with this precision. For practical wave predictions it is usually satis- 

 factory to regard the wind speed as reasonably constant if variations do 

 not exceed 5 knots (2.5 meters/second) from the mean. A coastline upwind 

 from the point of interest always limits a fetch. An upwind limit to the 

 fetch may also be provided by curvature or spreading of the isobars as 

 indicated in Figure 3-12 (Shields and Burdwell, 1970), or by a definite 

 shift in wind direction. Frequently the discontinuity at a weather front 

 will limit a fetch, although this is not always so. 



1004 '9°° 996 



996 



1000 



1004 



1004 



1020 1016 



., 1012 1016 



1006 



1020 ' 1016 



1012 '006 



1012 



Figure 3-12. Possible Fetch Limitations 



Estimates of the duration of the wind are also needed for wave predic- 

 tion. Computed results, especially for short durations and high wind speeds 

 may be sensitive to differences of only a few minutes in the duration. Com- 

 plete synoptic weather charts are prepared only at 6-hour intervals. Thus 

 interpolation between charts to determine the duration may be necessary. 

 Linear interpolation is adequate for most uses, and, when not obviously 

 incorrect, is usually the best procedure. 



3-28 



