Sculpting Invisible Air Currents with Windbreaks

You can control the wind to keep your house warmer in the winter by blocking the chilling effects of the wind or to cool your yard or house in the summer by scooping the prevailing breezes toward the house. The most common way to sculpt invisible air currents is to use a windbreak of trees.

A windbreak won't fit into every yard, however. To be effective, a well-designed windbreak requires a certain ratio of length and height. There are two ways to determine the length of a windbreak. First, a good windbreak is at least eleven times longer than its mature height. This keeps the wind from whipping around the ends of the windbreak, like a speedy back-eddy, at a greater speed than its original velocity. For a tree such as an oak, which can reach a height of 75 feet, this means a windbreak 825 feet long—not for your typical suburban lot! The second rule of thumb states that a windbreak should extend at least 50 feet on both sides of the area or house to be protected.

Ideally, the windbreak's height should be one-fifth to one-fifteenth the distance to be protected. In other words, the distance from your house to the windbreak should be five to fifteen times the height of the mature trees. While a windbreak can reduce the wind's speed by 50 percent at a distance of up to fifteen times its height, the best area of protection is a distance of about five to ten times the height of the windbreak. For example, an 8-foot hedge can provide a measure of protection up to 80 feet downwind. Plan your windbreak so that the mature height of the trees is sized to the area you wish to protect.

Windbreaks work best when their length is perpendicular to the prevailing winds. And they needn't have an angled slope of short to taller trees to be effective. In fact, vertical, narrow windbreaks are usually the most effective in lifting the strongest winds over the largest sheltered area. One row of the right tree is much more effective than a wide, multi-row planting.

Contrary to normal assumptions, allowing some air to pass through a windbreak reduces the wind's speed over the greatest distance and makes the best diversion. A completely solid windbreak causes some of the wind to whip up over the tops of the trees and then down to create a blustery vortex, like a sideways tornado, in the very area you're trying to protect. With a permeable windbreak, some of the wind can slip through to form layers of air, like an invisible form of the lamination found in plywood. This blanket of layered air helps to keep the blustery winds aloft after passing over the top of the windbreak. Remarkably, the most effective windbreaks are actually 50 percent permeable.

Other important considerations for windbreak designs include the following:

• Be sure to talk to a local nurseryman about the best windbreak trees for your soil and climate.

• Use evergreen trees for winter and summer protection. Deciduous trees help buffer only summer winds.

• Don't leave any large gap in the windbreak, such as a driveway, as the wind will be funneled through the opening at a speed up to 20 percent greater than its normal velocity.

• Make sure the windbreak is far enough from the house that it won't cast a shadow on south-facing windows during the winter.

• Remember, all the research and diagrams for windbreaks are based on flat land. Hilly property requires careful observation of the wind's patterns. Predicting the wind's flow over complex topography can't be done with simple two-dimensional drawings.

 

BIO: Robert Kourik's books, blogs, and articles bring science to gardening. He has designed gardens for three decades and is a pioneer of edible landscaping, the art of growing food plants among ornamental plants.  He works throughout California and the country. Visit Robert's blog Garden Roots or go here to buy his books.

 

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