We have seen how waves on the ocean are combinations
of simple waves. In an irregular sea the number of differing
wavelengths may be quite large. Even in regular
swell, there are many different wavelengths present but
they tend to be grouped together. In Figure 1.12 we see
how simple waves with close wavelengths combine to
form groups of waves. This phenomenon is common.
Anyone who has carefully observed the waves of the sea
will have noticed that in nature also the larger waves
tend to come in groups.