54 DEACON AND WEBB [CHAP. 3 



and this for y= 18 is shown in Fig. 2. Panofsky, Blackadar and McVehil (1960) 

 show that a large number of wind-profile data for unstable conditions are well 

 represented by a form of this equation in which y' Ri replaces yRf — a relation- 

 ship first proposed on theoretical grounds by Olukhov (1946). The value of 

 y' = 18 gave the best fit and is consistent with a = 4.5 in equation (18) for near- 

 neutral conditions (i.e. a = y'l4). This formula of Panofsky et al. gives a curve 

 similar to that of Ellison in Fig. 2 but some 5% lower at Ri= —0.4. 



C. Observational Considerations 



To obtain satisfactory exposures for accurate profile observations over the 

 sea is considerably more difficult than over the land, and the smallness of 

 the vertical gradients makes considerable demands on technique. In much of the 

 earlier work with rafts and small boats, it has been a matter of doubt as to the 

 extent to which the profiles have been influenced by such factors as distortion 

 of the wind-field by the boat, etc., motion of the anemometers and some un- 

 certainty as to the datum plane for height measurement — a factor of some 

 importance with wind speeds measured at relatively small heights above the 

 water. 



Work carried out at coastal sites using rafts or masts in shallow water near 

 sand spits, etc. is also somewhat suspect as, with such sites, the character of 

 the water surface is not representative of open sea conditions nor is it uniform 

 but varies from point to point owing to variation in depth of water and to 

 refraction and reflection of waves. In such circumstances there may be ap- 

 preciable departures from the desired equilibrium wind-profile from which the 

 shearing stress may be determined. 



As a result of observations over shallow water, Bruch (1940) obtained wind- 

 profiles over the height interval 1 8-200 cm but the parts of the profiles below 

 the 64-cm level were difficult to reconcile with the upper portions. This may 

 have been an exposure effect of the kind mentioned above but there is, of 

 course, the possibility that flow over a water surface disturbed by waves is 

 sufficiently different from that over a rigid surface for the logarithmic law not 

 to hold at all heights over water; see e.g. Stewart (1981). However, Roll (1948), 

 working with what would appear to have been a more uniform expanse of shallow 

 water over the Neuwerk shoals, found profiles following rather closely the 

 logarithmic form from close above the wave tops up to 2 m, his highest level, 

 as may be seen from Fig. 3, but the waves were only around 10 cm high. 



In another paper, Roll (1949) has shown how a number of earlier observa- 

 tions may have been vitiated by the disturbance to the wind-field caused by 

 a ship's hull, even with anemometers mounted on the bowsprit. 



To secure profiles characteristic of the Open sea, Deacon, Sheppard and Webb 

 (1956) fitted an anemometer mast to the outboard end of the jib-boom of a 

 small diesel-schooner and were able to investigate, and to correct for, the 

 extent to which the ship, head to wind, influenced the wind-profile. This was 

 done by taking profiles at the lowest possible speed consistent with maintaining 



