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 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 next higher layer may reach down 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, 196o). 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.