[Storm Steve]

Steven Gale  asked the Naked Scientists:
   Dear Chris et al,

Love the show! [nofollow]

My question is: Where does the energy for ocean waves come from?

Some suggestions:

* Tidal forces (extra-terrestrial gravity)
The tide does come in and go out, but in all phases of the tide there are waves.  So tides cannot be a complete explanation.

* Weather (wind)
Waves are more intense when a weather front comes through, but waves exist even when the weather is extremely calm. So weather cannot be a complete explanation.

* The Earth's rotation
I personally observed that Ocean waves come crashing in on both the east side and the west side of the Atlantic Ocean, and they appear to be just as powerful on one side as on the other (although I didn't measure them). If the energy source for waves is directional from one side or the other, like the earth's rotation, then I would expect to see big waves on one side and little waves on the other. So the earth's rotation cannot be a complete explanation.

* Geothermal energy

* Tectonic plate movement

* Currents (like the Gulf Stream)

* Echos from long ago events, like meteor impacts.
When you drop a pebble into a pan of water the ripples bounce outward and then back inward.

And a related question: Which side of the Hawaiian Islands has better surfing?

Thanks,
Steve

Hashmonaim, Israel

What do you think?

[LeeE]

The waves you're talking about are made by the wind.  Even the little ones you see on a calm day would have been made by the wind but they may have originated a long way away and even quite a long time ago i.e in terms of hours or even days.

While waves are big and are traveling fast they get slowed down and reduced in size quite quickly because they're having to move a correspondingly large amount of air around as the troughs and crests pass by but as they get smaller and slower they lose less and less energy to this, and so can travel further.

Sea waves created by the wind are not like the point-source waves you'd get by dropping a stone into some water either.  Such a point-source wave will weaken as it gets further away from its origin because the length of the wavefront in increases as the wave circle expands but wind generated waves tend to be more parallel and the length of the wavefront widens relatively slowly in comparison.

Re the best side for surfing: I don't know Hawaii - never been there - but as one of the things it's renowned for is surfing I'd check out some surfing websites and the Hawaiian tourist info sites.

[Storm Steve]

Thank you LeeE for your quick response.

I understand that weather patterns, in general, move from west to east (at least in the northern hemisphere).  If it is true that weather moves west to east, and wind is what causes waves, then we should expect to see larger waves on the east coast of an ocean, on average.

So can we test the wind/wave connection by measuring waves (height, intensity, etc) on both sides of a large body of water?

Steve

[LeeE]

I understand that weather patterns, in general, move from west to east (at least in the northern hemisphere).  If it is true that weather moves west to east, and wind is what causes waves, then we should expect to see larger waves on the east coast of an ocean, on average.

Hmm...   on average over what time period?

Just because weather systems generally move from west to east, it doesn't necessarily mean that the wind will always be coming from that direction.  The wind will blow from regions of high-pressure to regions of low-pressure, so as weather systems pass over and near the islands the wind direction will change according to where the regions of high and low pressure are located at the time.

So can we test the wind/wave connection by measuring waves (height, intensity, etc) on both sides of a large body of water?

Steve

I can't see why you can't test it - I don't think there's a law against it.  I don't think that was really the question you were asking though, which perhaps was 'is there a correlation?  I dunno.  NOAA might have some info about this though, and as it's U.S. gov funded body, like the USGS, all their data may be freely available (I have to applaud the reasoning that as the data was gathered using public funds it should be publically available).

[Storm Steve]

From http://www.hawaii.islands-travel.com/surfing.html [nofollow]

Hawaii surfing waves have two distinct seasons. The biggest hit the north shores of all the islands between November and March, generated from winter storms around Alaska. The first landmass the resulting waves hit to the south are the Hawaii Islands, a distance of over 5000km, by which time the waves can be massive. The lie of the land and ocean floor on Oahu's north shore are particularly favourable to receiving monster waves and endless barrels.

By summer, the waves on the north shores are as flat as glass and unimaginable for surfing. But things are different on the south shores. Tropical storms in the south pacific send waves northwards and reach the exposed south shores of all islands between June and October. These waves are not as intense as the winter waves that hit the north shores...