by the Gaussian case for storms with a fetch of finite length 
is then given by the value of S.F.G.F. which is one if equation 
(7.57) holds and zero otherwise. The functional form of S.F.G.F. 
is given by equation (7.60). 
Forecasting diagram for a storm of finite duration with fetch 
of finite length 
A forecasting diagram for the application of equation (7.60) 
to the power spectrum given at the source in order to find the 
power spectrum at the point and time of observation is given by 
figure 16. The upper part is a copy of the corresponding part 
in figure 15 except that the lines which pass through the point 
t= De are now labeled with the values of x + F. The diagram 
again applies for De = 10 hours. The lower lines are graphs of 
equation (7.55) for fixed x, with w and t,, variable. The upper 
lines are graphs of equation (7.56) for fixed (x + F), withpy 
and tb variable. 
As an example, suppose that a storm with winds that last 
for ten hours over a fetch one hundred miles long develops and 
then ceases. The fetch is defined over a strip on the x axis 
between x = O and x = - F with x = O the forward edge of the 
storm; and positive values of x define the area of decay. Suppose 
also that the power spectrum at x = O during the time O<t<D, 
is given by the same graph as in the previous figure. 
Then to forecast the power spectrum at the point x = 400 km 
for the time t., = 20 hours, with the use of figure 16, locates 
the lines for t,, = 20 hours, x = 400 km, and x + F = 500 km on 
the upper part of the figure. The intersection of top = 20 hours 
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