XV 



AEROBIOLOGY 



The literature of aerobiology is scattered, but extensive reference lists 

 are given in the following works: Cunningham (1873), Heald (1913), 

 Gardner (191 8), Sernander (1927), Stepanov (1935), Rempe (1937), 

 Pettersson (1940), Craigie (1941), Moulton (1942), duBuy et al. (1945), 

 Gregory (1945), Stakman & Christensen (1946), Wolfenbarger (1946, 

 1959), Jacobs (1951), Maunsell (1954), Wells (1955), Werff (1958), and 

 Hirst (1959). 



It now remains to review the conclusions of each of the preceding 

 chapters in the light of the whole, to consider their implications, and to 

 attempt a synthesis. 



The Phenomena 



Airborne microbes, whether carried singly, in groups, or on 'rafts', 

 are heavier than air. In still air they fall under the influence of gravity, 

 with constant terminal velocities ranging from 0-05 to 150 cm. per sec, 

 according to their size and density. This falling would lead to their sedi- 

 mentation out of the air if other forces did not oppose gravity. Two atmos- 

 pheric processes tend to prevent sedimentation through the quiet bound- 

 ary layer : turbulent diffusion in wind carries the spore-cloud horizontally, 

 at the same time diffusing it both horizontally and vertically; 

 and thermal convection can carry a spore-cloud to great heights in 

 the troposphere. Modes by which spores cross the boundary layer 

 of air near the Earth's surface and reach the turbulent wind layer, are 

 therefore of prime importance in the dispersal of microbes. Energy for 

 'take-off' may be supplied by the organism itself or may come from external 

 sources supported by a wide variety of mechanisms, such as wind or rain- 

 splash, and factors controlling the take-off mechanism also control the 

 occurrence of spores in the air. The more an organism is specialized 

 towards one dispersal route, the more unfit it becomes for dispersal by 

 another route. For many purposes, knowledge of the 'take-off' mechanism 

 is important. 



Rain-splash dispersal results in a local scatter because the larger splash 

 droplets, which are less easily carried by the wind, pick up more spores 

 than do smaller droplets. 



Frictional turbulence will diffuse the spore-cloud to the top of the 



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