Figure 6-2.-Swell waves.
Wave Length (L). The wave length is the
horizontal distance between two successive crests
or from a point on one wave to the corresponding
point on the succeeding wave. Wave length is
measured in feet, and it is found by the formula: L
= 5.12 T2.
Wave Speed (C). The wave speed is the rate
that a particular phase of motion moves along
through the medium. It is the rate that a wave
crest moves through the water. There are two
speeds used in ocean wave forecasting: group
speed and individual speed. The group speed of
waves is approximately one-half that of the
individual speed The individual wave speed in
knots is found by the formula C = 3.03 T. The
group wave speed is found by the formula C =
1.515 T.
Definitions of Other Terms
Other definitions that the Aerographer’s Mate
should be familiar are as follows:
Deep water. Water that is greater in depth
than one-half the wave length.
Shallow water. Water that is less in depth
than one-half the wave length.
Fetch (F). An area of the sea surface over
which a wind with a constant direction and speed
is blowing, and generating sea waves. The fetch
length is measured in nautical miles and has
definite boundaries.
Duration time (t). The time that the wind has
been in contact with the waves within a fetch.
Fully developed state of the sea. The state the
sea reaches when the wind has imparted the
maximum energy to the waves.
Nonfully developed state of the sea. The state
of the sea reached when the fetch or duration time
has limited the amount of energy imparted to the
waves by the wind.
Steady state. The state reached when the fetch
length has limited the growth of the waves. Once a
steady state has been reached, the frequency
range produced will not change regardless of the
wind.
Wind field. A term that refers to the fetch
dimensions, wind duration, and wind speed,
collectively.
Effective duration time. The duration time
that has been modified to account for the waves
already
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