Porosity is recognized as the most important single parameter describing shelterbelts. However, porosity is a very difficult variable to measure or define. This paper describes a method for categorizing wind breaks in terms of porosity the wind sees, using only measured minimum leeward-wind velocity.
Wind velocities at a height of 1.4 m were measured behind shelterbelts of different porosity. A double row of hardwood trees (high porosity), a medium porosity mixed stand, and a dense pine stand (low porosity) were used. Wind velocity declined to a minimum at 1--4 tree-heights leeward of the trees and then gradually increased to open-field velocity. The field data show that the wind measurements could be made at any height without affecting relative reduction in velocity. The data indicate that the minimum relative wind velocity behind the shelterbelt could be used as an index to compare the porosities of different shelterbelts.
Comparisons were made to Dyunin's analysis for surface-wind velocities leeward of uniformly porous barriers. The analysis indicates that tree-stand porosity is the dominant parameter governing the wind reduction obtained, while the range of sheltering is controlled by the amount of local turbulence and the turbulence present in the atmosphere beforehand.
INTROD UCTION
Wind breaks have been used many years for crop protection, soil erosion, and increased snow deposition. Numerous studies describe the effect of wind breaks on various atmospheric parameters such as temperature, evaporation, wind velocity, water-vapor content, and carbon dioxide concentration. But no suitable method has been found to compare the results of tests using different wind breaks. As mentioned in the abstract, porosity is recognized as the most important single parameter describing shelterbelts. However, porosity is a very difficult variable to measure or define. This paper describes a method for categorizing wind breaks in terms of porosity the wind sees, using only measured minimum leeward-wind velocity.