A wave is fundamentally a mechanism for transporting energy through a medium, not the movement of the medium itself. When a wave passes across the ocean, the energy travels horizontally, while the water molecules largely remain in place. These molecules perform small orbital or circular movements as the energy passes through them. The disturbance is propelled forward. Most waves visible on the surface begin as tiny, almost imperceptible disturbances.
Energy Transfer: From Wind to Water
The most common way a surface wave begins is through the friction between air and water. When a gentle breeze skims across a calm surface, the initial friction creates minute wrinkles known as capillary waves. These initial ripples are very short, typically less than 1.7 centimeters in wavelength, and their shape is primarily maintained by surface tension, the cohesive force between water molecules.
As the wind continues to blow and transfers more energy, the ripples grow in height and their wavelength increases. The transition occurs when the wavelength exceeds approximately 1.7 centimeters, at which point the force of gravity pulling the water down becomes stronger than the surface tension. At this threshold, the waves are redefined as gravity waves.
For continuous energy transfer to occur, the wind speed must be greater than the speed of the wave crest itself. This difference in velocity allows the air pressure on the upwind side of the wave to be slightly greater than the pressure on the downwind side. This process allows the initial ripple to accumulate energy and develop into larger surface waves. The waves will continue to grow as long as the wind feeds more energy into them than is lost through dissipation or breaking.
Elements Determining Wave Size
While a wave starts with the first breath of wind, it only grows when specific environmental conditions align to facilitate its growth. The ultimate size a wind-generated wave can achieve is dictated by three interconnected variables: wind speed, duration, and fetch. These factors determine the total amount of energy that can be transferred from the atmosphere to the ocean’s surface.
Wind speed is the velocity of the air moving over the water, and a stronger wind provides a much greater force for generating and sustaining larger waves. The duration refers to the length of time the wind has been blowing in a relatively consistent direction. Even a very strong wind will not produce large waves if it only blows for a few minutes.
Fetch is the uninterrupted distance over open water that the wind can blow without its direction changing substantially. A short fetch, such as in a small lake, limits the space available for a wave to accumulate energy, regardless of how strong or prolonged the wind is. Large, powerful ocean swells form only when high wind speeds blow over a vast fetch for an extended duration, allowing the waves to reach a state known as a fully developed sea.
Non-Wind Initiated Waves
Not all waves are created through the gradual transfer of energy from the wind; some of the most powerful waves begin instantaneously due to mechanical or astronomical forces. Tsunamis, for instance, are initiated by the sudden, massive displacement of a large volume of water. The most common cause is a large-magnitude, shallow earthquake occurring beneath the ocean floor, typically in subduction zones.
When one tectonic plate abruptly slips beneath another, the overlying seafloor experiences a rapid vertical movement, either rising or falling. This mechanical shift instantaneously displaces the entire water column above it, creating a dome or trough on the ocean surface. Gravity attempts to restore equilibrium, triggering long-wavelength tsunami waves that propagate rapidly across the ocean basin. Other events like underwater landslides, volcanic eruptions, or large meteorite impacts can also cause this sudden water displacement.
Tidal waves, by contrast, are initiated by the gravitational forces exerted by the Moon and the Sun acting upon the Earth’s oceans. These are long-period waves that affect the entire mass of the ocean simultaneously. The Moon’s influence is about twice that of the Sun, despite its smaller size, because the tidal force varies inversely with the cube of the distance, making the Moon’s proximity more impactful.
These gravitational forces create bulges of water on both the side of Earth facing the Moon and the side directly opposite it. As the Earth rotates beneath these bulges, the rise and fall of the sea level creates the rhythmic, predictable phenomenon of tides. The initiation of a tidal wave is a continuous, astronomical event, unlike the localized, mechanical, or frictional forces that drive other wave types.