The most visible manifestation of the dynamic nature of the ocean is its undulating surface. It is visible to anyone who watches the water while standing on a beach or travelling in a ship. The undulation is caused by the action of winds on the ocean surface, and hence is known as wind waves. Wind waves are the most common form of a class of waves called surface gravity waves. They are called gravity waves because the Earth’s gravity pulls the water particles back to equilibrium once the wind disturbs the ocean surface and perturbs it from equilibrium by injecting kinetic energy into the water. Wind waves occur in water that is deeper than half their wavelength; hence, they are called deep-water waves. The momentum gained by the water decays with depth, and there is no motion due to these waves in deeper water (Fig. 24). The pattern associated with these waves moves at speeds of 1-100 m/s, their wavelengths range from 1-1000 m, and their periods range from seconds to minutes.
At a given time, the link between the winds and waves is so strong that wind velocity can be inferred quite reliably from the characteristics of the waves. A scale, called the Beaufort scale, has been constructed for the measurement of waves.
The waves generated in the deep ocean travel thousands of kilometres (in the form of swells) across the oceans and reach the shore. The area where waves are generated in the open ocean is called ‘fetch’. The size of a wind wave depends on wind speed, duration, and fetch.