Cutting The Grass

Guest Post by Willis Eschenbach

Sufi stories of the wisdom of the foolish Mulla Nasrudin have been around for a thousand years or more. One of them tells how the Mulla was walking down the street one day. Three stories above him, a man was working on a roof. The workman slipped and fell. He landed on the Mulla. Fortunately, the workman was totally unharmed. The Mulla, however, was injured enough that he had to be taken to the local hospital to recover.

When the Mulla’s disciples came in to see him, they asked him what lesson could be learned from the incident.

“Shun reliance on theoretical questions,” said Nasrudin, “questions like, if a man falls three stories from a roof, will he have to go to the hospital?”.

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With that as prologue, in the Americas both North and South we have birds called “turkey vultures”, Cathartes aura. Where I grew up they were known as “buzzards”. In parts of the South they are known as a “carrion crow”. I’ve always felt a kinship with them because back in the 1800’s my great-grandfather, The Captain, named his tugboat the “Carencro” after the local name for buzzards.

Plus buzzards have a very important job. They’re the garbagemen, the cleanup crew for all the corpsicles left behind by nature, our allegedly loving mother nature who is also famously “red in tooth and claw” … you go, Mom. Here’s a turkey vulture eating a dead armadillo somewhere down south.

The buzzards visit our place a lot, but fortunately, not to eat. We live in a hillside clearing in a redwood forest. It’s the place with the tiny house icon, to the right of and above the center in the photo below. Because there are no trees, the cleared area around our house is warmer than the surrounding forest. This warm air rises.

The buzzards know about warm air rising, of course, because those jokers hate to flap their wings. They are champion gliders, going for mile after mile without ever flexing their wings. In their search for free rides, they regularly use the warm air off of our clearing to work their way up to the ridgetop behind us. They’ll come in and wheel round and round, gaining altitude with each pass Once they’ve gotten high enough on the warm air rising from our clearing, they drift off majestically along the ridge, in their perpetual search for the dead.

Here’s the curious part. A few days ago while I was working outside, I noticed that the buzzards were coming in over the clearing as usual.

But they weren’t gaining the altitude that they were expecting to gain. They weren’t able to get their usual lift. I watched as bird after bird tried their usual route without much success. One even had to flap his wings to get out of the top corner of the clearing. For a buzzard, that’s a sure sign of failure.

Finally, one buzzard ended up gliding so close to the ground that it couldn’t even make it over our eight-foot (2+ metre) deer fence. It flared out its wings, gained a little altitude, and settled on one of the wooden fence posts. This almost never happens. Nothing to attract them here.

All of this set me to pondering. Why were the buzzards getting fooled about the amount of lift? What had changed?

The only thing I could think of that had changed was that the previous day I’d had the grass cut. I couldn’t cut it earlier because of the endless rain. By the time I got back from Fiji, the grass had gotten up to about chest high in parts. Literally. Chest high.

But that explanation didn’t seem right to me. I’d always figured that if you had a grassy field, it kept the surface from getting too hot. And if there was no vegetation, like in the desert or on the beach, the surface would get hot. As computer modelers like to say, it’s just “simple physics” …

But if that were so … why were the buzzards getting fooled? I love the natural world for exactly this kind of puzzle.

My conclusion was that I was looking at the wrong metric. The issue for the buzzards was not how hot the surface got. It was how hot the air got … and that’s a very different question. The sun heats the surface, and the surface heats the air. So a big issue is not how hot the surface gets, but how well the surface acts as a heat exchanger. Here are the two surfaces in question, mowed and unmowed.

It seems to me that the field of grass on the right side of the photo is a pretty good sun –> surface –> air heat exchanger. The grass slows down the passage of the air over the surface, allowing it to be warmed. This warm air will then rise into the free atmosphere. The grass also increases the amount of surface area exposed to the air by orders of magnitude. Finally, the grass acts as a pretty good solar trap, where the solar energy goes in but not much is reflected back out.

Regardless of the explanation, I cannot deny that I expected that cutting the grass would increase the surface warmth and thus the lift for the buzzards. However, the buzzards proved me 100% wrong.

Now, suppose we were trying to model this on the computer. This is far below the size of a single gridcell in a climate model. So it would have to be “parameterized”, meaning that we’d put in numbers that we think are reasonable for this kind of a change … but that’s just putting numbers to a theory about what will happen.

Which brings me back to Nasruddin and the issue of relying on theoretical questions, like “If a field of tall grass is mowed, will the buzzards fly higher or lower?”.

In this case, to my surprise, the answer was “lower”. Which is how I found myself looking into the eye of a buzzard sitting on my fence post. Oh, I didn’t stare at him. That kind of reckless eyeballing makes any wild creature nervous. So I looked at him in glances and pauses, and generally pretended that I didn’t see him.

Now, the birds circling so low to the ground had attracted two cats to the action. And when the buzzard landed on the fence post, the neighbor’s cat crept slowly over to the base of the buzzard’s post. The buzzard paid little attention until the cat was directly below him (her?). Then they both froze, with the cat looking straight up along the post at the buzzard looking down, and the buzzard looking straight down between its toes at the cat looking up.

Our cat just sat and watched them both. I watched all of them.

Then some other poor sucker of a buzzard came in, and he couldn’t get enough lift either. As a result, he came right past the ear of the buzzard on the post. Startled, the post sitter took off. The two cats both immediately vanished into hiding.

Me? I laughed and shook my head at the wondrous intricacies of the climate, and I went back to work.

Best of the springtime to everyone,

w.

PS—Perhaps the most intriguing part of the experience to me was the exquisite judgment of the buzzards as to how much lift they could expect off of our clearing. Birds are quite surprising in their ability to learn. And in that vein, after avoiding the clearing for a couple of days, today for the first time I saw a buzzard come through. However, it didn’t push its luck, and it didn’t get close to the ground. The buzzard just came in, got the main lift from the center of the clearing, and wheeled back out again. What a world, amazingly intelligent beings on all sides.