Such permeable screens and hedges allow enough air to filter through, avoiding anything approaching a vacuum that creates turbulence. Wind speeds are at the same time adequately reduced for practical purposes. CAUSE AND EFFECT The next aspect of the wind behaviour that needs consideration is the extent of the effect of barriers or shelter on wind speed and direction. Taking 'FT as the height of the barrier, the speed of wind striking a barrier at right angles is reduced to a distance of some 30 H to leeward and some 5 H to windward. In other words, if no other barrier is present the wind reaches its original speed at 30 H to the lee of the barrier, but there is only a noticeable reduction up to about 7 H. Thus a screen юА(зт) high will considerably reduce the speed of the wind up to some 7oft(2im) and the wind will steadily increase in speed up to 30oft(c)om) when the original speed will be regained (see Fig.54). An understanding of the theory of wind behaviour is necessary to plan new hedges or screens to best advantage if there is a choice of planting plans. Such knowledge can be put to good use when planning new I plantings or deciding whether to move or uproot existing plants. Most I gardeners will also want to take into account the overall design of the I garden-the need to hide particular eyesores, provide backgrounds and fill gaps between buildings or established plants. NOISE SUPPRESSION In addition to the advantages already referred to, ] hedges and screens can reduce noise to a limited extent but as noise and wind barriers they need very different properties. Wind barriers are best when slightly permeable and sited close to the area to be sheltered. Noise barriers are best when broad, solid and thick and near the source of the noise. But one must state that trees are not very efficient in this respect. Nevertheless, all hedges and screens muffle some sound and may reduce the distress caused by noise if only by the psychological effect of shutting out the sight of the source of the noise. Recent research, repeated in this country, has shown that to be effective a very wide filter screen of trees is i needed, certainly not less than 45ft(i4m) and probably up to 15011(4514 Shrubs-particularly evergreens-by providing thicker cover low down can be more effective than trees with bare trunks low down. NATURAL AND MAN-MADE BARRIERS What can be done to provide such hedges and screens? They can range from the formality of artificial fences and severely clipped hedges to lines or groups of trees or shrubs of a single or several similar species. All need to be able to withstand the strongest winds to be expected on the site. Loss of shelter during a violent storm not only causes annoyance and involves the cost of replacement but can result in the loss of the cherished plants that were being protected. Wooden fences and trellis of assorted types are available in most districts and can form useful temporary screens against which more permanent screens of trees or shrubs may be raised.