Ocean waves are caused primarily by wind blowing across the water’s surface. Wind transfers its energy into the water through friction, creating ripples that grow into the swells and breakers you see at the beach. But wind isn’t the only force at work. Gravity from the moon and sun, earthquakes on the ocean floor, and even sudden shifts in air pressure all generate distinct types of waves.
How Wind Creates Most Ocean Waves
The waves you picture when you think of the ocean, the ones that roll toward shore and curl into whitecaps, are almost always made by wind. The process starts simply: as wind moves across calm water, friction between the air and the surface drags the water along slightly, forming tiny ripples called capillary waves. These small disturbances give the wind more surface area to push against, which lets it transfer even more energy into the water. The ripples grow into larger waves, and those larger waves catch still more wind.
Three factors control how big wind waves get:
- Wind speed: Faster wind pushes more energy into the water.
- Fetch: The uninterrupted distance over open water that the wind blows. A wind crossing thousands of miles of open Pacific has far more fetch than one blowing across a small bay.
- Duration: How long the wind blows in a consistent direction. A brief gust produces chop; a storm lasting hours or days builds massive swells.
All three factors work together. A strong wind blowing briefly over a short stretch of water produces small, choppy waves. The same wind blowing steadily across a vast ocean for days produces powerful swells that can travel thousands of miles from the storm that created them, arriving at distant coastlines as smooth, evenly spaced waves.
Gravity, the Moon, and Tidal Waves
Tides are technically very long, slow waves driven by gravity rather than wind. The moon’s gravitational pull tugs on Earth’s oceans, creating a bulge of water on the side of the planet facing the moon and a corresponding bulge on the opposite side. As the Earth rotates through these bulges, coastlines experience the rhythmic rise and fall of high and low tides.
The sun plays a role too, but a smaller one than you might expect. The sun is 27 million times more massive than the moon, yet its tide-generating force is only about half as strong. The reason is distance. Tidal forces weaken with the cube of the distance between objects, not just the square. Because the sun is 390 times farther from Earth than the moon, its tidal pull is reduced by roughly 59 million times compared to what its mass alone would suggest. The moon wins out as the dominant tidal force.
When the sun and moon line up during new and full moons, their gravitational pulls combine to produce especially high and low tides called spring tides. When they pull at right angles during quarter moons, the tides are more moderate.
Tsunamis: Waves From the Seafloor
Tsunamis form when something suddenly displaces a massive volume of ocean water. The most common trigger is an earthquake beneath the seafloor. When tectonic plates shift and the ocean bottom lurches upward or drops, the water column above it is shoved out of place, sending waves radiating outward in every direction. The size of the vertical seafloor motion is the key factor in how powerful the resulting tsunami becomes.
Underwater landslides, volcanic eruptions, and even meteorite impacts can also generate tsunamis. In 1958, a rockslide triggered by an earthquake in Lituya Bay, Alaska, displaced so much water that waves splashed up to an elevation of 1,720 feet on a nearby ridge, taller than the Empire State Building. That event still stands as one of the tallest tsunami waves known to science.
In the open ocean, a tsunami may be only a foot or two tall and virtually undetectable to ships. The wave’s energy is spread across an enormous wavelength, sometimes hundreds of miles from crest to crest. As it approaches shallow coastal waters, the wave slows down and compresses. Its energy has nowhere to go but up, and the wave can surge to devastating heights.
Internal Waves Below the Surface
Not all ocean waves are visible. Internal waves travel beneath the surface along boundaries where water layers of different densities meet. The ocean isn’t a uniform body of water. Temperature and salinity create distinct layers, and where a warm, lighter layer sits above a cold, denser layer, a boundary called a pycnocline forms. Waves can propagate along this hidden boundary much the way surface waves travel along the air-water interface.
Internal waves are commonly generated when tidal currents push stratified water up and down along sloping seafloor features like continental shelves. They tend to be slower and much taller than surface waves, sometimes reaching amplitudes of dozens of meters, though they cause little visible disturbance at the surface. Submarines and deep-sea operations care about internal waves because they can create unexpected currents and shifts in water density at depth.
Rogue Waves and Constructive Interference
Rogue waves are unusually massive waves that appear seemingly out of nowhere in the open ocean, sometimes reaching more than double the height of surrounding seas. For decades, scientists debated what caused them. Research from Georgia Institute of Technology has identified the primary mechanism: constructive interference enhanced by the ocean’s nonlinear behavior.
Ocean waves travel in many directions at once. In rare conditions, waves from different directions arrive nearly in phase, meaning their crests line up at the same point at the same time. This constructive interference can roughly double the height of the resulting wave. On top of that, the ocean’s natural asymmetry between crests and troughs (crests are peaked, troughs are rounded) adds another 15 to 20 percent to the wave’s height. The combination produces a wall of water that can be catastrophic for ships.
Weather-Driven Waves: Meteotsunamis
Rapid changes in atmospheric pressure can also push waves across the ocean. These weather-generated waves, called meteotsunamis, look and behave much like small tsunamis but have no connection to earthquakes or seafloor movement. They’re triggered by squalls, thunderstorms, frontal passages, or atmospheric gravity waves that create abrupt pressure shifts over the water.
The initial disturbance is tiny. A pressure change of less than 5 hectopascals over 10 minutes (roughly the weight of a few centimeters of water) is enough to set one in motion. What makes meteotsunamis dangerous is resonance. When the atmospheric disturbance moves at the same speed as the shallow-water wave it generates, the wave gains energy continuously, much like pushing a swing at exactly the right moment each time. As the wave enters shallower water near the coast, shoaling and the shape of the coastline can amplify it further, turning a few centimeters of open-ocean disturbance into a destructive surge.
How Waves Change Near Shore
Whatever their origin, waves transform as they move from deep water into shallow coastal areas. In deep water, waves travel as smooth, symmetrical shapes. As the water gets shallower, the bottom of the wave begins dragging against the seafloor. This friction slows the base of the wave while the top keeps moving, causing the wave to steepen, lean forward, and eventually break.
This process, called shoaling, also increases wave height. Energy that was spread through a tall column of deep water gets compressed into a shorter column as the depth decreases. The wave’s orbital motion near the seabed becomes asymmetrical, which is why breaking waves push sand and sediment toward shore more effectively than they pull it back. It’s the reason beaches constantly reshape themselves, building up in calm weather and eroding during storms.
Anatomy of a Wave
Regardless of what causes them, all ocean waves share the same basic structure. The highest point is the crest, and the lowest point is the trough. Wave height is the vertical distance from trough to crest. Amplitude is half the wave height, measured from the still-water line to either the crest or trough. Wavelength is the horizontal distance between two successive crests. Wave period is the time it takes for one full wave to pass a fixed point, and wave frequency is the number of waves passing that point in a given time. Long-period swells generated by distant storms carry more energy and travel faster than short-period wind chop, which is why surfers track storms thousands of miles away.