A windbreak (shelterbelt) is a

planting Sowing is the process of planting seeds. An area or object that has had seeds planted in it will be described as a sowed or sown area. Plants which are usually sown Among the major field crops, oats, wheat, and rye are sown, grasses and leg ...

usually made up of one or more rows of

tree In botany, a tree is a perennial plant with an elongated stem, or trunk, usually supporting branches and leaves. In some usages, the definition of a tree may be narrower, including only woody plants with secondary growth, plants that are ...

s or

shrub A shrub (often also called a bush) is a small-to-medium-sized perennial woody plant. Unlike herbaceous plants, shrubs have persistent woody stems above the ground. Shrubs can be either deciduous or evergreen. They are distinguished from tree ...

s planted in such a manner as to provide shelter from the

wind Wind is the natural movement of air or other gases relative to a planet's surface. Winds occur on a range of scales, from thunderstorm flows lasting tens of minutes, to local breezes generated by heating of land surfaces and lasting a few ...

and to protect soil from

erosion Erosion is the action of surface processes (such as water flow or wind) that removes soil, rock, or dissolved material from one location on the Earth's crust, and then transports it to another location where it is deposited. Erosion is d ...

. They are commonly planted in

hedge A hedge or hedgerow is a line of closely spaced shrubs and sometimes trees, planted and trained to form a barrier or to mark the boundary of an area, such as between neighbouring properties. Hedges that are used to separate a road from adjoi ...

rows around the edges of fields on

farm A farm (also called an agricultural holding) is an area of land that is devoted primarily to agricultural processes with the primary objective of producing food and other crops; it is the basic facility in food production. The name is use ...

s. If designed properly, windbreaks around a home can reduce the cost of heating and cooling and save

energy In physics, energy (from Ancient Greek: ἐνέργεια, ''enérgeia'', “activity”) is the quantitative property that is transferred to a body or to a physical system, recognizable in the performance of work and in the form of ...

. Windbreaks are also planted to help keep

snow Snow comprises individual ice crystals that grow while suspended in the atmosphere—usually within clouds—and then fall, accumulating on the ground where they undergo further changes. It consists of frozen crystalline water throughou ...

from drifting onto roadways or yards. Farmers sometimes use windbreaks to keep snow drifts on farm land that will provide water when the snow melts in the spring. Other benefits include contributing to a

microclimate A microclimate (or micro-climate) is a local set of atmospheric conditions that differ from those in the surrounding areas, often with a slight difference but sometimes with a substantial one. The term may refer to areas as small as a few squ ...

around

crop A crop is a plant that can be grown and harvested extensively for profit or subsistence. When the plants of the same kind are cultivated at one place on a large scale, it is called a crop. Most crops are cultivated in agriculture or hydropon ...

s (with slightly less

drying Drying is a mass transfer process consisting of the removal of water or another solvent by evaporation from a solid, semi-solid or liquid. This process is often used as a final production step before selling or packaging products. To be consid ...

and chilling at night), providing

habitat In ecology, the term habitat summarises the array of resources, physical and biotic factors that are present in an area, such as to support the survival and reproduction of a particular species. A species habitat can be seen as the physical ...

for wildlife, and, in some regions, providing

wood Wood is a porous and fibrous structural tissue found in the stems and roots of trees and other woody plants. It is an organic materiala natural composite of cellulose fibers that are strong in tension and embedded in a matrix of lignin ...

if the trees are harvested. Windbreaks and

intercropping Intercropping is a multiple cropping practice that involves growing two or more crops in proximity. In other words, intercropping is the cultivation of two or more crops simultaneously on the same field. The most common goal of intercropping is ...

can be combined in a farming practice referred to as alleycropping, or being deployed along riparian buffer stripes. Fields are planted in rows of different crops surrounded by rows of trees. These trees provide fruit, wood, or protect the crops from the wind. Alley cropping has been particularly successful in India, Africa, and Brazil, where coffee growers have combined farming and forestry. A further use for a shelterbelt is to screen a farm from a main road or motorway. This improves the farm landscape by reducing the visual incursion of the motorway, mitigating noise from the traffic and providing a safe barrier between farm animals and the road. Fences called "windbreaks" are also used. Normally made from

cotton Cotton is a soft, fluffy staple fiber that grows in a boll, or protective case, around the seeds of the cotton plants of the genus '' Gossypium'' in the mallow family Malvaceae. The fiber is almost pure cellulose, and can contain minor pe ...

nylon Nylon is a generic designation for a family of synthetic polymers composed of polyamides ( repeating units linked by amide links).The polyamides may be aliphatic or semi-aromatic. Nylon is a silk-like thermoplastic, generally made from pet ...

canvas Canvas is an extremely durable plain-woven fabric used for making sails, tents, marquees, backpacks, shelters, as a support for oil painting and for other items for which sturdiness is required, as well as in such fashion objects as handbag ...

, and recycled sails, windbreaks tend to have three or more panels held in place with poles that slide into pockets sewn into the panel. The poles are then hammered into the ground and a windbreak is formed. Windbreaks or "wind fences" are used to reduce wind speeds over erodible areas such as open fields, industrial stockpiles, and dusty industrial operations. As erosion is proportional to wind speed cubed, a reduction of wind speed of 1/2 (for example) will reduce erosion by 87.5%. Sheltered, windless areas created by windbreaks are called wind shadows.

Windbreak aerodynamics

In essence, when the wind encounters a porous obstacle such as a windbreak or shelterbelt, air pressure increases (loosely speaking, air ''piles up'') on the windward side and (conversely) air pressure decreases on the leeward side. As a result, the airstream approaching the barrier is retarded, and a 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 The Reynolds-averaged Navier–Stokes equations (RANS equations) are time-averaged equations of motion for fluid flow. The idea behind the equations is Reynolds decomposition, whereby an instantaneous quantity is decomposed into its time-averaged ...

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 ''f_i'' (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 8''H'' downstream a zone may exist that is actually ''cooler'' than upwind.Argete & Wilson, 1989, Agricultural & Forest Meteorology, Vol 48

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)
* {{Authority control Forestry and the environment Landscape ecology Agroforestry Agriculture

Windbreak aerodynamics

See also

References

Bibliography

External links