What am I seeing?
January 25, 2014 12:05 AM   Subscribe

It's a very rainy evening as our plane goes into the landing phase over Barcelona; my window seat is just over the front edge of the wing, and we're below the clouds. As I look outside, alongside the plane, lit by the steady wing lights, there are perfectly horizontal, continuous strings - of slightly variable thickness - of rain. (At the wing tip, the flashing white beacon singles out perfectly horizontal, short stringlets of rain.) I know that rain doesn't fall sideways, so: what, exactly, am I seeing?
posted by progosk to Science & Nature (12 answers total)
 
Your plane's traveling sideways at 120 mph. The raindrops are falling down at 5-20 mph.

You can basically neglect their downward motion.

So it looks like a big flow of material with specks suspended in it coming towards some widget in a wind tunnel at 120 mph.
posted by sebastienbailard at 12:31 AM on January 25 [1 favorite]


Also: I know that rain doesn't fall sideways

Yes it does.
posted by valkyryn at 4:04 AM on January 25


I know that rain doesn't fall sideways

From the point of view of an observer moving through it sideways at high speed, yes it does.
posted by flabdablet at 6:29 AM on January 25


What you describe sounds like the shape of the slipstream of air around the wing of the airplane. The droplets of rain are affected as well, kind of like a more particulate version of smoke in a wind tunnel test. There's a picture on this page that shows what that looks like.
posted by Sublimity at 7:30 AM on January 25 [2 favorites]


What you describe sounds like the shape of the slipstream of air around the wing of the airplane.

That's what would have come to my mind too, only the strings of rain were actually ahead of the front edge of the wing.

I know that rain doesn't fall sideways

From the point of view of an observer moving through it sideways at high speed, yes it does.


Well, that's not really a meaningful sense of "fall" here, though. (And in case valkyryn is being literal: had there been extremely strong headwind that we'd been flying in to, then one might hypothesize rain falling at a pronounced diagonal slant towards the plane - but surely never totally horizontal. It was a completely calm flight too, so I'd rule out that the rain itself had anything but a vertical direction.)

I think sebastienbailard likely has it - what was so surprising was the absolute regularity/coherence of the strings. So, to put it in another way, given our speed through the virtually suspended drops of rain, was it my brain picking out specific series of drops of a similar size at a specific vertical position, leaving me with the impression of continuous lines?

It's still a little puzzling to me how that sort of ordering selection in a (presumably?) random mass of drops comes about. What role would the light sources have played in this (given that the continuous light shone on continuous strings, and the short flashes lit only short lengths)?
posted by progosk at 7:58 AM on January 25


It's still a little puzzling to me how that sort of ordering selection in a (presumably?) random mass of drops comes about.

sebastienbailard is basically saying the same thing as others. You're not seeing a series of drops in relatively the same position that blur together to form one horizontal line (in most cases - there are probably a small amount of horizontal rain lines that you saw that could be described that way). You're moving so fast through the rain that from your perspective the rain is streaking by horizontally and you see it blurred, much the same way that individual spots on the road look when you're driving. Flying in a plane at ~300mph is pretty much exactly the same as standing still in a 300mph wind, in which case you would absolutely see raindrops fall horizontally.
posted by LionIndex at 8:33 AM on January 25


So the strings are individual drops that, because I'm whizzing by, I see as lines, much as at a standstill, fast-falling raindrops can look like (vertical) lines? So a kind of low-fi version of warp-/light-speed (well, the fictional kind, anyway)?
posted by progosk at 9:12 AM on January 25


Yes.
posted by flabdablet at 6:18 PM on January 25


Next time you're out driving in the rain at night, have a look at the rain as it appears in the beam of your headlights. Then pull over, stop, and have another look.

I expect you'll notice that rain on the first look appears to be falling distinctly more horizontally than that on the second, and that this effect will become more pronounced the faster you drive.

Now imagine you're driving at 300mph, and that's your aeroplane effect.
posted by flabdablet at 6:22 PM on January 25


Also: go out walking with an umbrella in the next heavy shower. Then, keeping the umbrella exactly where it is over your head, start to run. Your front is now getting wet, and yet the umbrella canopy is exactly above you. So how sure are you that, from your point of view, rainfall has no horizontal component?

Sure, you can argue that the rain is actually falling vertically, and that you're getting wet because you're running into it. That's true. But there's an equally valid description of what's going on that says that you're the fixed reference point and that running makes the entire world stream backwards past you, and that second description is what your brain's visual processing system actually models.
posted by flabdablet at 6:29 PM on January 25


Here's a chart showing terminal velocity (maximum falling speed) for raindrops of various sizes, presumably in still air. Note that even the biggest drops will fall at under 10m/s, which is roughly 20mph.

If the horizontal component of a raindrop's motion relative to you is 300mph, and the vertical component only 20mph, you're not going to notice the vertical component.
posted by flabdablet at 10:17 PM on January 25


Oh, another thing: given that the plane was descending to land, the vertical component of its own velocity could easily have been in the same ballpark as that of the raindrops. That would make the vertical component of the raindrops' velocity even smaller from a passenger's point of view.
posted by flabdablet at 9:50 PM on January 26


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