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Determining how long the wind has blown is
relatively simple when the wind speed has bee n constant
for the entire duration. If this does not occur, a
representative duration must be selected.
SLOWLY VARYING WIND.— Suppose the wind
has been blowing for 24 hours, with velocities of 10
knots for 6 hours, 15 knots for 12 hours, and 20 knots
for 6 hours. The duration is 24 hours but the speed value
is in question. The most consistent solution is to use
three durations with the corresponding wind speeds and
work up three successive states.
MORE RAPID VARIATIONS.— Suppose the
wind blows for 12 hours and during that time it increases
in velocity from 10 to 20 knots. Studies and experience
have shown that in cases of variable winds a single value
may be assigned for wind speed if the change has been
relatively small. The following rules can be applied
under these conditions:
. Average the speeds when the change is gradual
or increasing or decreasing. Apply the average to the
entire duration.
. Use the last wind speed when the speed changes
in the first few hours, then remains constant. Apply that
speed to the entire duration.
OBJECTIVE METHODS FOR
FORECASTING SEA WAVES
There are a number of different methods for
forecasting sea waves. Some of the methods are too
technical or time consuming to be of practical use of
Aerographer’s Mates.
A relationship between wave velocity (c), wave
length (L), and period (T) maybe indicated using the
equation C = 3.03 T. The length in feet of a deep-water
wave (L) may be computed using the equation L = 5.12
T. The period of a wave in seconds (T) may be
calculated using the equation T = 0.33 C, where (C) is
the wave velocity.
Sea state forecasts are divided into four categories:
significant wave height (HID), average wave height
(HAvG), one-tenth average wave height (HIJ1o), and
high wave (Hw).
For more information, refer to the practical training
publication Sea and Swell Forecasting, NAVEDTRA
40560, published by the Naval Oceanographic Office.
This publication presents a method for forecasting sea
waves, and a brief summary follows.
In order to prepare an accurate sea state forecast one
must frost determine wind speed over the fetch (U),
length of the fetch (F), and the length of time the wind
speed (u) has remained unchanged within the fetch (u).
These parameters are determined using current
and/or previous surface charts. Using these parameters
and the tables in NAVEDTRA 40560, an accurate sea
state forecast may be obtained.
FORECASTING SWELL WAVES
LEARNING OBJECTIVES: Explain swell
wave generation and recognize the two
fundamental modifications that sea waves
undergo as they leave the fetch area. Define the
terms associated with swell waves, and explain
the five rules used to determine how much of
the swell will reach the forecast point. Prepare
an objective swell wave forecast.
In the preceding portion of this chapter, we have
discussed the principles of sea waves and methods of
forecasting them. With sea wave forecasting we are
considering the point that we are forecasting to be within
the generating area, with the wind still blowing. This,
however, will not be the problem in the majority of the
forecasts that will be required. Normally the forecast
point will be outside the fetch area; therefore, it will be
necessary to determine what effect the distance traveled
is going to have on the waves. In this section we will
discuss the basic principles of swell waves as well as an
objective method of determining what changes will take
place in the spectrum of waves as they traverse from the
generating area to the forecast point.
GENERATION OF SWELL WAVES
After a sea state has been generated in a fetch, there
are many different wave trains present with different
periods, and most of them are moving out of the fetch
in slightly different directions. Because of these
different periods and slight differences in direction, the
propagation of swell waves follows two fundamental
processes. These processes are dispersion and angular
spreading.
Dispersion
An accepted fact about wave travel is that the waves
with longer periods move faster than waves with shorter
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