Wall a town to keep out the wind?
February 11, 2013 6:44 AM   Subscribe

Mad science/weather physicsfilter: I live in a small town in the Midwest right at the moment. It gets very, very cold, and most of the actual suffering part of the cold is due to high winter winds. Why can't towns like this just build city walls, ancient Athens-style, say 20 feet high or so (more?) to keep the wind out? There must be some obvious physics/ecology reason why this wouldn't work, someone explain it to me?
posted by paultopia to Science & Nature (20 answers total) 4 users marked this as a favorite
 
You can't keep the wind out.

An obstacle, like a wall, will create a low-pressure area behind it that just makes the wind blow in a different direction. Ever see someone with long hair driving a convertible? Air rushing against the windscreen creates a "vacuum" behind it, pulling the hair forward.
posted by three blind mice at 6:48 AM on February 11, 2013 [3 favorites]


You'll still have cold winds that blow in from above the wall. Walls only do so much to protect from wind; you also need a covering above.
posted by dfriedman at 6:48 AM on February 11, 2013


I can't think of a physics/ecology reason that's anywhere near as important as the psychological aspects of living in what is effectively a palisaded bailey. Whether or not the perimeter is 100% enclosed or not, it's still little more than a prison for those inside.

Anyway, what you describe is often practiced by more thoughtful homesteaders by planting rows of evergreens around the buildings and between fields.
posted by seanmpuckett at 6:50 AM on February 11, 2013 [5 favorites]


It would be very easy to create the phenomenon that is much of what makes Chicago nearly uninhabitable for most of the year, where structures that one might think would block the wind, slowing it down, only baffle it, speeding it up. A smart town would plant forests in extended parks just upwind.
posted by Blasdelb at 6:55 AM on February 11, 2013 [7 favorites]


You can do this with trees instead of walls. It is used in large-scale gardening. They're called wind breaks.
posted by vacapinta at 6:56 AM on February 11, 2013 [6 favorites]


If you think of the way downtown cities get winds in between the tall buildings -- the wind comes very strong in the gaps between buildings -- you can expand that to how it would work with a wall. Instead of coming straight at the town from outside, it will come down around the inside of the wall. Not sure if that would make it better/worse, but it wouldn't stop the wind for sure.
posted by DoubleLune at 6:57 AM on February 11, 2013


It can be done, in theory, but you need to adjust for scale. Mountains and large hills divert wind flow, and in nature this creates wind corridors and areas of wind acceleration. Large forests can also do the same. In built up areas tall buildings do this to a lesser degree. What you are talking about is less small scale wind breaks and more the creation of man made wind corridors, which don't really exist yet but may be more important/commercially attractive as oil prices rise and the cost of smaller scale wind power falls.
posted by MuffinMan at 7:06 AM on February 11, 2013 [2 favorites]


It is a scale thing. A 20 foot wall surrounding a good sized town is no barrier to wind. Think of about a typical NFL stadium. The field is surrounded by seating (walls) that rise up to great heights compared to the size of the field. And yet, if it is windy the wind still finds a way in....just in a swirling more turbulent way.
posted by Seymour Zamboni at 7:08 AM on February 11, 2013 [1 favorite]


Rule of thumb for farm windmill placing was to put it downwind 3× the height of the obstacle for unobstructed(ish) wind. So your 20' wall would be almost uselss 60' away.

Actually, MuffinMan, the buffeting wind you get between city blocks is pretty turbulent, and isn't really useful for generation, no matter what type of wind turbine you're being sold.
posted by scruss at 7:22 AM on February 11, 2013


Back when folks had patience they grew the wall.

Trees are better than a wall as they disturb the flow of the wind rather than creating a wind tunnel effect like walls and buildings.
posted by sammyo at 7:42 AM on February 11, 2013


To boil down the physics to the most essential point... You (and other people, it's a common misconception) seem to be imagining the are molecules as a bunch of little bullets. If you could just make a shield that blocked the little bullets, then you could stand behind the shield and they won't hit you, no matter the distance between you and the shield. This seems reasonable enough, and after all, that's how it would work if it were correct to think of the air molecules as little bullets.

But the fact is that the "bullet model" of aerodynamics doesn't work because air molecules hit each other far, far, far more often than they hit any solid object that they're flowing around. Thus air will flow around objects and fill in the area behind them. If one air molecule hits the wind screen, it then immediately hits a million other molecules and "communicates" the presence of the wind screen to them. The air will then flow around and fill in area behind the wind screen.

This is exactly the same as the description already provided upthread that an object creates an area of low pressure behind it that tends to pull the air flow into the area behind the object. I'm just describing the phenomenon in terms of the individual particle motions in the hope of fleshing out the concept a bit more.

The upshot is that unless you're prepared to build a wall whose height is between 1/3 and 1/10 the horizontal size of your town, then you should forget about walls.

You'll have better luck if you grow a forest in your town. The trees increase the thickness of the boundary layer between the surface of the earth and the wind at altitudes of hundreds or thousands of feet. You won't stop the wind, it'll still be howling over the trees, but the wind speed at ground level will be much lower.
posted by ngc4486 at 7:50 AM on February 11, 2013 [4 favorites]


I can't think of a physics/ecology reason that's anywhere near as important as the psychological aspects of living in what is effectively a palisaded bailey. Whether or not the perimeter is 100% enclosed or not, it's still little more than a prison for those inside.

Where I live in the Northeast U.S. there are noise barrier walls like these all over the place, anywhere there's a highway near a residential area, and people do not appear to be psychologically damaged by their presence.
posted by XMLicious at 7:51 AM on February 11, 2013


Seconding trees. Farmers often plant trees on the perimeter of their field as a windbreak to prevent soil and seeds from blowing off or onto their land.
posted by ceribus peribus at 7:54 AM on February 11, 2013


Here is a diagram of what happens when wind blows against a solid wall. The wind comes from the left, and it actually accelerates as it flows over the top of the wall (because more air is trying to squeeze into a smaller area -- much the way a river runs faster when it gets narrower). It then whips down at high speed into the space behind the wall. The sheltered area is relatively small, not at ground level, is surrounded by a higher-air-speed vortex, and in cold areas will get filled up with snow drifts. The turbulent zone will be if anything more unpleasant to be in than the areas of steady air flow, and will not extend a substantial distance beyond the wall anyway.

The windward (left) side of the wall actually will be more sheltered from the wind than the turbulent downwind side, but you have to be very close to the base of the wall for the effect to be worthwhile. And, again, snow drifts will build up here (though less than on the downwind side.)

Semipermeable wind breaks are much more useful than solid walls, in that rather than blocking the airflow completely they break up and absorb the wind's energy to some extent. Therefore trees.
posted by ook at 8:07 AM on February 11, 2013 [2 favorites]


Forgot to add: You could create a zone of relatively still air by building two walls relatively close together (the second wall would have to be before the vortex starts to pull down to the ground.) It would feel like living at the bottom of a well.

(Though if you did it just right you'd get a steady drone note like when you blow across the top of a bottle; that'd be kind of neat...)
posted by ook at 8:11 AM on February 11, 2013


Seconding trees, I live in a small town with towering old trees around my house that is one of the highest elevations in this hilly town. I love windy days where I can hear to wind whip through the trees. If I walk just a few streets away to low bungalows that have no trees the wind is very different and unpleasant. Microclimates for the win; even if you can't forest your whole town, encourage trees around your house.
posted by saucysault at 8:51 AM on February 11, 2013 [1 favorite]


Trees, by fluttering, flexing, and moving, are also taking some of the energy out of the wind, not merely buffering the pressure. I don't know if it's significant, it probably isn't, but maybe windmills? Then with each freezing blast, you can think how you're really the ultimate victor, and that asshole wind is warming your electric blanket waiting at home for you :)
posted by anonymisc at 10:25 AM on February 11, 2013


And build a lot of windmills to generate the electricity you need to heat your (very well-insulated) houses.
posted by mareli at 11:23 AM on February 11, 2013


I live in Long Beach, NY, which was one of many places hit hard by Sandy. The beach was flattened, the dunes washed away and now they took the boardwalk down. Everyone seems to agree that it's much windier (and I think my apartment is a bit colder). They built temporary dunes a while ago and now the wind seems less.

So it would seem one could simply wall off their town to reduce the wind.
posted by Brian Puccio at 3:30 PM on February 11, 2013


The reason that The Arbor Day Foundation, the world's oldest and perhaps most successful tree planting non-profit organization is still headquartered in what many think of as the nearly treeless state of Nebraska is simply that it was successful, early on, in encouraging Nebraska towns, cities, school districts, and individual farmers to plant trees in rows forming wind and snow breaks (also known as shelterbelts, some of which are designated scenic areas in the state) around major bridges and highway sections, and as a soil conservation strategy during the Dust Bowl era. The early projects were so successful that they became models for a large Federal program known as the Great Plains Shelterbelt. Later, they expanded into promoting tree planting as orchard stock for family farms, and as wildlife habitat, which still remain important small scale programs in Nebraska, and neighboring Plains states.

Trees, in large enough numbers, work amazingly well to mitigate the rampages of even continental scale winds.
posted by paulsc at 3:55 PM on February 11, 2013


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