Windbreaks force wind upwards and also slow it down in a spiral pattern, which drops dust. This was felt to be desirable by some after the North American Dust Bowl disaster in the 30’s as some of the fertile dust could be returned to the field.
Solid objects such as a line of thick foliage that completely blocks the wind or a fence can create turbulence on the lee side of the wind. This is because low pressure or vacuum is created by the wind moving upward, and it is pulled into the area. If any wind gets through, it will be slowed down, but will prevent such a drop in pressure, and thus, prevent turbulence.
Wind must be able to move through a wind break to some degree. Stand on the lee side of a solid fence when wind is blowing on the opposite side of the fence, to experience this effect. Good architects know this - in Malibu, you will see concrete fences cutting down on the sometimes quite strong ocean breeze, but they will have holes in them. This causes much less wind effect than a solid wall, and also allows a view of the ocean. It helps block the western sun, but allows some cooling breezes through. There are hundreds of varieties of “breeze blocks,” created with concrete, brick, bamboo, metal or wood in the tropics or subtropics throughout the world.
Wood and iron breeze blocks.
Concrete breeze blocks.
A screen is an effective windbreak as it slows the wind down rather than stopping it or forcing it upward.
You’ll often see tree windbreaks in the plains that have three tree lines - shorter trees, mid size trees and tall trees. This pattern sends the wind upward and out over the field. The below diagram shows two things. One is that a wind break compresses the wind upward and can keep it above the field for 10-15 times the height of the windbreak. If a windbreak is 20 feet tall, this could protect 200 feet of field. It can also slow the wind down for that distance.
But if the tree line is too thick, low pressure will be created on the lee side and Ekman spirals or other turbulence can result.
A wind tunnel (an opening in a line of trees, or trees lining a driveway on both sides) can also create turbulence that can become an Overbeck Jet, exploding the wind throughout the space you’re trying to protect.
In the derecho (straight line wind storm, the equivalent of a hurricane) that passed through Iowa in 2020, vortexes created by the winds caused damage to thousands of trees, twisting them until they jaggedly broke. If you watch a tree in a hurricane wind, it whips back and forth in a pattern similar to a Karman vortex.
Note that this was on an open edge, and did effectively act as a windbreak to trees and houses behind it. When winds are more mild, there is still a spiral effect in branch movement but it provides benefits to the tree, like circulation, which reduces fungal pressure.
Windbreaks are one of the main practical ways I apply this information about these spiral patterns. Another application is to create oxygenation in water. Another is when handling erosion, these patterns help predict how fast, hard flowing water patterns will erode the landscape.
Both wind and water create similar patterns when striking objects. Observing the patterns in running water such as a stream can teach you something about how wind behaves as well.
Soapy water flowing over a silk thread.
Hands On Activity
Observe patterns in wind and water with different objects interfering with them. You can experiment with water by using a hose down a driveway or sidewalk or board, and placing objects of different sizes in the way. Or fill up your sink or bathtub and move the water with different objects and different speeds.
To notice the effects you can stand behind and beside objects when the wind is blowing. For example, if the wind blows along the front of a house there will be a higher air pressure there from that wind. If you seek shelter from the wind by going around the corner you will find it will be windy as well. That is because around the corner there will be less wind which means a lower pressure. A lower pressure will draw in some of the passing wind to equalize the pressure. (See Ekman spiral.) Nature always tries to equalize.
Getting to know the sometimes seemingly mysterious patterns of flowing wind and water allows you to predict what they will do in various scenarios and allow you to take advantage of these patterns.