109 



The degree to which conditions in the air above the water departed 

 from neutral stratification was assessed by computing hourly values for the 

 bulk Richardson Number, B-, 



R = i h _a s. (3) 



^ T ^m2 u^2 



where g is the acceleration of gravity; T is the air temperature in degrees 

 Kelvin at hei^t a; z is the height of the observation^ (6.0 m) above the 

 sea surface; 0^ the potential temperature at 6.0 m; 63 the potential 

 temperature at the sea surface; ug the wind speed at 6.0 m; each of the latter 

 quantities averaged over 1 hour centered on the hour. The coefficients Th 

 and Tni represent the gradients of heat and momentum, and are defined as 



1 6u 



m ~ — 



u 6 In z 



and 



66 



^ e^ - e 6 In z 

 a 3 



i^) 



(5) 



According to Deacon and Webb (1962) at heights of about i|.0 to 8.0 m and for 

 conditions from neutral to moderate thermal stratification, r^ and T^ 

 have values around 0.1. Thus the ratio in (3) was taken as equal to 10. 



Ninety-six percent of the observations obtained on the CRAWFORD fall 

 in the range 



-0.325 < Rg < +0.025 ; 

 sixty percent fall in the range 



-0.040 < Rn < +0.020. 



Conditions are, therefore, relatively close to neutral stratification. The 

 departure from neutral stratification is a combined effect of lapse rate 

 and wind shear. If we consider the bulk of the observations in the range 

 -0.325< Rg <+0.025 and assume a mean wind speed of 5*0 m sec"-^, an air-sea 

 temperature difference ranging between -k.oSC (sea warmer than air) and 

 +0.21C (air warmer than sea) can be predicted from equation (3)« The actual 

 maximum range (except for three observations in showers) observed was -2.88c 

 to +0.17C . It is, therefore, concluded that the most critical parameter in 

 determining stability is the wind speed and, more important, the stronger 

 the wind, the more closely governing conditions are met . 



