Wikipedia:Reference desk/Archives/Science/2019 November 11

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November 11

Cleveland rat-squirrel ?

There seems to be a new species of rodent just SE of Cleveland, Ohio. It looks mostly like a squirrel, but is smaller, has a less fluffy tail, and is more aggressive towards humans. Any idea what it is ? SinisterLefty (talk) 06:49, 11 November 2019 (UTC)

Look like this? -- The source is dubious, but they call it a "sqrat". 2606:A000:1126:28D:25BA:5B02:33E7:A409 (talk) 09:04, 11 November 2019 (UTC)

Yea, that looks like it. The 2 useful suggestions there were that it was mange or that the squirrels plucked their tails for nesting material. The mange theory doesn't explain why it's only the tail. Maybe there's a new squirrel-tail mange mite that specializes in just that ? That might also explain the smaller size, if the parasites drain them of nutrients. But when I've seen mangy animals it's typically patchy hair loss, not as uniform as this. As for the plucking for nest material theory, that doesn't explain why there are so many more like that recently. Neither theory explains why they would be more aggressive towards humans. SinisterLefty (talk) 11:28, 11 November 2019 (UTC)

sorry, but this irresistibly draw this to my mind Gem fr (talk) 15:01, 11 November 2019 (UTC)

• Here's an article about them that seems pretty solid. --Jayron32 12:43, 13 November 2019 (UTC)

What size of sink is needed to truely demonstrate Coriolis effect at the equator?

There are these bogus demonstrations where two sinks are a meter apart with the equator between them, and water rotates one way in the north and the other way in the south.

What size of sink will really demonstrate it? — Preceding unsigned comment added by 77.124.56.240 (talk) 21:10, 11 November 2019 (UTC)

Somewhere between the size of the Great Lakes and the size of an ocean ? I say this because the Great Lakes [1] don't show the same rotation as ocean currents. Now the shape of the sea floor and beaches, and inflow and outflow points, do contribute, but still I'd expect to see more rotation than I do, if the Coriolis effect was a major factor in lake currents. SinisterLefty (talk) 22:00, 11 November 2019 (UTC)

See[2] where it says:

Coriolis forces are best observed at a large scale...In your tub, such factors as any small asymmetry of the shape of the drain will determine which direction the circulation occurs. Even in a tub having a perfectly symmetric drain, the circulation direction will be primarily influenced by any residual currents in the bathtub left over from the time when it was filled. It can take more than a day for such residual currents to subside completely. If all extraneous influences (including air currents) can be reduced below a certain level, one apparently can observe that drains do consistently drain in different directions in the two hemispheres.

You can estimate the magnitude effect by applying conservation of angular momentum. For simplicity assume that the basin of water is the same shape as the plug hole (circular) and that the hole is central. For more simplicity let the experiment initially be performed at the north or south pole of the earth (which will maximize the effect).

Before the plug is pulled, the basin and water are rotating with the earth, once in just under 24 hours. When the plug is pulled, the water moves towards the centre and the radius of rotation of each particle of water ultimately decreases in proportion to the radius of the hole to the radius of the basin - say a factor of 1/n. Conservation of angular momentum demands that the angular velocity increases by a factor of n², so the period of rotation decreases by a factor of n². Thus, if the radius of the basin is 100 times that of the hole, the water ends up rotating once in about 24hr/100² ~ 8.6sec - not hugely fast, but possibly discernible.

If the experiment is performed at a lower latitude, the period of rotation is multiplied by (probably) the sine of the latitude (see Foucault pendulum). catslash (talk) 00:24, 12 November 2019 (UTC)

Further to the above, note that the original poster asked about bogus demonstrations with sinks "a meter apart with the equator between them". Now one nautical mile is defined as 1,852 m and corresponds pretty nearly to one minute of latitude. Therefore a position 0.5 m from the equator corresponds to a latitude of 0.5/1,852 of a minute north or south. According to units this angle is equal to about 1/12,700,000 of a radian, and therefore, using the small-angle approximation, its sine is also about 1/12,700,000. So the force is so slight that no such practical demonstration is possible using anything smaller than a sizable lake. (And of course, a sizable lake would have to be farther from the equator to be all one one side of it, requiring a new calculation. But even at say 100 km from the equator, the sine of the latitude is about 1/64,000.) --76.69.116.4 (talk) 09:49, 12 November 2019 (UTC)

In OR, an actual bath tub filled nearly to the top, at above 52°N and drained after standing for a few days showed no obvious rotation. The water appeared to go straight down, right to the dregs. catslash (talk) 00:34, 12 November 2019 (UTC)

Thanks for that. I've never tried leaving the water to stand like that and the bath water has always had a vortex one way or the other for me - except for one time when it disappeared straight down the plughole just like you describe. I was really astonished. It emptied far faster than usual. If somebody could get it to happen again and take a video they'd have a winner. Dmcq (talk) 19:40, 13 November 2019 (UTC)

• Look at Rossby number for a way to quantify this. It's the ratio of the forces for the inertial and Coriolis forces. If Coriolis is greater, then rotation follows the relevant direction for each hemisphere, otherwise it depends on the initial conditions instead. For weather systems, the larger they are, the more likely they are to follow the Coriolis direction. Smaller systems can (and do) go either way. Andy Dingley (talk) 01:12, 12 November 2019 (UTC)

OP here, I am confused. I do not feel ashamed to admit the math and physics in the articles are above me. Is there any straight answer? — Preceding unsigned comment added by 77.124.56.240 (talk) 10:04, 12 November 2019 (UTC)

• We really do need better coverage in the weather articles which shows a comparison list of low-pressure system by their size, so that we can easily see how their behaviour changes according to size. Most large scale weather arises through a similar means, warm air cooling down and its pressure dropping, but what that then turns into depends so much on how big a mass of air is acting as one.

You need to get to the point of having an idea what Rossby number means (even without the maths). It's the ratio between the non-Coriolis forces and the Coriolis forces. The Coriolis forces are weak, but increase with size of the system. So small systems ignore Coriolis, large ones (hurricane and upwards) are affected by it. Middle-size ones might be. Your bathroom sink is so much smaller than any of this that Coriolis is just insignificant.

Smaller than a hurricane (and that includes tornadoes) and they rotate either way, depending on the starting conditions. Try a circular bathroom sink in the UK. Sometimes you can demonstrate this as "Hot water rotates one way, cold water rotates the other". The trick is that in the UK we still mostly have two separate taps, not single mixers, and that water enters the basin with pre-existing spin, depending on which side the tap was mounted. This rotation continues for enough time for you to then pull the plug and demonstrate rotation during emptying, with either direction of rotation, and predictably.

There's also the ice skater effect. The rotational energy stays mostly constant (it's lost, but only slowly) and so if a rotating system shrinks in diameter, then it has to speed up. So in your "big funnel by the side of the equatorial road" sideshow example, it might start off with no discernible rotation in the water, but as it approaches the plughole it speeds up so obviously - we've all seen this in the bathroom. Now what you're seeing there is conservation of angular momentum, not Coriolis, but it's great sideshow patter to present it that way. Andy Dingley (talk) 11:21, 12 November 2019 (UTC)

As Andy Dingley explains, it won't work with a draining sink. The forces are just too small, especially near the equator. These forces are strongest at the poles and zero on the equator. At higher latitudes the effect can be seen from a Foucault pendulum (if you have patience, at my location it is takes over a day). Rmvandijk (talk) 12:55, 12 November 2019 (UTC)

What with, say, 500Km sink (well, reservoir). The question came form an argument I had with guys who saw this YouTube vid. So for the sake of the argument i wanted to say what terribly large "sink" will be required.

I think I got the general idea of this Mossby number: Larger than 1 - Coriolis not effective. lower than one - effective. But to get proper answer I need to do calculation like the angles sinus from above, and even than I will surely mess things up. If you guys can lend a hand that will be great. Just think of enlighting the uneducated... — Preceding unsigned comment added by אילן שמעוני (talk • contribs) 13:35, 12 November 2019 (UTC)

Did some moss grow on the Rossby number ? :-) SinisterLefty (talk) 13:46, 12 November 2019 (UTC)

• It depends on the relative density of air and water too. But for air you need something the size of a hurricane, so even though denser water allows you to do the same at a smaller scale, that's still bigger for "the sink" than any building to put it in.

Also, as noted, the Equator itself (nearby to it on either side) is just the wrong place to do this, as it's where the forces are smallest.

I've built Foucault's pendulums myself. They're not trivial either, but you can do it for a school science project, if you have a way to hang a weight from the top of a gym roof. One of mine is still running in a science museum - that's about 50 lbs of concrete in an oil drum, dressed up on the outside. Andy Dingley (talk) 15:23, 12 November 2019 (UTC)

I'm trying to get to the point where I can say confidently "Sure mate, you only need like 500Km (approx. 300 miles) wide sink and it will work out just fine...". With air it wouldn't have that impact. — Preceding unsigned comment added by אילן שמעוני (talk • contribs) 17:36, 12 November 2019 (UTC)

It's not going to have a sharp cut-off, due to all the factors listed, but 500 km would be a fine approximation. SinisterLefty (talk) 17:39, 12 November 2019 (UTC)