Tuesday, June 28, 2011

Breaking Wind

If you are cool enough to have garnered an invite to our place you know that we have a pretty sweet (yet somewhat small) south facing roof deck which is the perfect place to grow potted plants, exposing them to unobstructed sunlight while safely keeping them out of the reach of curious neighborhood kids and peeing dogs.

There is one major drawback however, our garden located 10 stories above street level where it considerably more windy than traditional terrestrial gardens. This can be a problem considering tomato plants usually can’t support their own weight let alone stand up to a constant onslaught of high winds.

A little about wind:

The wind speeds you see on the local weather reports are calculated as the average speed recorded 10 meters above ground level over the course of 2 minutes. A wind gust is the highest instantaneous wind speed recorded over 10 minute window. This can be a good source of information if you have a garden in a large open area at ground level (ie the burbs). However, wind speeds increase the higher you are in the troposphere (the lowest layer of the atmosphere). So the more stories above ground level the more wind you will have to deal with. This phenomenon can be observed by looking at the swaying of treetops and the racing of clouds through the sky on what feels like a calm day at ground level.

An additional challenge for city rooftop gardens is the urban canyon effect. The urban canyon effect is created by city streets that cut through large blocks of densely populated tall buildings. Air is funneled through the smaller street area creating faster and more turbulent winds. You have probably experienced the canyon effect in the avenues of New York City. While DC’s buildings pale in comparison to the skyscrapers of NYC, they are still tall enough to speed up the wind.

At the rooftop, the naturally occurring faster wind and the canyon effect wind come to a head. The wind speeds up even more as the air is forced over the top of a building. As the air makes its way over the edge of the roof it begins to swirl adding to the turbulence. This is why on roofs wind feels as if it comes from all directions at seemingly random bursts of speed. Check out this virtual wind simulation to see rooftop wind turbulence in action. 

So how does one overcome this windy problem? Windbreaks have been used since the beginning of agriculture. Farmers have long planted rows of trees or shrubs throughout their fields to diffuse damaging winds and in turn reduce soil erosion and wind damage to crops. According to a couple of farm sources, a windbreak with a density of 60% or more would get the job done. For rooftops it is not a good idea to do a solid windbreak like a piece of plywood because it can blow over onto your plants or catch a gust and fly off the roof. Plus a solid piece will only add to the turbulence as air makes its way over the windbreak. The goal for a rooftop windbreak is to diffuse the turbulent gusts into something more like surface winds which  the plants can handle. 

I began searching craigslist for used picket fencing. The results had me paying to do people's yard work (taking down old fencing) way out in the country, so I began trolling the large home improvement stores for fencing options. Turns out actual fencing is expensive and not worth the investment considering our modest anticipated produce yield and fact that we are renting. Plus the logistics of getting 3 sections of fence from the store, into my Accord, up the elevator and onto the roof deck was daunting. I eventually stumbled upon Reed Fencing (the stuff that frat boys put around their tiki bars). It is inexpensive ($24), easy to work with (it comes rolled up and can be cut with a good pair of scissors) and dense enough to get the job done. With some zip ties I was able to make a decent looking wind break for about $35.

It was very easy, just had to roll out the 6 foot tall reed fencing along the 3 sides of the roof deck's railing, secure it with some zip ties and cut it down to size. The results are pretty substantial, the wind break diffuses the gusty wind creating a little oasis 10 stories up.


  1. Mad props on incorporating lower-tropospheric meteorology into your post.

  2. You know, you're probably also exposing your vegetation to all kinds of urban pollution.... just a precaution.

  3. In fact your creative writing abilities have inspired me to start my own blog now.
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