THE MICROBIOLOGY OF THE ATMOSPHERE 



As the air is coldest and densest at the bottom of the inversion, gravity 

 tends to prevent it from ascending and mixing with warmer air above. 

 The air in the inversion becomes stratified according to temperature and 

 remains very stable, in contrast to the instability that is apt to develop 

 when the ground is heated. Such a layer of cold, heavy air may flow slowly 

 downhill as a nearly laminar katabatic wind, filling hollows with cold air 

 and aiding the formation of frost pockets (Geiger, 1950, p. 203). 



In a temperature inversion, spores and dust particles tend to settle 

 out, leaving the air relatively clean although the air above the inversion 

 may continue to carry a normal spore-load. 



ROLE OF CONVECTION 



When the surface of the ground is heated by sunshine the lowest 

 layer of air may be heated in turn. If a large temperature lapse-rate is 

 established, the atmosphere becomes unstable, because the less dense 

 ground-layer of air tends to rise and carry its load of microbes upwards, 

 being replaced by cooler air from above. The pattern of this overturn 

 is not yet clear. A regular 'cellular' pattern of ascending and descending 

 air has been suggested, but more recently the ascending air has been 

 pictured as taking the form of 'chimneys' or of 'bubbles'. 



Glider pilots are familiar with the properties of warm ascending 

 currents of air or 'thermals', as described by Yates (1953). In still air a 

 glider sinks at about 90 cm. per sec. (about 20 times the terminal velocity 

 of a pine pollen grain). On dull days thermals do not develop. They reach 

 their maximum upward velocity of 3 metres per sec, or up to 25 metres 

 per sec. in cloud, at midday in summer. Yates indicates that, depending 

 on the type of soil, on Mind strength, and on sun height, a sizeable thermal 

 is released from an area of 1-25 sq. km. every 5 to 15 minutes in summer. 

 At a height of 300 metres, thermals may be 300 metres in diameter, 

 though they are probably often smaller when lower, whereas at still 

 greater heights a diameter nearer 2 km. was reported by Ludlam & Scorer 

 (1953). Their vertical movement may cease at a temperature inversion, 

 or may continue to from 10,000 to 50,000 ft. The temperature in a thermal 

 appears to average i°-2°C. higher than the surrounding air through which 

 it is ascending. 



The theory of thermals is still a matter of controversy, but there seems 

 no doubt that air rises from some surfaces more often than from others. 

 Green vegetation and wet soils may be relatively cool, while a ripe cereal 

 crop, buildings, roads, or bare rock, may heat up rapidly in the sun and 

 become the source of rising warm air. Thermals can also arise at a cold 

 front, and glider pilots regard hilly country as the best source of thermals. 



The pattern by which cool air sinks to replace the ascending warm air 

 is also a matter of debate. Downward draughts reported in the neigh- 

 bourhood of thermal upcurrents appear to be comparatively feeble. 

 Probably the downward movement occurs over a much wider area than 



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