(a) The location of nearby anemometer sites should be checked to see 

 if they are sheltered by major topographic features. The method should 

 not be used for thunderstorms or any other condition that violates the 

 assumption that the winds over the water are driven by the same pressure 

 gradient at the land. The overland anemometer should be located at a 

 large clearing, such as at an airport. 



(b) The windspeeds should be adjusted for different observation 

 elevations with equation (3-26). Note that the elevation of wind 

 instruments at a site may have been changed sometime during the period of 

 record. This possibility must be checked carefully and the wind data 

 adjusted accordingly. 



(c) If the anemometer is located inmiediately adjacent to the water- 



body, then onshore winds do not require adjustment for 



For sites 



some distance from the water or for winds blowing offshore at a site 



adjacent to the water, the windspeeds should be adjusted by 



Rt 



from 



Figure 3-15. If the fetch is less than 16 kilometers, then Rt can be 

 set to 1.2, with the assumption that the boundary layer is not in full 

 adjustment to the water surface. 



(d) The adjustment for stability 

 applied. 



Rj from Figure 3-14 should be 



(e) The duration adjustment in Chapter 3, Section IV,l,b should be 

 made. 



(f) The windspeed should be converted to the wind stress factor by 

 Chapter 3, Section IV,l,e. 



This method is an approximation that can vary as the landscape character- 

 istics change. It is highly desirable to obtain local wind data to calibrate 

 the method for specific sites. Topographic funneling effects should not be 

 present, or the wind data must be adjusted to account for the funneling. 



4 . Wind Information from Surface Pressure . 



Direct observations of wind may not always be available. It is possible 

 to estimate windspeeds by analysis of pressure charts. The free air windspeed 

 is first estimated from sea level pressure charts. Corrections to the free 

 air wind are then made. Estimation from pressure charts can be subject to 

 considerable error and should be used only for large areas over which pressure 

 gradients can be smoothed. This method is not recommended for areas of high 

 topographic relief; estimated values should be compared with other obser- 

 vations to confirm their validity. 



a. Free Air Wind. Surface wind-field estimates that are fairly accurate 



can often be determined from analysis of the isobaric patterns of synoptic 

 weather charts. 



Horizontal pressure gradients arise in the atmosphere primarily because of 

 density differences, which in turn are generated primarily by temperature 



3-33 



