is generated (the longer period wave energy travels faster than the energy of the shorter period waves, so the initial "random" wave field disintegrates into regular swell).

enter image description here

Shallow water wave processes like diffraction and refraction will curve the paths of the waves that start reaching the coast and "feeling the bottom". The waves appear to be heading almost perpendicular when they reach the coast, even if 1 km out to sea they're moving almost parallel to the long distance coast.

How the waves appear at the coast to the observer depends on the direction of the local wind. Generally, if the local wind blows in the same direction as the waves (i.e. onshore wind), the waves appear "mushy" as the wind helps to break the waves (see example image). If the local wind blows in the opposite direction of the incoming waves (i.e. offshore wind), the waves maintain their shape due to opposing wind and they break later (see example image). These conditions are favourable to surfers.

[1] Holthuijsen, L. H. Waves in oceanic and coastal waters. Cambridge University press, 2010.

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    $\begingroup$ Welcome to this site. Upvoted your answer. Could you clarify on whether if the sea breeze and much farther wind blow in a) same direction b) opposite direction how the waves are affected ? $\endgroup$ – gansub Feb 13 at 13:05
  • $\begingroup$ It is very much dependent on how strong the local breeze is. Generally, in case a), the waves appear "mushy" as the wind helps to break the waves (see example image). In case b), the waves are favourable to surfers, as the waves maintain their shape due to opposing wind and they break later (see example image. This is of course a simplified explanation of shallow water wave theory :). $\endgroup$ – Ingvar Lukas Feb 13 at 13:56
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    $\begingroup$ As waves, these water waves with a long fetch will experience diffraction and refraction. Diffraction will let those in the diagram enter the Irish Sea and even the North Sea. Refraction is more noticeable close to a beach where the water shallows dramatically. This is what causes waves to almost always appear as if they're heading almost perpendicular towards the beach, even if 1km out to sea they're moving almost parallel to the long distance coast. Refraction will curve them into the local bay and towards a beach. (not quite perpendicular - this is where longshore drift comes in) $\endgroup$ – winwaed Feb 13 at 15:52
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    $\begingroup$ Remote generation of ocean surface waves is the topic of "Waves Across the Pacific [1]," a science documentary about work done in the early 1960s. [1] youtube.com/watch?v=MX5cKoOm6Pk $\endgroup$ – John Feb 13 at 20:27
  • $\begingroup$ winwaed's comment is very good and I forgot to include this in my answer - shallow water wave processes like diffraction and refraction will curve the paths of the waves that start "feeling the bottom" , so that the waves appear to be heading almost perpendicular when they reach the coast. $\endgroup$ – Ingvar Lukas Feb 14 at 7:00
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I'm making my comment an answer.

Waves are ubiquitous, except on land ;-). Waves in the open sea are a mix — a superposition — of waves in different directions.1 The dominating direction of large waves is, after a while, the wind direction; but that's not absolute. You have some omnidirectional background "noise" of chaotic movement as well as waves which have traveled a long distance and have nothing to do with local conditions.

What you see at the shore is the part of the waves on sea which run towards the shore. That's of course less when the wind is off-land, but it is not zero. In addition you have refraction on the coast line; the waves "bend around" the coast line.

In essence, that you see a dominating wave direction towards the land at any wind direction is a result of zero waves emanating from the land.


1 In this study there is a nice image of the directional spectrum of wind generated waves in Figure 16.

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    $\begingroup$ please add a source for "Waves in the open sea are a mix — a superposition — of waves in different directions." $\endgroup$ – gansub Feb 14 at 10:51
  • $\begingroup$ @gansub Have you ever been on sea? $\endgroup$ – Peter - Reinstate Monica Feb 14 at 11:22
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    $\begingroup$ Certainly not disputed but for people who want to foray into this field just some introduction would be helpful $\endgroup$ – gansub Feb 14 at 11:48
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    $\begingroup$ Yeah, I'd like to see a citation for "omnidirectional background noise". Yes, there will be waves in many directions in many places with deep water, but they all originate somewhere. As you've largely identified elsewhere in the answer, the important things for answering this question are (a) that waves don't necessarily relate to local wind, and (b) that we won't see wind waves going offshore as they'd have zero fetch $\endgroup$ – Semidiurnal Simon Feb 14 at 15:58
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    $\begingroup$ @Peter-ReinstateMonica I don't question the idea that wind waves can come from multiple directions. I'm just a bit skeptical about the idea that every location has an "omnidirectional background" that is always there.I mean, sometimes it's calm! And as you yourself noted, there won't be much wave energy going off a beach due to lack of fetch. But possibly we're arguing about wording rather than concepts, and it doesn't feel important to the question :-) $\endgroup$ – Semidiurnal Simon Feb 14 at 20:24
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Wind blows from sea to land in day and land to sea in night due to pressure and temperature difference.

Not so much. This is common in the Mediterranean in summer, for example, where the area sits under a stable area of high pressure and there is little wind caused by the weather system. Go to the Med in winter though, and you'll find the weather systems are completely different as the weather systems which create the Meltemi kick in. If you go elsewhere in the world - Britain, for example - you'll find that the wind at the coast is generally dominated by changeable weather systems blowing across the country, and not by coastal winds. And it should be obvious that you can go somewhere without a coast - say, Kansas - and still experience winds, and see waves on the surface of lakes.

Other answers have already covered the mechanics of waves better - but I thought this additional misunderstanding in the question was worth clearing up.

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I am not a wave expert, but i thought waves were caused by the gravitational pull of the moon and sun ? As the ocean is pulled up and down every night and the bulge from the pull being dragged across the ocean and then released sets the waves in motion and the reason they always go towards the shore is because the mass of the ocean and its shape. Once the waves comes in and then underneath it is cycled back out because the top wave has more mass and is the dominate one. The wind just adds to motion or partially cancels out the motion of the wave depending on its direction. I know this is a old thread but i thought it was very interesting. Here is some citation i found while replying and it seems both gravitational pull and wind both cause the wave direction

https://oceanservice.noaa.gov/facts/wavesinocean.html

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    $\begingroup$ You are confused between waves and tides. $\endgroup$ – Semidiurnal Simon Feb 17 at 9:17
  • $\begingroup$ @SemidiurnalSimon Cant the tide influence the wave ? $\endgroup$ – gansub Feb 17 at 12:40
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    $\begingroup$ @gansub yes, but that should be the topic of a different question :-) $\endgroup$ – Semidiurnal Simon Feb 17 at 17:02

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