and the curves for x + F = 500 km determine the value of mp, , 
and the intersection of top = 20 hours and the curve for x = 400 km 
determines the value of wy . The forecasted power spectrum then 
equals the power spectrum at the source between these values and 
zero otherwise. This case is illustrated in the first graph below 
the forecasting diagram. 
For later values of te and with F fixed, the band width of 
the power spectrum observed at the point x, becomes wider. In 
general, the longer the fetch the more rapidly the band width widens 
at the point of observation. 
Figure 16 includes two special cases. If F equals zero, x + F 
= x, and the special case considered in figure 15 is the result. 
Lf De is zero, then the upper lines passing through the point 
t = Dg coincide with the lines passing through t = 0. Then by 
considering the lines appropriate to, say, x = 400 km and 
x + F = 500 km and top = 20 hours, the spectrum at the point 
x = 400 and time ton = 20 hours can be found in the same way as 
described above. This special case could occur when a strong wind 
blows for a very short time over a long fetch. 
In general, the upper curves shift up and down for different 
values of Dg, and F varies from storm to storm. In some storms, 
the effect of Do dominates the effect of F. For other storms, 
the effect of F dominates the effect of D,. For the usual weather 
situation, the values of F and D, must both be considered in order 
to forecast the power spectrum of the waves. 
- 164 - 
