As the wind blows over the surface of the ocean, there is the transfer of energy from the wind to the ocean water. As energy is put into the ocean surface by wind, the ocean surface is deformed into small rounded waves with V-shaped troughs. The wavelengths of such initial waves are less than 1.74 cm.
These small waves are known as the Capillary waves. Such waves are easily destroyed by the surface tension. Later on, as the surface water gains more and more of energy from the wind blowing over it, the waves increase in height and length.
When the lengths of such waves exceeds 1.74 cm, their shape resembles a sine curve. Such waves are called the gravity waves. The steepness of these waves increases due to the gain of increased energy. There is a radical change in the shape of these gravity waves. As more energy is gained, there is a corresponding increase in their height.
Now, the wave height increases more rapidly than the wave-length. The gravity waves are distinguished by pointed crests and rounded toughs. With increasing height of the wave, there is also an increase in the wave-velocity.
This is due to the fact that as more and more energy is put into the waves, their height and velocity both register an increase. As soon as the wave velocity equals the wind speed, there is no further change in either the velocity or the height of the waves. At this stage the wave is at its maximum height with the resultant stoppage of net energy exchange.
The term ‘sea’ refers to the wind-generating area which is characterized by irregular waves running in different directions. These irregular waves have dissimilar characteristics caused by the frequent changes taking place in wind speed as well as direction.
The more important factors that determine the increasing amount of energy supplied to the waves are the following: (i) wind speed, (ii) the time during which the wind blows in one direction, and (iii) the fetch, the distance over which the wind blows in one direction.
The height of a wave is directly controlled by the amount of energy gained by it. The usual height of waves in a sea hardly exceeds 2 meters, but there are exceptions. Wave heights measuring 10 meters are a common sight.
It is interesting to note that sometime in February, 1935, the crew of the U.S. Navy tanker observed a 34 meter wave with a period of 14.8 seconds in the North Pacific. When the steepness of sea waves obtains a critical value of 1/7, open ocean breakers known as whitecaps are produced.
Another point to be kept in mind in connection with the growth of the waves is that there is the maximum fetch and duration of wind beyond which the waves will not grow. When for a given wind velocity the maximum fetch and duration is reached, the sea is said to be ‘fully developed.’
Let us remember, the wave velocity being equal to that of the wind, waves cannot absorb more energy from the wind. The following table shows the fetch and duration required to produce fully developed sea for a given wind speed.