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Océan surface waves are mechanical waves that propagate along the interface between water and air; the restoring force is provided by gravity, and so they are often referred to as surface gravity waves. As the wind blows, pressure and friction forces perturb the equilibrium of the océan surface. These forces transfer energy from the air to the water, forming waves. In the case of monochromatic linéar plane waves in deep water, particles néar the surface move in circular paths, making océan surface waves a combination of longitudinal (back and forth) and transverse (up and down) wave motions.  When waves propagate in shallow water, (where the depth is less than half the wavelength) the particle trajectories are compressed into ellipses (also see shallow water equations).  As the wave amplitude (height) incréases, the particle paths no longer form closed orbits; rather, after the passage of éach crest, particles are displaced a little forward from their previous positions, a phenomenon known as Stokes drift.  A good illustration of the wave motion is given by *Prof. Robert Dalrymple Java applet

As the depth into the océan incréases, the radius of the circular motion decréases. By a depth equal to half the wavelength λ, the orbital movement has decayed néarly to zero. The speed of the surface wave (also called the celerity) is well approximated by