THE MICROBIOLOGY OF THE ATMOSPHERE 



instantaneous or continuous. Furthermore, we may have an 'area source' 

 (emitting Q_ spores per square centimetre), or a 'volume source'. Real 

 sources in the field that correspond approximately to these ideal sources 

 would be a single plant (point), a hedge (line), a ground crop (area), and 

 an orchard or forest stand (volume). The dimensions of the source must be 

 treated as relative to their distance: thus a field would be regarded as 

 effectively a point source when considered from distances many times its 

 own width. 



Fig. 6. — Diffusion of spore-cloud during horizontal travel in wind. O = origin of 

 co-ordinates at source of liberation; x, y, z= down-wind, cross-wind, and vertical axes, 

 respectively. Growth of cloud is measured by increase in standard deviation after the centre 

 of the cloud has travelled to three positions down-wind. 



All these sources may be instantaneous or continuous. The cloud from 

 an instantaneous point source is a puff or spherical cloud, whereas the 

 conical cloud arising from a continuous point source is familiar in the 

 smoke plume from a chimney. A continuous point source can be viewed 

 as made up of a succession of overlapping instantaneous emissions. 



(iv) Standard deviation. Suppose that a 'puff' of spores has been liber- 

 ated at an instant from a point source into a wind and has become subject 

 to the action of atmospheric eddies which move individual spores apart 

 at random. After a short time the particles composing the cloud will show 

 a scatter around their origin (Fig. 6). At any instant such a cloud has two 

 characteristics which we could compute if we had all the data: (i) the 

 mean position of the particles, i.e. the centre of the cloud, which can be 

 expressed as a point on the system of x, y, and z ordinates ; and (2) the 

 standard deviation, o-, of the particles from their mean position. When the 



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