1 A typical record of wave traces is shown in
figure 1 below.
2 The record is, in general, complex and shows
immediately all the difficulties inherent in eye observation. For
example, are all the waves to be considered on an equal footing or
are only the big waves to be counted? Since the wave characteristics
vary so much, what average values shall be taken? It is obvious that
if comparable results are to be obtained the observer must follow
a definite procedure. The flat and badly formed waves (“A”
in figure 1) between the wave groups cannot be observed accurately
by eye and different observers would undoubtedly get different results
if an attempt were made to include them in the record. The method
to be adopted, therefore, is to observe only the well-formed waves
in the centre of the wave groups. The observation of waves entails
the measurement or estimation of the following characteristics:
3 Reliable average values of period and height
can only be obtained by observing at least twenty waves. Of course,
these cannot be consecutive; a few should be selected from each succeeding
wave group until the required number has been obtained. Only measurements
or quite good estimates are required. Rough guesses have little value
and should not be recorded. It will often be found that there are
waves coming from more than one direction. For example, there may
be a sea caused by the wind then blowing and a swell caused by a wind
that has either passed over or is blowing in a distant area. Or there
may be two swells (i.e. cross swells) caused by winds blowing from
different directions in distant areas. In such cases, the observer
should distinguish between sea and swell, and report them separately,
giving two groups for swell when appropriate. The direction, height
and period of the sea wave may be quite different from that of the
swell wave. It will, however, often happen − particularly with
winds of Beaufort force 8 and above − that the sea and swell
waves are both coming from the same direction. In that case, it is
virtually impossible to differentiate between sea and swell, and the
best answer is to look upon the combined wave as being a sea wave
and log it accordingly.