A belt of trees and/or shrubs arranged as a protection against strong winds; a type of windbreak. The trees may be specially planted or left standing when the original forest is cut. A shelterbelt decreases the force of the wind near the ground, both upwind for a distance of up to six times the height of the barrier, and downwind for a distance of fifteen to twenty times the height. These ratios are roughly constant, irrespective of the height of the belt. The lowest wind speed is found downwind at a distance of three or four times the height of the belt. If the trees are too dense, the air beyond this quiet zone is often turbulent, with downdrafts that may flatten crops. The best form is a moderately dense belt of mixed conifers and deciduous trees, five to ten yards wide, containing at least three rows of trees at right angles to the prevailing winds. A system of shelterbelts to give protection to crops over a large area should be planted at intervals of about twenty-five times their expected height.