An interactive field guide to the sea
Why? Where do waves actually come from — and why do they keep rolling in, even while the tide goes out? Play with the wind, earthquakes, the sea floor, the Moon and the Sun, and see for yourself.
This sea is 48 waves added together, running live. The pattern you're looking at has never existed before.
Begin ↓1
Look at any patch of sea and you're not seeing a wave. You're seeing dozens of them — long and short, fast and slow, from slightly different directions — all passing through the same water at the same time, and adding up.
Where crest meets crest, the sea leaps up. Where crest meets trough, they cancel and the water goes briefly flat. Physicists call this superposition. Surfers call it sets.
Drag the slider and add waves one by one. Even in an imaginary, perfectly tidy sea, the wait for a repeat explodes. In the real one, it never comes.
2
Almost every wave you will ever see was made by wind. Air dragging over water snags on tiny ripples, shoves at their backs, and hands them energy. Ripples become wavelets, wavelets become waves, waves become swells.
How big they get depends on three things:
The weakest ingredient sets the limit. A gale over a lake only makes chop — it runs out of water. A short squall over the ocean runs out of time. Only a strong wind, blowing for days, over thousands of kilometres of sea, builds the giants.
Try a , a , or a .
3
Here's the strangest thing about a wave: the water hardly goes anywhere.
A wave is energy passing through water — like a stadium wave passing through a crowd. Nobody leaves their seat. Each bit of water just rolls around a small loop and comes back to almost where it started. Watch the duck: it bobs in circles while the crests race past.
The loops shrink fast with depth. Half a wavelength down, the water barely moves at all — which is why submarines simply dive to ride out a storm.
And because waves carry energy rather than water, they can cross whole oceans. Which brings us to the waves that arrive on a perfectly calm day…
Switch to shallow water and watch the loops squash into ellipses: the wave has started to feel the bottom. Remember that — it's what makes waves break.
4
So why are there waves on a windless morning? Because they were made somewhere else, days ago.
Once waves leave the storm that made them, they sort themselves out. Long waves travel faster than short ones, so they outrun the rest and arrive first: smooth, evenly spaced lines called swell. The short, choppy waves lag behind and slowly fade.
This is why there are always waves at the coast. Somewhere on the planet a storm is blowing, and its swell is crossing the ocean toward you — whatever the weather is doing where you stand.
Press play. Then move the storm further away and watch the swell spread out — the longest waves arrive first.
5
Not every wave is made by wind. When a big undersea earthquake strikes, a slab of seafloor the size of a country can jump up several metres in seconds — and lift the entire ocean above it, all four kilometres of it. Gravity pulls that hump back down, and a tsunami spreads out in both directions.
Because it moves the whole water column, a tsunami is a completely different beast from a wind wave. Out at sea it's hundreds of kilometres long but only a metre or two high near the quake — and less than a metre once it spreads across the ocean. It travels at jet-airliner speed, yet its slope is so gentle that a ship would never notice it pass.
Then it reaches shallow water. Just like swell on a beach, it slows down and the energy piles up — but now it's a hundred kilometres of water piling up. It arrives not as a curling breaker but as a flood that keeps rising for minutes, and then comes again, and again.
Often the sea pulls back first, emptying bays and harbours. That's the trough arriving ahead of the crest — and the most important thing to know about tsunamis: if the sea suddenly retreats, go to high ground.
Pick a magnitude and trigger the earthquake. Watch the ship far out at sea, then the coast — and the little sea-level gauge in the corner.
6
Waves are fast — a crest every few seconds. The tide is something else: the whole sea slowly rising and falling, about twice a day. It isn't made by wind. It's made by gravity.
The Moon pulls the near side of Earth a little harder than the centre, and the centre a little harder than the far side. That difference stretches the oceans into an egg shape: one bulge facing the Moon, and a second one on the opposite side.
Earth spins underneath both bulges once a day — so most coasts get two high tides and two low tides a day. The Moon moves along its orbit meanwhile, so the tides come about 50 minutes later each day.
The Sun does the same thing at about half the strength. When Sun and Moon line up — at new and full moon — their bulges add up: spring tides (nothing to do with the season). At half moon they pull at right angles and partly cancel: neap tides.
Scrub through a month. Switch off the Moon, then the Sun. Turn on the stretch to see the forces that make the bulges.
7
Now bring it ashore. As swell runs into shallow water it starts to feel the bottom. The front slows down, the waves behind catch up, the wavelength shortens — and because the energy has nowhere else to go, the waves grow taller.
A wave can only stand about as tall as the water is deep. When it reaches roughly 0.8 × the depth, the crest outruns its base and it breaks. Gentle slopes make long, crumbling spilling waves; steeper ones make hollow, curling plungers; very steep ones just surge up the shore.
Now let the tide go out. The waves keep coming. The tide only moves the stage — the place where waves break slides up and down the beach — while the swell, made by winds far away, keeps arriving on its own clock.
Press Run the tide and watch the surf zone wander across the beach while the waves keep rolling in. Try the sandbar and the reef.
8
Two forces pull a disturbed sea back to flat. For big waves it's gravity. For the tiniest ripples — shorter than about 1.7 cm — it's surface tension, the "skin" of the water.
Those capillary ripples are where every wave starts. Glassy water gives the wind nothing to grip. The first gust raises a shimmer of ripples — sailors call them cat's paws — and those give the next gust something to push on. Energy then flows to longer and longer waves, all the way up to ocean swell.
Notice the curious dip: there is a slowest possible wave, about 23 cm/s at 1.7 cm long. Anything shorter is driven faster by surface tension; anything longer, by gravity.
Slide across ten powers of ten — from a 1 mm ripple, past a tsunami, all the way to the tide itself.
9
Every wave you've watched was a crowd of waves, carrying energy from a wind that may have blown days ago, a world away — riding on a sea lifted and lowered by the Moon. The next one will be different. They always are.
Built with linear wave theory, JONSWAP wind-wave growth and the equilibrium tide — simplified to be felt, accurate enough to teach. How it's modelled.