Wind waves, swell and generating histories

Wind transfers energy to the water surface and can generate waves. Response depends on intensity, duration and generating distance in a direction, called fetch.

These factors act together: recent and prolonged winds of the same strength need not produce the same sea state. Water can also receive waves formed far from the observer.

Wind waves are associated with generating wind in the region considered. Swell describes waves that have propagated beyond their generating area.

The distinction concerns generation rather than a universal height boundary. Swell can coexist with wind waves from another direction.

Total sea combines these components; its statistical representation is not obtained simply by stacking and adding every height number.

Height, period and wavelength are different dimensions. Period describes time between equivalent phases in the stated frame; wavelength is distance between them.

For an ideal regular wave, phase speed equals wavelength divided by period. It is not automatically water-particle speed or boat speed.

An irregular sea contains components with different periods and directions, so one drawn sine curve is only a conceptual tool.

Approaching shallower water, waves can change speed, length, direction and form. Transformation depends on bathymetry and components, not just local wind.

The existing lesson explains significant height and period; this one adds generating and propagating history. Interpreting a breaking area or passage is applied material, without an operating rule in this diagram.

Wave components can cross. One crest direction combined with another produces a surface more complex than a single profile.

Observed wave height depends on that combination and is not constant from wave to wave. A sea statistic summarises part of the set.

Mean period, peak period and the ideal example wave’s period are different quantities too.

An advancing crest does not imply that one water parcel travelled with it throughout. Local water motion and pattern propagation are distinct relationships, with mean transport and current potentially superimposed.

The diagram does not show exact circular particle paths valid at every depth. Separating wave and current allows sea interpretation without turning every propagation arrow into water velocity.