I noticed a stange phenomenon on the bus one day. Whenever the wind blows at a pile of leaves, the leaves tends to go in circular motion around, giving me the impression as if it was a “mini tycoon”. Further obsevations leads me to realise that this does not apply solely to leaves, but to objects which floats easily on air (plastic bags) Can someone be kind enough to explain the workings behind this?
I don’t know if I can express it properly, but these light-weight objects easily get caught in the eddy currents created within the wake of, say a passing vehicle. Heavier objects not be as easily caught in these eddies.
I can see it; wish I could explain it better!
- Jinx
Y’don’t get these much in my neck of the woods, but I’ve heard of a certain phenomenon known as a dust devil - basically the little tornadoes of dust or rubbish that whip up on those days when grumpy ol’ Mr Weather can’t seem to choose a hat.
They did an X-Files novel about them. I had to swallow my pride but I enjoyed it once I read it.
The real point here is that little cyclonic winds or vortices are actually very common.
When two bodies of air are in relative motion, the area where they rub one another is in shear - that is, if you measure at various points along a line from one body to the other, you’d find the velocity perpendicular to the line was changing rapidly.
In a substance whose viscosity is high relative to its density, like tar, shear dissipates energy by friction. But in a substance whose viscosity is low relative to its density, like air, shear induces vortices. Likewise with water, which is why you see whirlpools spring up around a paddle.
(Note that “viscosity high relative to its density” takes considerable liberties, as it refers to a relationship that also includes speed and distance - to get a more complete picture, look up “Reynolds number” in a book on fluid mechanics.)
Air is doing this vortex stuff all over the place, but you usually can’t see it because there are not enough visible markers along for the ride. A dust devil is a great place to see it - I once drove through one about 50’ in diameter, which rocked my Jeep back and forth pretty hard. There’s a documentary on tornados that closes on an amateur videotape of some children playing in a hay devil; they are running along with it, holding up armloads of cut hay which it carries up and scatters around them. Another way to witness vortices is by driving perhaps 40’ behind a tractor trailer at highway speeds with your car windows open on both sides. A couple of times a second you will feel the air pressure shift back and forth between sides, as the truck, or more properly the column of air it is dragging behind it, sheds vortices alternately on either side and they wash over you. Of course if you do this too long and too close behind the truck, some trucker wrath may start washing over you too.
Just a minor nitpick: Dust devils and things of like scale are not properly described as cyclones. A cyclone is a region of circulating air caused by a combination of a low-pressure eye and the Coriolis effect (which we all know is only significant for large systems), and rotates counterclockwise in the Northern hemisphere and clockwise in the Southern. An anticyclone has a high-pressure “eye”, and rotates the opposite direction.
Also, a “tycoon” is someone with a lot of money. I think the OP meant “typhoon”, which is a storm.
On the subject of nitpicks, you’re describing “cyclones”, not “little cyclonic winds”. A “little cyclonic wind” is a little wind that is like a cyclone.
And AceRegen refers to a windy phenomenon that “tends to go in circular motion around”. There are more tycoons fitting that description than typhoons at any given moment. For my part I’m happy to take the statement at face value…
No no no, you are all wrong. The leaves blow around in circles because someone turned on a shower while sucking on a piece of spaghetti. 