usually produce seas with small wave heights, small
wave lengths, and short periods; higher winds usually
produce waves with higher heights, longer wave
lengths, and longer periods. When the winds over the
water produce sea waves, the wave crests are generally
aligned perpendicular to the direction the wind is
blowing. The continuing force of the wind on the waves
distorts the ideal sine wave pattern, forming sharper
crests (fig. 1-33). The waves move in the direction the
wind is blowing, with wave crests and troughs
perpendicular to the wind direction.
In a given sea condition, many different size waves
are present. Observers determine significant wave
height, or the average wave height of the highest 1/3 of
all the waves present. Ideally, the heights of 50 to 100
waves should be recorded on a piece of paper, then the
highest 1/3 of the recorded heights should be averaged
to obtain significant wave height for the seas. In
practice, taking the average height of the "most well
defined" waves approximates the significant wave
height. Attempt to observe 50 or so waves as a
minimum, and then average the height of the "best" 16
or 17 waves.
The average significant wave period in an area of
sea waves gives analysts and forecasters a better idea of
the total wave energy present in the area than does the
observation of the significant wave height.
Observations of the average significant period should
be made by timing the passage of "well defined" wave
crests past a fixed point, such as a buoy, clump of
seaweed, wood block, or square piece of cardboard
mentioned earlier, and then dividing to find the average.
The observer should attempt to time the passage of the
same "significant" wave crests that were used in the
determination of average significant wave height. If,
for example, you timed the passage of 17 "well defined"
waves out of 50 waves of various size passing the
cardboard square (before you lost sight of the square) in
120 seconds, the average wave period is 120/17, or 7
seconds. The important factor in determining both the
average height and the average period for sea waves is
that only the highest 1/3 of the waves, the significant
waves, are evaluated.
A direction is always determined for sea waves, and
the direction found should be in general agreement with
the wind direction. If the sea wave direction does not
agree within plus/minus 20° of the wind direction,
recheck both sea and wind direction. The sea direction
is usually not recorded or reported, since it is assumed
that the sea direction is nearly the same as the recorded
wind direction.
SWELL WAVES
Swell waves are seas that have moved out and away
from the area in which they were formed. Because of
their different wave lengths and wave speeds, waves
move outward from the windy areas where they formed,
and separate into groups of waves with distinct wave
periods. Since the winds are no longer pushing on the
waves, they take on a more typical sine wave pattern
with generally equally rounded crests and troughs, and
thus are smooth and regular in appearance.
Typically, when only one group of swell waves is
present, the wave heights and the wave periods are
fairly uniform. Determinations for the average swell
wave period, the average swell wave height, and the
wave direction may be easily made. When determining
Figure 1-33.—Typical sea wave pattern. Note the sharper crests and the irregular wave pattern caused by the superposition of
many different wave length/wave height patterns.
1-48
