The idea that waves are inherently larger during high tide is a widespread misconception. Tides and the wind-generated waves observed crashing onto the shore are two distinct physical phenomena. While high or low tide does not determine the initial size or energy of an incoming wave, the change in water level significantly alters how that wave behaves as it approaches the coastline. High tide modifies the local conditions waves encounter, changing their shape and where they break, which can lead to the perception of a difference in wave size.
Understanding the Independent Forces
The forces that create tides and the forces that create typical surface waves operate independently. Tides are long-period waves with wavelengths spanning thousands of kilometers, resulting from the gravitational pull of the Moon and the Sun on Earth’s oceans. This astronomical influence causes the predictable rise and fall of the global sea level over periods of approximately 12.4 or 24.8 hours.
Wind-generated waves, in contrast, are short-period waves, typically lasting only seconds or minutes. These waves originate when wind transfers energy to the water surface through friction, creating ripples that grow into larger wave forms. Because their formation depends entirely on meteorological conditions, not celestial mechanics, their energy and size are separate from the tidal cycle. The energy content of a wave in deep water is set long before it encounters the coast.
How Tides Influence Nearshore Wave Dynamics
Although tides do not generate the wave energy, the resulting change in water depth profoundly affects how waves transform in nearshore environments. As a wave approaches the coast and the seafloor begins to restrict its movement, a process known as shoaling occurs. During shoaling, the wave’s base slows down due to friction with the seabed, causing the wave energy to compress vertically and the wave height to increase.
At high tide, the increased water depth means the wave travels further over the submerged shore before seabed friction significantly impacts it. This greater depth allows the wave to maintain its form and energy for a longer distance, often resulting in a more gradual, “softer” break closer to the beach. Conversely, low tide brings the seabed closer to the surface, causing waves to encounter shallow water sooner. The rapid shoaling forces the wave to steepen and break further out from the beach, sometimes appearing more powerful or “hollow.”
This interaction of depth and wave energy can sometimes lead to an actual increase in wave height, particularly for longer-period waves, when tidal currents flow in the same direction as the wave propagation. Studies have shown that this interaction can modify the significant wave height by as much as 25 percent in certain areas. However, this effect is localized and depends on the specific bathymetry, or underwater topography, and the direction of the tidal current relative to the incoming swell. The visual impression of waves being bigger or smaller is largely due to the change in the breaking point and the shape of the wave face, not an overall increase in the wave’s initial energy.
Factors That Actually Determine Wave Size
The true size of a wave, meaning its overall energy and height in the open ocean, is dictated by three primary meteorological factors: wind speed, duration, and fetch.
Wind speed is the first factor, as faster wind transfers more energy to the water surface. Duration is the length of time the wind blows consistently over the water. The third factor is the fetch, the uninterrupted distance over open water that the wind blows in a single direction. Large waves require the combination of strong winds, blowing for a long duration, over an extensive fetch. If any one of these three elements is limited, the wave size will be restricted.
Oceanographers also consider the wave period, the time interval between successive wave crests, as a major indicator of a wave’s power and origin. Waves generated by distant storms, known as swell, have longer periods and carry their energy across vast distances. These long-period swells often result in the largest and most organized waves at the coast, independent of the local tidal phase.