5 Waves and Tides
Learn how wind generates ocean waves, how waves reshape coasts, and how lunar and solar forces, together with local geography, shape tides and tidal currents.
How wind generates waves
Most ocean surface waves form when wind transfers energy to the water. Wave size depends chiefly on wind speed, how long the wind blows, and —the distance the wind travels over open water in one direction. Stronger winds, longer-lasting winds, and greater generally produce larger waves.
Waves are described by their height, measured from trough to crest; their period, the time between successive crests passing a point; and their direction. A wave carries energy forward, but in ordinary surface waves the water particles mostly move in small circular or elliptical paths rather than traveling forward with the crest.
Waves that travel away from the area where the wind created them are called . They can cross long distances and arrive after local winds have weakened.
Wave changes and coastal effects
As waves enter shallower water, their interaction with the seafloor slows them and changes their shape. Their wavelength shortens, and wave crests often bend to align more nearly with the shoreline; this change in direction is called . As waves steepen and become unstable, they break and release energy in the surf zone.
Breaking waves move water and sediment. Waves approaching at an angle can drive a parallel to the shore. This current transports sand along the coast through .
Wave action can build or erode beaches and reshape shorelines. Its effects vary with wave conditions, sediment supply, and coastal form.
Tides and tidal currents
Tides are regular, long-period changes in sea level produced mainly by the Moon’s gravitational pull, with a smaller contribution from the Sun. Their actual timing and height vary from place to place: ocean-basin shape, coastlines, seafloor depth, and friction affect how tidal water moves.
As water rises toward high , the incoming flow is the . As water falls toward low , the outgoing flow is the . The interval when the turns between rising and falling is . Tidal heights are measured relative to a reference level called a tidal datum.
Daily tidal patterns
A lunar day is about 24 hours and 50 minutes. Where the local pattern is semidiurnal, successive high tides are commonly about 12 hours and 25 minutes apart.
Three broad tidal patterns occur:
Semidiurnal: Two high tides and two low tides of roughly similar heights per lunar day.
Mixed semidiurnal: Two high tides and two low tides per lunar day, but successive highs or lows differ noticeably in height.
Diurnal: One high and one low per lunar day.
Local geography and basin dynamics help determine which pattern occurs. Much of the U.S. Atlantic coast has semidiurnal tides, while much of the Pacific coast has mixed tides.
Spring and
The is the vertical difference between high water and low water. It tends to be greatest during , which occur around new and full moons when the Sun, Moon, and Earth are nearly aligned. It tends to be smaller during , around the first and third quarter moons, when the Sun and Moon act from roughly right angles.
“Spring ” refers to the greater , not to the season.