THE ATMOSPHERE AS AN ENVIRONMENT 



next higher layer may reach do\Mi nearly to the surface. In very calm 

 weather the laminar layer may thicken considerably. 



In comparison with the relatively equable air at a metre or two above 

 the surface, the eco-climate of the laminar boundary layer is violently 

 changeable (Monteith, i960). Unless protected by a layer of vegetation, 

 small organisms at ground-level may be subject to extreme heat from the 

 sun's rays by day, followed by a rapid drop in temperature as heat is lost 

 by radiation to a clear sky at night. 



The laminar boundary layer acts as a dust trap. Particles which have 

 sunk through it and come to rest in the still or slowly moving air at the 

 surface, are out of reach of eddies — until some unusual condition arises 

 which thins the laminar layer enough for eddies to penetrate down and 

 sweep away the dust particles. High winds may do this ; or local heating of 

 the surface, perhaps on a micro-scale, may set up 'dust-devils' — smaller 

 or larger whirlwinds raising dust into the air. 



LOCAL EDDY LAYER 



For biological purposes we need to add the 'local eddy layer'. Even in 

 streamlined air, local stationary eddies may exist behind small rough- 

 nesses; and, as will be shown on page 35, air-flow over a cup-shaped 

 depression may set up a rotation pattern sufficient to throw dust up from 

 the bowl. This layer is probably important in nature, where ideally smooth 

 surfaces are rare. A special r\'pe of boundary at the top of a plant layer or 

 crop has been called the 'outer active surface', or, in forests, the 'crown 

 layer'. 



TURBULENT BOUNDARY LAYER 



In this layer, where flux of momentum decreases linearly with height, 

 solid obstacles, arising at the surface in the laminar boundary layer, 

 project into the wind and cause eddies which break away from the surface 

 and travel downwind. A surface is aerodynamically smooth in conditions 

 when the laminar layer is thick enough to submerge projections from the 

 surface; but if the irregularities project through the laminar layer, the 

 surface is considered rough. As the thickness of the laminar layer depends 

 both on the wind speed and the stability of the atmosphere, it is clear 

 that a particular surface such as a grass sward or a hairy leaf may be 

 aerodynamically smooth under one set of conditions and rough under 

 another. Each surface has a characteristic roughness parameter. Air-flow 

 over calm water may be smooth ; but, except at extremely low wind-speeds, 

 flow over land is normally rough and disturbed by surface irregularities 

 which cause turbulence. 



Eddies of two types may occur : local or stationary eddies which may 

 arise on both the windward and leeward sides of a bluff" obstacle, and 

 eddies which break away and travel \^ith the wind in the obstacle's wake, 



25 



