proportion of it is displaced up and over the barrier, resulting in a jet of higher wind speed aloft. The remainder of the impinging airstream, having been retarded in its approach, now circulates through the barrier to its downstream edge, pushed along by the decrease in pressure across the shelterbelt's width; emerging on the downwind side, that airstream is now further retarded by an adverse pressure gradient, because in the lee of the barrier, with increasing downwind distance air pressure recovers again to the ambient level. The result is that minimum wind speed occurs not at or within the windbreak, nor at its downwind edge, but further downwind - nominally, at a distance of about 3 to 5 times the windbreak height H. Beyond that point wind speed recovers, aided by downward momentum transport from the overlying, faster-moving stream. From the perspective of the Reynolds-averaged Navier–Stokes equations these effects can be understood as resulting from the loss of momentum caused by the drag of leaves and branches and would be represented by the body force fi (a distributed momentum sink). Not only is the mean (average) wind speed reduced in the lee of the shelter, the wind is also less gusty, for turbulent wind fluctuations are also damped. As a result, turbulent vertical mixing is weaker in the lee of the barrier than it is upwind, and interesting secondary microclimatic effects result. For instance, by day sensible heat rising from the ground due to the absorption of sunlight (see surface energy budget) is mixed upward less efficiently in the lee of a windbreak, with the result that air temperature near ground is somewhat higher in the lee than on the windward side. Of course this effect is attenuated with increasing downwind distance and indeed, beyond about 8H downstream a zone may exist that is actually cooler than upwind. See also Agroforestry Buffer strip Dead hedge Desertification Energy-efficient landscaping Erosion control Great Plains Shelterbelt Hedgerow Macro-engineering Rain shadow Sahara Forest Project Sand fence Seawater Greenhouse Tugay References Wilson, 1985; Journal of Wind Engineering & Industrial Aerodynamics, Vol. 21 Argete & Wilson, 1989, Agricultural & Forest Meteorology, Vol 48 Bibliography External links National Agroforestry Center (USDA) Forestry and the environment Landscape ecology Agroforestry Agriculture