April 18, 2013

Where the waves get started

WAVES ARE FASCINATING. If you’re out on the water, they’re all around you; waves everywhere. But how do they start?  What kick-starts a wave into motion?

Well, if you blow on a saucer of water, your breath will make a little dent. If you keep blowing, the pressure of your breath on the back of the dent will move it forward. It looks as if the molecules of water are trying to get away from you. But that’s deceptive.  In fact, the water in a wave just moves up and down, or in a slight circular motion. The wave you have created is in fact simply moving energy through the water, just as you can make loops snake through a length of rope. The rope itself doesn’t move forward.

Waves are visible energy, but they don’t move water forward either — at least not until they reach shallow water near shore and trip over their own feet. That having been said, a minimal amount of water moves forward when a whitecap breaks out at sea.

Out there, it’s the wind that dents the surface of the water, and the wind, as we know, is caused by unequal heating of the earth’s atmosphere by the sun’s rays; so we can safely say that waves are actually caused by radiation from the sun.

That still doesn’t explain how a little dent in the surface tension of a sheet of water can grow into a wave large enough to sink big ships. What’s going on here? How do little waves become big waves, and then swells?

Three phenomena make waves grow:  1. Wind speed. The harder the wind blows, the bigger the wave it forms. 2.  Wind duration. The longer the wind blows in the same direction, the larger the waves become. Roughly speaking, the biggest waves form after the wind has been blowing for the number of hours equalling the wind speed in knots. For example, a 20-knot wind needs to blow for 20 hours to form the biggest waves it’s capable of. And the maximum height of a wave in feet is roughly one half of the wind speed in knots. 3. Fetch. For waves to grow to their maximum size, a fetch of at least 600 miles is needed. (A fetch is a stretch of deep water with no intervening land masses.)

It seems to me that waves also grow bigger by eating other waves. Big ones bite and swallow little ones, but, as you’ll know if you’ve ridden out a gale at sea, the little ones don’t disappear. They ride along on the backs of the bigger ones.

It reminds me of what the Victorian-era mathematician Augustus De Morgan once wrote:
“Great fleas have little fleas upon their backs to bite 'em,
“And little fleas have lesser fleas, and so ad infinitum.
“And great fleas themselves, in turn, have greater fleas to go on,
“While these again have greater still, and greater still, and so on."

­ Eventually the wind will die down, but the energy in the waves will carry on. The waves flatten out, round off, and turn into what we call swells; and swells can travel for thousands of miles from the areas where storms created them. Thus, what started off as a tiny ripple in a calm sea can turn into a pack of energy capable of upsetting stomachs on ocean liners and cruise ships far, far away.

Incidentally, if you have any questions about waves, the University of California at Santa Cruz (UCSC) has some answers for you:  

Today’s Thought
The waves came shining up the sands,
As here today they shine;
And in my pre-pelasgian hands
The sand was warm and fine.

— Frances Cornford, Preëxistence

There was a young lady of Natchez
Whose garments were always in patchez.
When comment arose
On the state of her clothes
She drawled: “Where Ah itchez Ah scratchez.”

(Drop by every Monday, Wednesday, Friday for a new Mainly about Boats column.)

1 comment:

san blas said...

If one has experience he definitely enjoys with sailing but with trick is to watch the waves as they approach and make your maneuver on those that threaten to induce a slam, particularly the large, steep waves.