THE MICROBIOLOGY OF THE ATMOSPHERE 



perhaps the most reliable records — because hailstones can be surfiice- 

 sterilized. The first rain after a dry spell is heavily contaminated and, 

 even during prolonged wet weather, the spore numbers in rain remain 

 substantial. 



A spore liberated near ground-level has a high probability of being 

 deposited dry; but wash-out by rain, hail, or snow, probably mosj: often 

 terminates the journey of spores reaching the tail-end of the dispersal 

 gradient. Microbial sampling of precipitation is still in the naive stage. 

 Methods have not been tested, and we still do not know how a collecting 

 vessel should be placed to avoid contamination from soil and vegetation. 



Conceivably, spores may undergo re-concentration ^^■ithin a cloud. 

 Rising convection bubbles may bring new spores to the top of the cloud, 

 where they can be collected and washed down in raindrops to the base 

 of the cloud. Here the drops might evaporate, allowing the spores to be 

 carried up again — perhaps eventually to be brought down to earth in 

 hailstones. The abundance of microbes in hail, and the reports of a 

 'biological zone' at several thousand metres, supports the suggestion that 

 convective clouds may be spore-concentrators. 



Exploration of organisms in precipitation needs an experimental study 

 of methods of sampling from ground, ships, and aircraft. Systematic 

 sampling could then be attempted with some prospect of learning what 

 part such precipitation plays in terrestrial microbial circulation. 



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