the microbiology of the atmosphere 

 The Troposphere 



The troposphere is the name given collectively to the lower layers of 

 the atmosphere extending from the ground to a height of approximately 

 10 km., and is a region characterized by a decrease in temperature with 

 increasing height — the temperature lapse. 



Air is relatively transparent to the short-wave radiation of sunlight 

 which therefore heats the air very little as it penetrates the lower layers 

 of the atmosphere. On a sunny day, solar radiation falling on the Earth's 

 surface is in part temporarily absorbed, and in part reflected back as a 

 radiation of longer wave-length that is more readily absorbed by air. 

 This reflected radiation now heats the layer of air near the ground and the 

 heat later becomes diffused through the lower layers of the atmosphere 

 from below upwards. Air temperature is thus highest near the ground 

 and decreases with increasing height, unless a 'temperature inversion' is 

 formed under conditions described below. The normal temperature 

 decrease (or 'lapse rate') is about o-6°C. per lOo metres. At the top of the 

 troposphere is the tropopause — the boundary between troposphere and 

 stratosphere. 



The troposphere comprises the five following layers. 



laminar boundary layer 



In contact with the surface of the earth and all projecting bodies is a 

 microscopically thin layer of air held firmly by molecular forces. Except 

 for molecular diffusion this layer is still and windless. 



Above this windless film the atmosphere is usually in motion, set 

 going either by pressure differences of distant origin, or by convection 

 currents produced by local heating. The lowest layer of moving air, next 

 to the still layer, is known as the 'laminar boundary layer' (or sometimes 

 the ' laminar' layer). This again is a thin layer, of the order of a millimetre 

 thick, in which there is no turbulence and the air flows in streamlines 

 parallel to the nearest surface; heat, gases, and water vapour can move 

 across the streamlines by molecular diffusion. Wind speed is negligible 

 at the still surface film, and in the laminar boundary layer wind speed 

 increases linearly with height (momentum being transmitted through the 

 layer by molecular diffusion only). Particles, droplets, or spores getting 

 into the laminar layer will sink through it, following trajectories deter- 

 mined by wind speed and gravity, and will come to rest at the Earth's 

 surface. 



A laminar layer also exists at the interface around any solid body, and 

 much of the foregoing description applies equally to the air layer at the 

 surface of a leaf or stem. 



The thickness of the laminar boundary layer varies with the wind speed 

 and with the roughness of the adjacent surface. In a high wind it may be 

 thinned down to a fraction of a millimetre, and turbulent air from the 



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