Woods 



HOLE 



Mass.* 



X 



THE UPPER-AIR SPORA 



The air-spora near the ground is dominated by fluctuations in its im- 

 mediate local sources. In the upper air, however, the effects of local sources 

 are smoothed out and attention can be focussed on organisms under- 

 going long-distance transport. Concentrations in the upper air are sparse, 

 the necessity of keeping samples free from contamination is paramount, 

 and sterile technique for enumerating the microscopically small particles 

 becomes exacting when they are exceedingly dilute. 



Vertical Diffusion 



Whereas spores and pollen grains are heavier than air, and tend to fall 

 under the influence of gravity, atmospheric turbulence and convection 

 tend to work in the opposite direction. As a result, the atmosphere is in a 

 sense a spore suspension that generally decreases in concentration from 

 ground-level up to the base of the stratosphere. Eddy diffusion will bring 

 spores to the top of the outer frictional turbulence layer: above this, 

 convection will operate and, in the upper part of the troposphere, we 

 would expect to find mostly components of the day-time spora. When it 

 first became possible to explore the air overhead, it was a matter of surprise 

 to find how far up microbes could go. As methods have been developed 

 for exploring greater and greater heights, we can begin to form a picture 

 of the changes in concentration with height and of the circulation of spores 

 of micro-organisms over the surface of the globe. 



Evidence that concentration decreases with increasing height comes 

 from two distinct sources of information which have often been confused : 

 (i) observations at a standard height above local ground-level at a chain 

 of stations differing widely in altitude above sea-level; and (2) observations 

 at widely different altitudes above local ground-level at a single station. 



GROUND STATIONS AT DIFFERENT ALTITUDES AB0\T: SEA-LEVEL 



Observ-ations in this category are extremely fragmentary and have the 

 flavour of holiday tasks on fine days in summer. Samples at various 

 altitudes are taken at successive times as the climber reaches a suitable 

 station— as in Pasteur's visit to the Mer de Glace, where the relative 

 purity of mountain air was convincingly demonstrated {see p. 4). Using a 

 volumetric method, Miquel (1884, p. 524) confirmed this conclusion. 



131 



