44 DEACON AND WEBB [CHAP. 3 



of the order of a millimetre in thickness. Above this laminar layer there is a 

 transitional layer in which turbulent exchange increases rapidly and becomes 

 the over-riding process at the base of the region of fully developed turbulent 

 flow. Much of the difficulty in attempting theoretical treatment of the problem 

 of heat transfer between sea and air in relation to the temperature difference 

 between the two media (and the corresponding problems for water vapour, 

 momentum, etc.) resides in this complexity of regimes close to the surface, a 

 complexity aggravated by the wave motion at the interface. That much of the 

 resistance to transfer exists in the air layers close to the sea surface is exhibited 

 by the fact that typically more than half of the difference in temperature 

 between the sea and the air at, say, 5 m height takes place over the lowest few 

 millimetres. 



For diffusion through the laminar layer the following molecular transfer 

 coefficients of dimension L^T"! are operative for heat, momentum and water 

 vapour respectively : 



Thermometric conductivity k = thermal conductivity -=-/)Cp 

 Kinematic viscosity v = viscosity/p 



Diflfusivity of water vapour in air, D, 



where p is density and Cp is the specific heat at constant pressure. For air the 

 values of these coefficients are given in Table I, reproduced from Montgomery's 

 (1947) critical review of published values. 



Table I 



Density, Kinematic Viscosity and Thermometric Conductivity of Air and 

 Diflfusivity of Water Vapour in Air at 1000 mb 



The molecular transfer coefficients are inversely proportional to pressure at 

 any given temperature. Ratios of quantities in Table I are very nearly iil- 

 dependent of both temperature and, pressure and are as follows : 



v/k (Prandtl number) = 0.711 + 0.003 

 v/D = 0.596+0.008 



k/D = 0.84+0.01 



The specific heat, Cp, for dry air at 0°C has the value 1.004 x 10'^ erg g-i °C-i. 



