THE ATMOSPHERE AS AN ENVIRONMENT 

 TRANSITIONAL OR OUTER FRICTIONAL TURBULENCE LAYER 



Here frictional turbulence, generated in the layer below, still domin- 

 ates vertical diffusion, but it dies out gradually until, at the top of the 

 layer, both turbulence and diurnal temperature changes disappear. Both 

 layers may be dusty, and the top of the transitional zone is sometimes 

 visible as a distinct dust horizon at 500-1000 metres, marking the upper 

 limit to which spores are raised by frictional turbulence (though much 

 greater heights may be attained by convection). 



In dynamical meteorology this zone is defined as the region where the 

 wind structure is determined partly by surface friction and partly by the 

 Earth's rotation. 



CONVECTIVE LAYER 



This layer extends from about i km. above the ground to the top of the 

 troposphere at about 10 km. As in all the layers constituting the tropos- 

 phere, the temperature continues to decrease with height to the top of the 

 convective layer, though diurnal temperature variation is almost absent. 

 Frictional turbulence does not reach here, but, as already indicated, 

 particles from the Earth's surface can be carried into this layer by large- 

 scale convection currents when the ground is heated by sunshine. 



The height above the ground attained by a mass of heated air before 

 it loses buoyancy, depends on the temperature gradient and water-vapour 

 content of the air at the time, as explained in the standard works on 

 dynamical meteorology. Ascent may be halted if there is a temperature 

 inversion layer in the upper atmosphere. Under conditions of thermal 

 instability, 'bubbles' of heated air may arise intermittently from areas 

 where the ground or vegetation is being heated by the sun. These bubbles 

 may rise to the convective layer, carrying spores and other particles as 

 well as water vapour to the level at which cumulus clouds are formed, and 

 at times reaching to the base of the stratosphere (Mason, 1957). 



NIGHT RADIATION AND TEMPERATURE INVERSION 



At night, wind speeds tend to diminish; the laminar boundary layer 

 then becomes thicker than by day and the turbulent boundary layer 

 may become thinner, being reduced to perhaps only 10 to 15 metres in 

 thickness. 



These changes may be carried still farther if the sky is cloudless, thus 

 allowing radiation from the ground to escape into space. Loss of heat by 

 radiation cools the ground and this in turn cools the air lying nearest to 

 the ground. Thus, instead of temperature decreasing with increasing 

 height, a 'temperature inversion' is set up: over the cold air near the 

 ground lies air at a higher temperature — up to a certain height, the top 

 of the inversion, above which the usual lapse rate is again encountered. 



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