GUIDE WAVE ANALYSIS AND FORECASTING - WMO

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Figure 8.1 — Example of a wave record. In an analysis the
waves marked with arrows are ignored
sea surface above a fixed point plotted against time, i.e.
it represents the up-and-down movement of a floating
body on the sea surface as it is seen by the observer, and
is included to illustrate the main difficulty in visually
observing height and period — the irregularity of the
waves. The pattern shown is typical of the sea surface,
because waves invariably travel in irregular groups of
perhaps five to 20 waves with relatively calm areas in
between groups.
It is essential that the observer should note the
height and period of the higher waves in the centre of
each group; the flat and badly formed waves (marked
with arrows) in the area between the groups must be
entirely omitted. The analysis should therefore include:
• Height and period: that is the mean height and
period of about 15–20 well-formed waves from the
centres of the groups; of course all these waves
cannot be consecutive;
• Wave direction: the direction from which the waves
come should be noted, and reported to the nearest
10° on the scale 01–36, as for wind direction. For
example, waves arriving from the west (270°)
should be reported as direction 27. Where more
than one wave train is clearly identifiable, the direction
as well as an observed height and period
should be reported for each such train.
The observer must bear in mind that only good
estimates are to be reported. Rough guesses will have
little value, and can even be worse than no estimate at
all. The quality of observations must have priority over
their quantity. If only two, or even one, of the three
elements (height, period, direction) can be measured, or
really well estimated (e.g. at night), the other element(s)
could be omitted and the report would still be of value.
When different wave trains — for example sea and
swell or several swells — are merged, the heights do not
combine linearly. Wave energy is related to the square of
the wave height and it is the energy which is additive.
Consequently, when two or more wave trains are
combined, the resultant height is determined from the
square root of the sum of the squares of the separate
trains:
2 2
H = H + H . (8.1)
combined sea swell
More than one swell train can be combined if necessary.
The following possible systematic errors in the
observation of waves should be borne in mind by an
observer:
(a) Waves running against a current are steeper and
usually higher than when in still water, while they are
GUIDE TO WAVE ANALYSIS AND FORECASTING
lower when running with the current. Observers may
not necessarily be aware of this in a given case;
(b) Refraction effects due to bottom topography in
shallow water may also cause an increase or
decrease in wave height;
(c) Waves observed from a large ship seem smaller
than those same waves observed from a small ship.
8.3.1.1 Observations from merchant ships
A traditional source of wave information has been from
so-called ships of opportunity. Given the lack of surfacebased
measurements of waves on the open ocean, the
coverage offered by merchant shipping has been utilized
in the WMO Voluntary Observing Ship (VOS)
Programme. The participating ships report weather
information including visual observations of waves. The
guidelines for making these observations are set down in
the WMO Guide to meteorological instruments and
methods of observation (WMO, 1996, see Part II,
Chapter 4).
HEIGHT
With some experience, fairly reliable estimates of height
can be made. To estimate heights of waves which have
relatively short lengths with respect to the length of the
ship, the observer should take up a position as low down
in the ship as possible, preferably amidships, where the
pitching is least violent, and on the side of the ship from
which the waves are coming. Observations should be
made during the intervals, which occur every now and
then, when the rolling of the ship temporarily ceases.
In the case of waves longer than the ship, the
preceding method fails because the ship as a whole rises
over the wave. Under these circumstances, the best
results are obtained when the observer moves up or down
in the ship until, when the ship is in the wave trough, the
oncoming waves appear just level with the horizon. The
wave height is then equal to the height of the eye of the
observer above the level of the water beneath him. By far
the most difficult case is that in which the wave height is
small and the wavelength exceeds the length of the ship.
The best estimate of height can be obtained by going as
near to the water as possible, but even then the observation
can only be approximate.
An observer on a ship can often distinguish waves
coming from more than one direction. Wave characteristics
from each direction must be reported separately;
only the waves under the influence of the local wind (i.e.
of the same direction as the local wind) are termed sea,
the others are, by definition, swell and should be
reported as such. Sometimes it is possible to distinguish
more than one swell train, and each such wave train
should be reported separately as swell.
PERIOD AND DIRECTION
Period and direction can be reported as outlined in
Section 8.3.1. The period of waves can often be estimated
by watching a patch of foam or other floating