THE MICROBIOLOGY OF THE ATMOSPHERE 



would not be collected by drops less than i mm. in diameter, and I have 

 shown that drops of less than this diameter do not operate the bellows 

 mechanism of this species (cf. p. 60, and Gregory, 1949). According to 

 Langmuir, collection efficiency is at a maximum with drops of about 2 mm. 



No of 



drops 



5000 



IPOO 



500- 



Diameter of drops (mm) 



Fig. 15. — Number of raindrops of various diameters passing per second through i sq. 

 metre (horizontal), with rainfall of intensities varying from 0-5 to 25 mm./hour. (Smoothed 

 curves from data of Best, 1950). 



in diameter for all spore sizes, and is then about 25 per cent for spores 4/x 

 in diameter, and 80 to 90 per cent for spores 20 to 30 ju. in diameter. 



Nothing is known of the pick-up of non-spherical spores and how 

 properties other than size or terminal velocity affect collection by rain- 

 drops. The spores of Ustilago perennans, although not easily wetted by 

 water, are evidently readily collected by rain. The work of Burges (1950) 



86 



