make the opposite assumption from the one 

 necessary to determine the formation of these 

 waves, i. e., that the wind stops blowing- every- 

 where over the fetch at the same time, the only 

 difference between the behavior of the waves at 

 the downwind and upwind ends of the fetch is 

 that the latter will have to travel farther to 

 reach the shore or target in which we are in- 

 terested, or in other words, the decay distance 

 will be longer. This extra decay distance is 

 the entire area in which waves of constant 

 characteristics were previous established, or 



There remains to consider the behavior of 

 the waves within the minimum fetch as they 

 pass through the constantly lengthening decay. 

 Proceeding upwind, each wave is slightly lower 

 than the one ahead of it and has a shorter 

 period. Thus, a wave farther from the target 

 will not only have to travel farther, but also 

 will have less energy to perform this travel. 



Consequently, as soon as the waves generated 

 in the minimum fetch begin to arrive at the 

 target we can expect a more rapid decrease in 

 height than was experienced with the waves 

 from the fetch greater than the minimum. It 

 will usually be found that if the minimum fetch 

 is successively halved the wave heights at the 

 target soon become so small as to be negligible. 

 This method is also used when the minimum 

 fetch is greater than the actual fetch. 



Example: To determine diminution of swell. 

 Given U = 20 knots 

 ti = 20 hours 

 F = 300 nautical miles 

 D = 200 nautical miles. 

 (1) Hp =8 feet (pi. IV) 



Tf = 5.6 seconds (pi. IV) 

 Ho/Hp = .50 (pi. VI) 

 To = 7.6 seconds (pi. VI) 

 to = 18 hours (pi. VI) 



Hd = 4.0 feet 



(2) 



(3) 



(4) 



Since wave decay is not linearly proportional 

 to the decay distance, direct linear interpola- 

 tion between these values is not strictly correct. 

 However, it appears to be sufficiently accurate 

 for most purposes and, if greater precision is 

 desired, intermediate computations may be 

 made for other fetches both greater and less 

 than the minimum. 



The complete forecast for this situation 

 would then be for waves with significant height 

 of 4 feet to arrive 18 hours after map time, 

 diminishing to 3 feet in the following 10 hours, 

 to 2 feet after 4 more hours, and to less than 

 1 foot in the following 6 hours, or 38 hours 

 after map time. If other values are desired, 

 these may be plotted on a graph of wave height 

 against time, and the height at any time or the 

 time for any height read from the graph. 



ALIGNMENT CHARTS AND GRAPHS 



The theoretical basis and practical use of the 

 alignment charts and graphs are discussed else- 

 where in the text. This section deals with the 

 mechanics of their use. 



Plate I is a sea level, geostrophic wind scale 

 for 3-millibar spacing of isobars. Enter the 

 bottom of the graph with the degree interval 



between isobars in the fetch. Proceed verti- 

 cally to the diagonal representing the mean 

 latitude of the fetch. Proceed horizontally and 

 read wind speed in knots at the right-hand 

 edge of the graph. 



Plate II is an alignment chart to determine 

 the surface wind from the geostrophic wind. 



23 



