If the waves move out of the margin of the generating or wind area the supply of energy is cut off, and the waves are subjected to the loss of energy. The loss of energy is due to turbulence rather than the molecular viscosity.
Since the waves now move with a velocity greater than that of the wind, the steepness of the waves diminishes, and they become long-crested waves and their periods are relatively longer. Such waves are called swell. The swell travels over large stretches of the surface of the ocean with no loss of energy.
However, as soon as the swell moves out into the calmer water the shorter waves disappear, because their small amount of energy is lost rapidly. Now, the swell appears more uniform and longer crested than the sea from which it originated. The swell becomes sinusoidal in form with smooth rounded crests.
Away from the generating area the height of this type of wave is drastically reduced. This process is accelerated if there is a strong wind from the opposite direction. The swell waves lack short wave lengths.
Sometimes, a series of small waves are superimposed on the swell with the result that it is hardly visible in the open ocean because of their very low heights. The swells that originated in the Antarctic because of storms have been observed breaking along the coast of Alaska after travelling a distance of more than 10,000 kilometers.
A very characteristic feature of the swell is that it leaves a sea in a group called a swell wave train (a group of swells). After a swell travels out from the generating area, it gets modified. However, there are different views regarding its modifications.
But it is true that as a swell wave train moves away from the generating area, the individual wave train retains its wavelength. Some authorities are of the view that the group (of swells) moves across the ocean surface at only half the velocity of an individual swell in the group. Finally, the leading swell dies out.
But there will always be the original number of waves in the group, because after the disappearance of the front wave, an altogether new one replaces it at the rear of the group.
From the preceding discussion it is evident that the movement of the ocean water, once started, will continue even far beyond the generating area, especially if there are large waves with a velocity of 40 to 80 km an hour. It is due to this that great waves lose their identity and become long, deep swells.
Such swells are often felt on a smooth and calm sea where there is no wind. Thus, it is true that the ocean surface cannot remain calm and motionless, because a swell may come from more than one source in any direction simultaneously.
With the aid of a diagram, it is possible to determine the exact geographic location of a generating area and the wind force from the time of the arrival of the swell, its amplitude and period, and also the direction of the crests. This method is used for storm predictions at sea.