behind the y axis. The lower part of figure 23 illustrates 
two possibilities which could occur. Suppose that an area ofr 
relatively strong winds covers the region between x = O and 
ena y= W./2 and y = - wo/2. Also suppose that the 
winds last Ds units of time. Then at the time t = D, an ele- 
mental wave which has left the storm area would occupy the area 
ahead of the y axis which is bounded by the solid lines. When 
the winds stop at t = De» the area behind the y axis bounded by 
the solid lines might be occupied by an elemental wave component 
which could travel off in the 6. direction after t = D,- This 
al 
would imply a rather peculiar behavior of the wave component, and 
the two obliquely oriented areas would sweep out a rather peculiar 
area as they travel along. 
In an actual storm the area covered by the winds merges 
gradually into the area of relative calm. In addition the wind 
direction varies turbulently over the storm area. It would there- 
fore be equally consistent to assume that the area occupied by 
the wave element traveling in the 8, direction could be given by 
the dashed area at the time, t., = Doe At any time after t,, = 0, 
one observation, sufficiently detailed to determine E,(p,©) at 
x = 0, y = 0, would yield only enough information to show that 
either of the two assumed areas could be used. A system of weather 
maps which could yield such a pattern and which could occur in an 
actual meteorological situation, will be described in a later 
chapter. The filter under discussion will be derived under the 
assumption that the elemental wave system occupies the area 
bounded by the dashed lines. 
= 235 - 
