Ill 



THE ATMOSPHERE AS AN ENVIRONMENT 



Aerobiology is a synthesis: just as the geographer draws upon astronomy 

 and geology, so the aerobiologist draws upon many sources. To under- 

 stand the environment of the air-spora we must go to meteorology. A 

 fuller account of the relevant features of the atmosphere than that given 

 here can be found in works on dynamical meteorology, e.g. SutclifTe 

 (1940), Geiger (1950), Sutton (1953), and U.S. Weather Bureau (1955), 

 the last including many excellent diagrams. 



The Atmosphere and its Layers 



The atmosphere is usually recognized as layered; some of its main 

 features are illustrated in Fig. 3, in which altitude is drawn on a logarith- 

 mic, instead of a linear, scale in order to allow the various layers to be 

 represented together on one page and to illustrate vividly how the proper- 

 ties of the atmosphere change most sharply near the ground. 



Barometric pressure, density of the air, and (as a rule) temperature, 

 decrease with increasing height above the Earth's surface. These changes 

 are all quantitatively important in aviation, and calculations are based on a 

 table of an agreed 'International Standard Atmosphere'. Changes with 

 altitude in temperature, humidity, density, and viscosity will be complex 

 in their effect on a suspended spore, but are not likely greatly to affect 

 its terminal velocity. 



The three vertical panels of the diagram represent conditions in 

 contrasting weather types. The central panel represents a dull, windy day, 

 with a cloud layer shielding the ground from direct sunlight (conditions 

 on a cloudy night are not very different). The right-hand panel represents 

 a sunny day, and the left-hand panel a still, cloudless night. The thickness 

 of each individual layer of air varies according to conditions ; the boun- 

 daries between them vary in definiteness: sometimes transitions are imper- 

 ceptible, but there is sometimes even a visible interface between layers. 

 The layers are variously named in the literature and this may be confusing 

 unless the following approximate equivalents are borne in mind {see p. 23). 



It is convenient to describe these layers in the reverse order, from 

 ground-level upwards, beginning in the troposphere with the laminar 

 boundary layer. 



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