As seen in this simple case, the velocity at an elevation z is dependent 

 upon the shear stress through U* , the surface roughness, and the air-sea 

 temperature difference. To complicate matters, the surface roughness is 

 directly related to the friction velocity. Since the shear stress is most 

 directly related to wave growth, the relationship between observed windspeed 

 and shear stress must, at a minimum, be dependent upon local windspeed and 

 air-sea temperature difference, A T 



To more accurately estimate the effect a particular windspeed will have on 

 wave generation, AT ,11^^, and z must be known. The wave growth 

 curves in Chapter 3, Section VI are given in terms of an equivalent windspeed 

 observed at z = 10 meters for neutral stability so that the values are 

 commensurate with units of measurement in normal use. Thus, observed wind- 

 speeds must be increased or decreased to account for the effect of the other 

 factors. 



In Chapter 3, Section IV, 2 to 6, specific instructions for estimating 

 winds for use in the wave growth curves and formulas of Chapter 3, Section V 

 will be given for the major wind observation conditions with which the 

 engineer must normally deal. In addition, a procedure for estimating surface 

 winds from pressure charts will be given. To make the wind transformations 

 required in Section IV, 2 to 6, combinations of five adjustment factors will 

 be used. These adjustment factors are discussed below. 



a. Elevation . If the winds are not measured at the 10 meter elevation, 

 the windspeed must be adjusted accordingly. It is possible, but normally not 

 feasible, to solve equation (3-25) for U* at the observed elevation z and 

 then estimate U at 10 meters. The simple approximation 



in 1/^ 

 U(10) = U(z) (-i^) (3-26) 



can be used if z is less than 20 meters. 



b. Duration-Averaged Windspeed . Windspeeds are frequently observed and 

 reported as the fastest mile or extreme velocity (considered synonymous). 

 (Daily fastest mile windspeed equals fastest speed (in miles per hour) at 

 which wind travels 1 mile measured during a 24-hour period.) 



Studies have indicated that the fastest mile windspeed values are obtained 

 from a short time period generally less than 2 minutes in duration (U.S. Army 

 Engineer Division, Missouri River, 1959). It is most probable that on a 

 national basis many of the fastest mile windspeeds have resulted from short 

 duration storms such as those associated with squall lines or thunderstorms. 

 Therefore, the fastest mile measurement, heaause of its short duration, should 

 not he used alone to determine the windspeed for wave generation. On the 

 other hand, lacking other wind data, the measurement can be modified to a 

 time-dependent average windspeed using the procedure discussed below. 



To use the procedures for adjusting the windspeed discussed later, which 

 are ultimately used in the wave forecasting models, the fastest mile windspeed 

 must be converted to a time-dependent average windpseed, such as the 10-, 25-, 

 50-minute average windspeed. 



3-26 



