Table 3-1. Correction for Sea-Air Temperature^ 



*Pore and Richardson (1969) and Hasse and Wagner (1971) 

 report recent studies designed to refine the above table. 

 Neither were able to find enough high quality observa- 

 tions of large differences between air and sea tempera- 

 tures to rigorously establish any effect of the sea-air 

 temperature difference on the ratio of the surface wind 

 speed to the gradient wind speed over the ocean. Both 

 recommended the use of a constant value near 0.6 for 

 most routine work. 



If there are several observed wind reports within the fetch region, 

 and these consistently deviate in the same manner from wind speeds arrived 

 at by the instructions given above, an average between the reported values 

 and those computed by the above instructions will usually be the best 

 estimate. 



Over the Great Lakes and some coastal regions, large temperature 

 inversions (temperature increasing with elevation) may be observed. 

 Bellaire (1965) reports air temperatures more than 15°C (27°F) greater 

 than the water temperature in May 1964. When air temperature is much 

 greater than that of the sea, all turbulent motion in the lower atmosphere 

 is suppressed, and the wind near the surface has little relation to the 

 wind estimate determined from a synoptic weather chart. Near mountainous 

 coasts and particularly in fjords, the wind near the sea is often channeled 

 to flow parallel to the mountains. The local temperature contrast between 

 snow-covered mountains and relatively warm open water may have more control 

 over the wind near the water than the isobaric pressure pattern from 

 weather maps. In these cases, the wind determined from the pressure 

 analysis on a weather map has little if any value for wave prediction. 

 For these exceptional cases, there is no valid substitute for wind 

 observations. 



3.42 DELINEATING A FETCH 



The fetch has been defined subjectively as a region in which the wind 

 speed and direction are reasonably constant. Confidence in the computed 

 results begins to deteriorate slightly when wind direction variations 

 exceed 15°, and deteriorates significantly when direction deviations 

 exceeding 45° are accepted in the fetch area. The computed results are 

 sensitive to changes in wind speed as small as 1 knot (0.5 meter/second), 

 but it is not possible to estimate the wind speed over any sizable region 



3-27 



