What causes waves? The short answer
Most ocean waves are caused by wind. As wind blows across the sea, friction between the moving air and the water pushes up tiny ripples.14 Those ripples make the surface rough, which gives the wind something to grip, so it pushes harder and the ripples grow into waves.5 How big they get depends on three things: how fast the wind blows, how long it blows, and how far it blows over open water (the fetch).3
The surprising part: a wave is energy travelling through the water, not water travelling across the sea. The water itself mostly goes round in circles and stays where it is.12
How are waves formed? Step by step
- Wind meets calm water. Air pushes against the smooth surface and friction raises tiny ridges and ripples.4
- Ripples catch more wind. A rougher surface is easier for the wind to grip, so the ripples grow into waves.5
- Waves keep growing as long as the wind moves faster than the wave crests, because only then does the wind keep handing over energy.5
- Growth stops when the wind's extra energy is lost as fast as it arrives, mostly in breaking whitecaps. Scientists call this a "fully developed sea".45
- Waves leave as swell. Once they roll out of the windy area they turn into smooth, regular swell that can travel thousands of miles.45
- They break at the coast. In shallow water the seafloor slows the bottom of the wave, it gets taller and steeper, and it topples over.26
Go back far enough and waves are powered by the Sun: sunlight heats Earth unevenly, and that uneven heating is what makes the wind blow.6
What decides how big waves get?
A strong wind on its own isn't enough. NOAA puts it simply: if the wind is slow, you only get small waves, however long it blows. A strong wind that lasts just a few minutes won't build big waves either. And a strong, long-lasting wind blowing across a short stretch of water still can't. Large waves only form when all three factors combine.3
Here's a real example from the U.S. National Weather Service: a steady wind of 30 knots (about 55 km/h) blowing for 24 hours over 340 miles (about 550 km) of sea gives waves averaging about 3 metres, with the bigger third of them averaging about 5 metres. About one wave in ten is higher than that.5 This is also why storms in the huge, open Pacific can make bigger waves than storms of the same size in smaller oceans: the wind has more fetch.5
The "Build a wave" chapter above lets you try this yourself. Its heights come from a simplified version of the curves forecasters use, tuned to match that example, so treat them as rough guides.
Waves move energy, not water
If waves carried water, a floating ball would be swept to the beach. It isn't. As a crest passes, the ball rises and moves forward. In the trough it sinks and slides back. It traces a circle and ends up almost where it started.26 OpenStax Physics describes the same thing with a cork: the water mostly moves it up and down. If you feel pushed in the sea, what you're feeling is the wave's energy.7
The water below the surface moves in circles too, but they get smaller the deeper you go. At about half a wavelength down, the motion has almost disappeared.2
The parts of a wave
- Crest: the highest point. Trough: the lowest point.4
- Wave height: the vertical distance from trough to crest.4
- Wavelength: the distance from one crest to the next.4
- Period: the time it takes one whole wave to pass a point. Wind waves are usually about 5 to 20 seconds apart.49
What is a swell?
Out in a storm the sea looks messy: short, steep, choppy waves of all sizes. Oceanographers call that a sea. Once waves travel away from the wind that made them, they become swell: smooth, rounded waves with long periods.4
Longer waves travel faster, so as swell spreads out it sorts itself, with the longest waves arriving first.45 Short waves lose their energy quickly, while long-period swell (about 8 to 20 seconds) can travel huge distances.8 Swell can cross thousands of miles almost unchanged, and waves can travel across an entire ocean basin.15 Hawaii's famous winter surf, from November to March, is swell sent out by big winter storms in the North Pacific.5
Why do waves break?
A wave doesn't break because it hits the beach. It breaks before that. As it moves into shallow water, the seafloor starts to drag on the circling water at the bottom of the wave. The bottom slows down, the water behind piles up, and the crest is forced higher.26 Waves can grow to about one and a half times their deep-water height as they run ashore.5
Eventually the wave is too steep to hold its shape. In shallow water that happens when the wave's height reaches roughly 0.8 times the water depth. The crest tips forward and the wave breaks into surf.4 That's why big waves break farther out, where the water is deeper, and small waves break close to the sand. Out in deep water, waves can also break on their own as whitecaps when they get steeper than about 1 in 7 (height compared with wavelength).8
Types of ocean waves
Sources for the table: tsunami.gov (wind waves and tsunamis),9 NOAA (tides and storm surge).1 Tides really are waves: huge, slow ones pulled along by the Moon and the Sun. See what causes tides. Tsunamis are often called "tidal waves", but that's wrong: they have nothing to do with tides.1 See what causes a tsunami. Wind itself comes from uneven heating, which you can explore in what causes wind.
Now and then sailors meet a rogue wave: one more than twice as tall as the waves around it. They often form when swells travelling at different speeds and directions line up and add together for a short time.10
Rip currents: the wave danger at the beach
Breaking waves push water toward the shore, and it has to get back out. Where waves break less, for example at a gap between sandbars or next to a pier, the water can rush seaward in a narrow, fast channel called a rip current.1112 Rips are usually 50 to 300 feet (15 to 90 m) wide and typically flow at 1 to 2 feet per second, but they can reach 8 feet per second, faster than an Olympic swimmer.1213 In the U.S. about 100 people die in rip currents each year, and lifeguards rescue tens of thousands more.13
If a rip current catches you: don't fight it by swimming straight back. Stay calm and float, wave and call for help, or swim
parallel to the shore until you're out of the current, then swim back to land at an angle.
13 Better still, swim at beaches with lifeguards, and check the beach forecast. Rips often form on calm, sunny days.
14 ("Rip tide" is the wrong name: rip currents aren't caused by tides.
12)
Common myths about waves
"Waves carry water across the ocean." They carry energy. The water mostly goes round in circles and stays put.
12
"A strong wind is all you need for big waves." You need wind speed, duration and fetch together. Miss one and the waves stay small.
3
"Waves break because they crash into the beach." They break in shallow water before the beach, when the seafloor slows their base and the crest topples.
26
"A tsunami is a tidal wave." Tsunamis come from earthquakes, landslides and eruptions, not tides.
1
"Rip tides pull you under." Rip currents flow along the surface, away from shore, and they aren't tides. Don't fight it: float and wave for help, or swim parallel to the beach to escape.
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FAQ
What is the cause of most waves?
Wind. As wind blows across the sea, friction between the air and the water pushes up ripples. The ripples give the wind more to grip, so they grow into waves. Tides and tsunamis are waves too, but they have different causes.
How are waves formed, step by step?
1) Wind blows over calm water and friction raises tiny ripples. 2) The rough surface lets the wind push harder, so ripples grow into waves. 3) Waves keep growing while the wind blows faster than they travel. 4) They leave the windy area as smooth swell. 5) Near the coast the seafloor slows them, they get taller and steeper, and they break.
Where do waves start in the ocean?
Wherever the wind blows over open water, and big waves usually start in storms far out at sea. The waves you see on a calm, sunny beach may have been made by a storm thousands of kilometres away and travelled to you as swell.
Do waves ever stop?
A single wave loses energy as it travels and finally gives it up when it breaks on a shore. But the sea is almost never flat, because somewhere the wind is always making new waves, and long swell can travel across whole ocean basins.
Does the water move with the wave?
Hardly. The energy moves forward but the water mostly goes round in circles. A floating ball rises and moves forward on a crest, then sinks and slides back in the trough, ending up almost where it started. Only when a wave breaks does water really rush up the beach.
Why do waves break?
In shallow water the seafloor drags on the bottom of the wave and slows it down. The water behind piles up, so the wave gets taller and steeper. When its height is roughly as big as the water depth (about 0.8 times the depth), the crest topples forward and the wave breaks.
What is the difference between a wave, a tide and a tsunami?
Wind waves are a few seconds apart (about 5 to 20). Tides are huge, slow waves caused by the pull of the Moon and Sun, with high tides usually about 12 hours apart. Tsunamis are caused by undersea earthquakes, landslides or eruptions, with waves minutes apart and up to hundreds of kilometres long.
Sources
- NOAA Ocean Service, Why does the ocean have waves?
- NOAA Ocean Exploration, What causes ocean waves?
- NOAA Ocean Service, Currents tutorial: Coastal currents (wind and waves)
- Scripps Institution of Oceanography, CDIP: Wave measurement
- NOAA National Weather Service JetStream, Wind, swell and rogue waves
- Woods Hole Oceanographic Institution, What causes ocean waves?
- OpenStax Physics, 13.1 Types of waves
- Encyclopaedia Britannica, Wave (water)
- NOAA/NWS U.S. Tsunami Warning System, Tsunami FAQ
- NOAA Ocean Service, What are rogue waves?
- NOAA National Weather Service, Rip current science
- NOAA SciJinks, How do I escape rip currents?
- NOAA Ocean Service, What is a rip current?
- NOAA National Weather Service, Rip current safety