A Straight Dope Classic from Cecil's Storehouse of Human Knowledge

How can waves be crashing on all sides of an island at once?

March 31, 2006

Dear Cecil:

This is going to take a while to explain, so bear with me. My kid is a fan of the Age of Empires series of computer games, which give you a bird's-eye view of the landscape on which your armies cavort. You can see an amazing amount of detail, but a while back I noticed something I found comical: whenever you come across an A-of-E island, you see waves crashing on the beach on all sides. Damn computer geeks who wrote this need to get out more, I thought--don't they know waves are driven by the wind, and the wind can't blow in all directions at once? I felt smug till we visited the Hawaiian island of Kauai over Christmas. Our cottage was on the south side of the island--we were lulled to sleep by the waves crashing on the beach. We visited the famous Na Pali coast on the north side of the island, and were impressed but a little puzzled (I was, anyway) by the massive waves crashing on the beach. Finally we took a helicopter tour, and you know what? On every side of the island there were waves crashing on the beach. I'm stumped. OK, the waves on the north were bigger than those on the south--supposedly this is due to winter storms. But shouldn't the waters on the lee side of an island be calm?

Cecil replies:

Obviously not, but don't let that get your hopes up, pops. Just because a computer game faithfully reflects reality once in a while doesn't mean you're about to meet Lara Croft.

Yes, waves crash on all sides of an island, and no, that doesn't require the wind to blow in all directions at once. (Well, from all directions at once.) In the big-picture sense you're right that the chief force driving waves is the wind. However, the ocean is a capacious place with a lot of water in it, and once set in motion a sequence of waves, or wave train (actually, in midocean it's more of a deep-swell train), isn't an easy thing to divert. So while waves move in the direction of the prevailing winds as a general proposition, what the wind is doing right this instant is largely immaterial. Instead, how a wave behaves on encountering an island is mainly a function of its interaction with the surrounding sea floor.

No doubt you can find more elegant explanations, but the following homely metaphor should get the matter across well enough. Imagine two roller skaters holding a beach towel between them as they skate due south toward a flagpole. They pass on either side but the middle of the towel snags on the pole. The skaters are swung in a circle with the pole at its center (we'll assume they're sufficiently nimble to avoid crashing into each other), while the towel wraps tightly around the pole. A bug on the south side of the pole looks up to see incoming terry cloth and thinks: Gosh, that looks like a towel heading north at high SPLAT.

You get the picture. The towel and the pole represent a wave striking an island. Initially the wave is heading uniformly south, but as it brushes the sloping sea bottom surrounding the island, friction causes it to slow and bend (or refract, I guess we should say) and in effect wrap around the obstructing landform. The practical result is that though the waves strike most forcefully on the north side of the island, they crash on the beach even on the south side, in all cases (barring topographical eccentricities) looking essentially like they're coming from straight offshore. Luckily for us traditionalists, this life-imitates-Age-of-Empires thing goes only so far. Who really wants human reproduction to be a matter of simply selecting a town center and pressing V?

QUESTIONS WE'RE STILL THINKING ABOUT

Don't ask how this came up, please. Which weighs more: a cubic foot of unsalted butter, or a cubic foot of an adult blue whale? --JKP, via e-mail

When a deaf person is also schizophrenic, do they hear people talking to them, or do they just imagine people signing to them? --Zack, via e-mail

Did atheists first come from Athens?--Nighthawk5000, via e-mail

How much deeper would the ocean be without sponges? --Justin Lefto, via e-mail

Way back in the late 1970s there was some guy in Italy that was going to eat a whole car, piece by piece. Any idea what happened to him? --grg2, via e-mail

Personally I haven't the vaguest. However, according to my assistant Bibliophage, who impresses me more each day, he finished oily.

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