Where do beach waves come from?

The full narration of the video.

At the beach, waves roll onto the sand one after another, and they never stop. Where do they actually come from? Let's start with the wave at your feet, and trace it backward. First, something odd: some days there's no wind at all on the beach, and the waves keep coming anyway.

So whatever is pushing them, it isn't the breeze around you. Drop a little rubber duck on the sea. A wave comes: the duck rises and drifts forward a bit. The wave passes, and it slides back, almost exactly where it started. The water under the duck does the same thing, just tracing little circles in place. The deeper you go, the smaller the circles.

The wave travels, but the water stays behind. What travels is energy. It's like a stadium wave: each person just stands up and sits down, yet the wave goes all the way around. So where does that energy come from? Keep tracing it back, far out to sea. There's a storm out there. Wind sweeps over the flat sea and first raises tiny wrinkles.

Once the wrinkles bulge up, the wind has something to push on. One push, and they become small waves. Keep pushing, and small waves grow into big ones. How big they get depends on three things: how strong the wind is, how long it blows, and how far it blows across open water. The storm dies down, but the waves don't. They leave the storm behind, line up into long, neat rows, and head off across the ocean.

Scientists have actually followed them: waves from a storm near Antarctica traveled fourteen thousand kilometers. They reached Alaska, the fastest in just twelve days. So the wave at your feet may have come from a storm days ago, thousands of kilometers away. That's how you get waves with no wind. One last step: when a wave reaches the shore, why does it tip over and crash down?

Remember those little circles under the water? Closer to shore the sea gets shallow, and the circles start scraping the bottom. The bottom of the wave gets dragged and slows down. The waves behind keep catching up and bunching together, so the wave piles up taller and steeper. Nothing is holding back the top, so it keeps rushing forward while the bottom lags. Roughly when the water is only a little deeper than the wave is tall, the top pitches forward and tumbles over.

All the energy it carried across the ocean spills onto the sand: splash! Run it forward once more: far-off wind raises waves. The waves carry energy across the ocean while the water just circles in place. At the shallows they trip, and tumble into the white surf you see. When wind sweeps a wheat field, waves ripple across it, yet every stalk stays rooted. Same thing. Where else have you seen a wave travel while the stuff stays put?