Waves

Ocean water is dynamic. Its temperature, salinity and density, and outside forces from the sun, moon and winds, keep it moving. Horizontal motions are currents and waves; vertical motion is the tide (and the upwelling of cold water and sinking of surface water). In a current, water itself flows from place to place; in a wave, the water does not travel, only the wave train moves on.

Waves are really energy moving across the surface. Each water particle only moves in a small circle as a wave passes: up and forward as it comes, down and back as it goes. The surface motion hardly touches the still deep water.

• Wind gives waves their energy. A breeze of two knots or less makes small ripples; stronger winds grow them until white caps break. Waves grow as they travel and absorb more wind energy, and the largest are in the open ocean.
• Maximum wave height depends on the wind's strength, how long it blows, and the area it blows over in one direction.
• Waves travel because wind pushes the water while gravity pulls the crests down, pushing the old troughs up.
• A wave's shape tells its origin: steep waves are young, probably from local wind; slow, steady waves come from far away, perhaps another hemisphere. Waves may travel thousands of kilometres before breaking as surf.
• Near the beach, friction with the sea floor slows the wave; when the water depth is less than half the wavelength, it breaks.

Parts of a wave:

• Crest (highest point) and trough (lowest point).
• Wave height: vertical distance from trough bottom to crest top. Amplitude: half the wave height.
• Wave period: time between two successive crests (or troughs) passing a fixed point.
• Wavelength: horizontal distance between two successive crests.
• Wave speed: how fast the wave moves through water, in knots.
• Frequency: number of waves passing a point per second.

Example (new): a ball floating beyond the surf bobs up, forward, down and back as each wave passes, but stays roughly in the same place: the wave's energy moves on towards the beach, the water under the ball does not.