What if waves had no wind?

At the beach, waves splash and crash.

The wind helps make them. What if there were no wind at all?

At the beach, waves roll in and crash, one after another. On a windy day they're rough and choppy. On a calm day they can be small and gentle.

So here is a thought experiment: what if the wind stopped blowing everywhere on Earth? Would the sea go perfectly flat, like a mirror? You can find out how wind makes waves, how some waves travel far from where they were made, and why the tide keeps moving even without wind.

Several different processes move the sea surface: local wind, swell from distant weather, tides and, rarely, sudden disturbances of the seafloor. On an ordinary day, wind is the most common cause of the waves we see.

This thought experiment sets the wind to zero everywhere. Its value is in separating causes: which motions of the ocean would stop, which would continue for a while, and which have nothing to do with wind at all.

Wind blows over the water and makes little ripples.

If it keeps blowing, the ripples grow into waves!

NOAA, the U.S. ocean agency, says most ocean waves are made by wind. As wind rubs across the water, it ruffles the surface into ripples, and if it keeps blowing, the ripples grow into waves.

The University of Hawaiʻi explains that three things make wind waves bigger: faster wind, wind that blows for a long time, and wind that blows across a long stretch of water. That's why a pond only gets ripples while a stormy ocean can build huge waves.

Wind-driven surface waves are created by friction between wind and water (NOAA). Over calm water, wind first raises ripples, which grow into waves that group into sets (University of Hawaiʻi).

Wave size depends on wind speed, duration and fetch, the distance over which the wind blows. For example, 25-knot winds blowing for 60 hours across 510 nautical miles of sea can raise a significant wave height of 4.3 m with an 8.2-second period. Waves transmit energy, not water: the water mostly moves in circles as the wave passes.

Let's pretend the wind stops everywhere.

No new waves are made. Slowly, the sea gets calmer and calmer. But the tide still comes in and goes out!

Imagine the wind stopping all over the world. No new wind waves would form, so the choppy, splashing waves near the shore would die down.

But the sea wouldn't go flat right away. Swells, waves that have left the storm that made them, could keep arriving from far away for a while. And the tide would keep rising and falling, because it's pulled by the Moon and the Sun. These are ideas about what might happen in our pretend world.

With wind at zero, no new wind sea would form, and existing short waves would decay as they broke and lost energy. Swell already in transit, which needs no wind to propagate, might continue to arrive at coasts until it had dispersed or broken.

Tides would be unaffected in their cause: they are very long-period waves forced by the gravitational pull of the Moon and Sun (NOAA). The thought experiment ignores how stopping the wind would change ocean currents and weather, which is far beyond its scope.

A wave is energy moving through the water.

Wind gives the water a push. No push, no new waves!

A wave is energy traveling through water. NOAA explains that the water itself doesn't travel along with the wave; it mostly moves round in a circle as the wave passes, like a floating ball bobbing up and down.

Wind is the push that puts energy into most waves. Without that push, new wind waves can't form. But once a wave has energy, it can keep traveling. Swells can cross whole oceans long after the wind that made them has stopped.

Waves are energy passing through water, which moves in near-circular orbits; a buoy rises, moves forward, falls and returns near its starting point (University of Hawaiʻi). Wind transfers energy to the surface through friction, so with no wind there is no new input to wind waves.

Swells are waves that have gained enough energy to travel beyond their fetch without further wind (University of Hawaiʻi). Their energy is lost mainly when they break, so they can outlast the wind that made them.

The tide is a very long, slow wave. The Moon and the Sun pull on the sea to make it.

So even with no wind, the tide would still go in and out.

Not every wave comes from wind. NOAA says tides are very long, slow waves that move through the ocean because of the pull of the Moon and the Sun. When the top of a tide wave reaches a beach, it's high tide; when the bottom arrives, it's low tide.

NOAA's tide station in San Francisco has been measuring tides for more than 150 years. Rare, very long waves can also be started by earthquakes or landslides under the sea, but those have nothing to do with wind either.

NOAA: tides are very long-period waves that move through the ocean in response to the forces of the Moon and Sun, appearing at coasts as the regular rise and fall of the sea surface. Isaac Newton explained this in 1687; because tidal force falls with the cube of distance, the Sun's tide-generating force is only about half the Moon's.

Other non-wind waves include storm surge from hurricane winds and pressure, and tsunamis from earthquakes, landslides or volcanic eruptions that displace water. A tsunami is not a tidal wave: its cause is unrelated to tides.