Most ocean waves are caused by wind. Friction transfers energy from moving air to the water surface, first making ripples and then larger waves. The water mostly moves in circular orbits as the energy travels; it does not normally move across an entire ocean with each crest.
Storms, earthquakes, landslides and the gravitational pull of the Moon and Sun can also create waves, but they produce different kinds of motion. Understanding the source helps explain why ordinary surf, tides and tsunamis behave differently.
How wind builds waves
Three main factors determine the size of wind waves: wind speed, how long the wind blows, and fetch—the uninterrupted distance over water. Strong wind blowing for a long time across a large fetch can transfer more energy and create higher, longer waves.
Chaotic waves within the windy area are called sea. Once waves leave the storm, they organise into smoother groups called swell. Long-period swell can travel thousands of kilometres and reach a coast when local winds are calm.
Wave height, wavelength and period
Wave height is the vertical distance from trough to crest. Wavelength is the horizontal distance between successive crests, and period is the time between them. Long-period waves generally carry more energy and begin interacting with the seabed in deeper water than short-period waves.
Wave forecasts consider all these measures. Height alone cannot describe how powerful or hazardous conditions will feel at a particular shore.
Why waves break near shore
In deep water, wave energy travels with little interaction with the bottom. As a wave enters shallower water, friction slows its lower part. The wavelength shortens and the crest grows steeper until it becomes unstable and breaks.
The slope and shape of the seabed determine whether a wave spills gradually, plunges into a curling breaker or surges up a steep shore. Reefs, headlands and bays refract and focus energy, so conditions can differ sharply along a short stretch of coast.
Other kinds of ocean waves
- Tides are very long-period waves driven mainly by the Moon and Sun.
- Tsunamis result from rapid displacement of a large water volume, usually during an undersea earthquake.
- Storm surge is an abnormal rise in water pushed toward shore by a storm’s winds and low pressure.
- Internal waves move along boundaries between water layers of different density.
A tsunami is not a “tidal wave.” It has no connection to the daily tide-generating force. The restored UNESCO Tsunami Ready article shows why that distinction matters for coastal education.
Do waves move water forward?
A floating object in deep water usually rises, falls and moves in a small loop as a wave passes. There can be a modest net drift, and breaking waves do move water toward shore. That water must return seaward, sometimes concentrating into dangerous rip currents.
Frequently asked questions
Can waves form without wind?
Yes. Earthquakes, landslides, volcanic activity, tides and changes in atmospheric pressure can all generate waves.
Why are some beaches wave-free?
They may be sheltered from the wind and swell by land, reefs or harbour structures, or the current weather system may not be sending energy toward that coast.