Profiles of wind speed over extended surfaces

Profiles of wind speed over extended surfaces 203 

A Lycopodium spore, or pollen grain of similar size, has an energy of 1(T4 ergs (10-11 J) in a wind speed of 0.75 m s-1, which is typical of conditions near the top of crop canopies or in open woodland. Hence it is likely that pollen grains will bounce off dry leaves or stems, but will be trapped on the stigma of flowers, which are sticky. Spores of plant pathogens, however, are usually themselves moist or sticky, enabling them to adhere to leaves on impact. 

Before considering experimental and theoretical data on transfer of vapours and particles to crop canopies, it is necessary to give a highly simplified discussion of boundary layer theory. 

The shearing stress, r, exerted by the wind on the ground entails a downwards flux of momentum. In the aerodynamic boundary layer above the surface, the momentum is transferred by the action of eddy diffusion on the velocity gradient. The friction velocity is defined by w = t/pa and is a measure of the intensity of the turbulent transfer. Near to a rough surface, the production of turbulance by mechanical forces 

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