CURVATURE 



Figure 11. Boundaries of the fetch for different 

 tyoes of Isobars. 



k- Duration of wind (9) . The duration of the wind Is 

 determined from a comparison of current and preceding weather 

 maps. The duration of the average wind velocity equals the 

 time Interval between the last two maps plus a correction 

 determined from the height of the waves present at the beginning 

 of that time interval. These waves should be known from the ex- 

 amination of the preceding map. Only the waves which travel 

 at an angle less than 45° from the average wind direction 

 should be examined. The correction is found from Plate IV 

 by the following procedure: 



Enter the graph with the average wind velocity and follow 

 a horizontal line to the curve which gives the wave height on 

 the preceding day. The corresponding duration, as read off 

 from the top or bottom scale, represents the correction to be 

 added to the time interval between the maps. 



4/, 



