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Shelter Belts and their Impact on Wind Speed, Solar Radiation, and Air Temperature Inversions (05/12/16)

Tree shelterbelts were planted to reduce near-surface wind speed, to decrease soil erosion or control snow drifting.

Shelter Belts and their Impact on Wind Speed, Solar Radiation, and Air Temperature Inversions

Tree shelterbelts were planted to reduce near-surface wind speed, to decrease soil erosion or control snow drifting. They are most effective when the wind blows perpendicular to them. For dense shelterbelts with little through-flow, winds are reduced greatly near the windbreak, but wind speed fully recovers at about 15 times the height of the trees downwind.

More open shelterbelts (single tree rows) allow more through-flow so wind speed is reduced less near the trees, but the downwind effect persists for about 30 times the shelterbelt height. Wind speed in both cases also is reduced upwind for a distance equal to three to four times the tree height.

Reduced wind speed near shelterbelts causes reduced air turbulence or mixing day and night. As a result, daytime surface and air temperatures in sheltered areas are greater, compared with those in open areas. However, when the surface is cooling at night, reduced mixing causes lower minimum temperatures, compared with open areas.

The result is often earlier inversion formation in the evening and later dissipation in the morning. Reduced turbulence also causes higher humidity and dewpoint temperature day and night. This combination often leads to earlier dew formation at night and slows evaporation in the morning.

However, this is somewhat determined by shelterbelt orientation. Shelterbelt orientation affects solar radiation. A crop planted on the south side of an east-west-oriented shelterbelt receives full sunlight, plus an extra 15 to 25 percent solar radiation that’s reflected from the shelterbelt. This causes greater surface and air temperatures, and greater evapotranspiration, compared with the cooler, shaded areas on the north side. Thus, Northside soils tend to be cooler and wetter, and dewpoint temperatures are greater than on the south side.

North-south-oriented shelterbelts also are affected even though they are symmetrical with respect to solar radiation. Plants on the east side of the windbreak receive full and reflected solar radiation in the morning. Some of this energy is used to heat the plants and air, and to evaporate any dew. The shaded Westside plants also are warming slowly and dew is evaporating slowly during the morning.

During the afternoon, the west side plants receive similar amounts of solar energy, but now all of this energy is used to further heat the plants and air. This causes greater afternoon plant and air temperature on the west side, compared with the shaded east side. As a result, the west side of the north-south shelterbelt will tend to be hotter and soils will be drier than on the east side. This may result in earlier inversion formation on the west side.

You can read more on this subject in the NDSU Extension publication: Air Temperature Inversions Causes, Characteristics and Potential Effects on Pesticide Spray Drift - AE1705 

Andrew A. Thostenson

Pesticide Program Specialist

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