SECT. 2] 



SMALL-SCALE INTERACTIONS 



71 



Turning now to humidity profiles, some values of Fe obtained by Mont- 

 gomery (1940) from his and from Wiist's (1937) observations, and by the 

 present authors from measurements made in May, 1958, are plotted against the 

 bulk Richardson number in Fig. 14. The value of Fe is seen to be about 0.1 in 

 neutral conditions, with a dependence on the stability somewhat as shown by 

 the curve drawn in by eye. There does not appear to be any marked variation 

 of Fe with wind speed, but in view of the paucity of data and the experimental 

 scatter, no definite conclusion on this point is possible as yet. 



d^-0^. deg F 



Fig. 12. Potential temperature difference between 2.8 and 4 m plotted against difference 

 between sea and air — observations in Port Phillip Bay and Bass Strait, November, 

 1956. The slope of the straight line, which is fitted by eye, indicates 7/^ = 0.10. 

 Symbols denote wind speed at 13m: 



X > 9 m/sec A 3 to 4.5 m/sec 



+ 6 to 9 m/sec 

 O 4.5 to 6 m/sec 



2 to 3 m/sec 

 ^ 2 m/sec 



There are three complicating factors which may influence the behaviour of 

 Fh and Fe, though to what extent is largely unknown at present. These are : 



i. Spray. As has been remarked by Montgomery (1940), the evaporation of 

 spray, by providing a source of water vapour between the sea surface and the 

 measurement level, will operate to increase Fe. While negligible in light winds 

 this effect may become of some importance at higher wind speeds. 



ii. Cool surface skin. Montgomery (1947a), Ball (1954) and others have 

 pointed to the existence, in at least some conditions, of a large temperature 

 gradient through a water-skin layer of a millimetre or so depth immediately 

 below the interface ; this will normally mean that, on account of evaporation, 



