THE MICROBIOLOGY OF THE ATMOSPHERE 



The forward velocity of a turbulent wind is thus the net result of a com- 

 plex movement; the wind has vertical and lateral components as well as 

 the horizontal movement. Further, vertical and horizontal turbulence 

 may differ in intensity (non-isotropic turbulence). 



Occurrence of mechanical or frictional turbulence depends on the 

 wind speed being high enough, and the object large enough, to cause 

 eddying. Whether or not flow will be turbulent can be calculated by the 

 method of Osbert Reynolds, who found that flow is turbulent when the 

 Reynolds number, 



, _ , length X wind velocity , , 



Re, denned as — 7-. -. — ; -. , exceeds about 2,000. 



kmematic viscosity 



Here 'length' is taken as a characteristic dimension of the object, and 

 kinematic viscosity for air under average surface conditions is 0-14 cm^. 

 sec.~^ Thus for a leafy bush 100 cm. high in a wind of 100 cm. per sec. 

 we have 



„ 100 cm. X 100 cm. sec.-^ 



Re = 2 = 7,100 



0-14 cm."^ sec.~^ 



so flow would be expected to be turbulent. 



In the turbulent boundary layer, properties such as temperature, 

 amount of water vapour, and wind velocity, change much less rapidly 

 with increasing height than in the laminar boundary layer beneath. Eddies 

 mix the different parts of the layer much more rapidly than do the slow 

 processes of molecular diffusion. Particles can also be carried by eddies 

 upwards and laterally in a manner impossible in the laminar layer. In the 

 turbulent boundary layer the wind velocity, temperature, and amount of 

 water vapour show a change which is linear with the logarithm of the height. 

 In this layer diurnal changes of temperature are less pronounced than 

 in the laminar boundary layer underneath, and diurnal changes decrease 

 still further with increasing height until, at the top of the next layer, they 

 have almost disappeared. 



An increase in wind-speed increases the thickness of the turbulent 

 boundary layer both downwards, by thinning the laminar boundary 

 layer, and upwards, by pushing into the transitional layer as turbulence 

 increases. The turbulent boundary layer is thinnest on clear calm nights 

 and thickest on hot sunny days, when it may reach to a height of 150 

 metres. 



The turbulent boundary layer is the part of the atmosphere most 

 familiar to us. While our feet are planted in the violently fluctuating 

 climate at ground-level, our heads, and the weather-recording instruments 

 of the conventional Stevenson's screen, inhabit the relatively equable 

 turbulent layer. 



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