After a sea state has been generated in a fetch, there are many different wave trains present with different periods, and most of them are moving out of the fetch in slightly different directions. Because of these different periods and slight differences in direction, the propagation of swell waves follows two fundamental processes. These processes are dispersion and angular spreading. Dispersion An accepted fact about wave travel is that the waves with longer periods move faster than waves with shorter periods.
T is the wave period in the wave train.
At first all of the runners start out at the starting line at the same time. As they continue on, however, the faster runners move ahead and the slower runners begin to fall behind. Thus the field of runners begins to string out along the direction of travel. The wave trains leaving a fetch do the same thing. The stringing out of the various groups of waves is called dispersion.