1 Negative values of (Of 



G) indicate stability and positive values indicate instability. 



d. Make correction for the curvature of the 

 isobars on the approximate surface wind ac- 

 cording to table II.5 to obtain the surface wind 

 speed Us- 



The stability and curvature corrections can 

 most conveniently be made by use of the align- 

 ment chart of plate II. The example illustrates 

 the use of this chart. 



The wind speed found in steps a through d 

 above applies only to the current weather map 

 and may differ from the wind speed over the 

 same area according to the preceding map. A 

 constant wind speed was assumed in the prep- 

 aration of plates IV and V which are used to 

 determine the wave height, and it is therefore 

 necessary to introduce an average wind speed 

 which can be considered applicable to the entire 

 time interval between the two maps. Forecast- 

 ing experience has indicated that a time inter- 

 val of 12 hours between maps is generally most 

 satisfactory. 



The average wind speed can be obtained by 

 first finding the component of the wind on the 

 current map. Then for decreasing winds add 

 one-fourth of the difference between these two 

 speeds to the later speed, that is, 



u = V4,[{u,)-Us^ + u, (ii.i) 



For increasing winds determine the differ- 

 ence between the two speeds and subtract the 

 corresponding value given in table II.6 from 

 the later wind speed. 



Ships' observations should be used as a check 

 on the computed wind speed. A difference of 

 not more than one force on the Beaufort scale 



Table II.6. — Correction for increasing winds 



between computed and observed wind speed is 

 a satisfactory check. 



Fetch 



The fetch is the horizontal length of the gen- 

 erating area in the direction of the wind, that 

 is, the distance between the rear and the front 

 boundaries of the generating area. In general, 

 the fetch boundaries are determined by coast 

 lines or by one of the following: (a) Fanning 

 out of the isobars, (&) meteorological fronts, 

 or (c) curvature of the isobars as shown sche- 

 matically in figure II.I. When the boundaries 

 have been decided upon, the fetch is measured 

 on the map. 



The determination of the generating area or 

 the fetch is probably the most subjective factor 

 in the entire process of wave forecasting. It 

 is difficult to set up hard and fast rules for 

 determining these elements because of the great 

 variety of weather situations that can be en- 

 countered. A few rules of thumb and examples 

 will be given here, but proficiency in determin- 

 ing the fetch can come only from first hand 

 experience. 



The following steps are suggested as a guide 

 in selecting a generating area and fetch : 



a. Decide on the boundary limits of the 

 generating area by drawing a few lines out- 

 ward from the forecast locality toward the 

 generating area. The exact boundaries of 

 the generating area are delineated in the case 

 of moderate to straight isobars by winds 

 whose directions are within 30 degrees of a 

 line joining the generating area and the fore- 

 cast locality or, in the case where the isobars 

 are curved, by the winds whose directions are 

 within 45 degrees of a line joining the gen- 

 erating area and the forecast locality. (See 

 fig. ILL) 



b. When a decay distance of 500 miles or 

 greater exists between the forecast locality 

 and the generating area, the portion of the 

 generating area with winds of 20 knots or 



17 



