Figure 6-7.-Measuremeuts of angles for angular spreading.
2. Extend the top and bottom edge of the fetch
outward parallel to the main direction of the wind. This
is shown as dashed lines in figure 6-7.
3. Draw lines from the top and bottom edges of the
fetch to the forecast point.
4. The angles to the forecast point are designated
Theta 3
(q3)
and Theta 4
(q4).
Theta 3 is measured from
the top edge of the fetch and Theta 4 from the bottom
edge.
5. Any angle that lies above the dashed line is
negative while any angle that lies below the dashed line
is positive.
After the angles Theta 3 and 4 have been measured
they are converted to percentages of the swell that will
reach the forecast point. This conversion is made by
entering sea and swell graph 7, figure 6-8, with the
positive or negative angles and reading the
corresponding percentages directly. The percentages
are then subtracted ignoring the plus or minus to find the
angular spreading.
OBJECTIVE METHOD FOR
FORECASTING SWELL WAVES
A number of terms used in dealing with forecasting
sea waves will be used again in this process; however,
a number of new terms will be introduced. Table 6-3
lists most of these terms with their associated symbol
and definition.
As with objective forecasting of sea waves there are
a number of different methods for forecasting swell
waves. Some of the methods are too technical or time
consuming to be of practical use.
When ship operations are conducted outside a fetch
area it becomes necessary to forecast swell conditions
at that location. Prior to computing swell conditions the
height and period of the significant waves departing the
fetch area must be determined. For more details refer
to Sea and Swell Forecasting, NAVEDTRA 40560.
FORECASTING SURF
LEARNING OBJECTIVES: Explain the
generation of surf and describe the two changes
that occur upon entering intermediate water.
Recognize the characteristics of the three types
of breakers. Define the terms associated with
surf. Describe an objective method for surf
forecasting and the calculations of the modified
surf index.
Thus far we have discussed the generation of sea
waves, their transformation to swell waves, some of the
changes that occur as they move, and objective methods
of forecasting both waves.
The Navy is greatly involved in amphibious
operations, which requires the forecasting of another sea
surface phenomena: surf. Senior Aerographer’s Mates
will occasionally be called upon to provide forecasts for
amphibious operations, and accurate and timely
forecasts can greatly decrease the chance of personnel
injury or equipment damage. Therefore, it is important
that forecasters have a thorough understanding of the
characteristics of surf and a knowledge of surf
forecasting techniques.
GENERATION OF SURF
The breaking of waves in either single or multiple
lines along the beach or over some submerged bank or
reef is referred to as surf.
6-11
