What-If 0093: "Windshield Raindrops"

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algorerhythms
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What-If 0093: "Windshield Raindrops"

Postby algorerhythms » Wed Apr 23, 2014 5:25 am UTC

http://what-if.xkcd.com/93/

At what speed would you have to drive for rain to shatter your windshield?

Daniel Butler


Living in Tornado Alley, I'm well acquainted with how precipitation can destroy a windshield even if the car is not moving at all, and with how expensive it can be to fix the windshield.
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JohnLeary
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Re: What-If 0093: "Windshield Raindrops"

Postby JohnLeary » Wed Apr 23, 2014 8:07 am UTC

I have to comment on "Luckily, cars can't drive at Mach 1 without lifting off" - It was done by Thrust SSC on 15 October 1997, when it achieved a speed of 1,228 km/h (763 mph) and became the first car to officially break the sound barrier.

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Re: What-If 0093: "Windshield Raindrops"

Postby Djehutynakht » Wed Apr 23, 2014 9:09 am UTC

Immediately harkened back to this:

http://xkcd.com/283/

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Tualha
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Re: What-If 0093: "Windshield Raindrops"

Postby Tualha » Wed Apr 23, 2014 10:53 am UTC

Randall saith:

Even in the heaviest rainstorms, the water in the air weighs less than the air itself ...


I don't understand that statement at all. He's talking about the drops of liquid water, not water vapor, right? If they weighed less than the air, why would they fall?

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Re: What-If 0093: "Windshield Raindrops"

Postby karhell » Wed Apr 23, 2014 10:56 am UTC

Tualha wrote:Randall saith:

Even in the heaviest rainstorms, the water in the air weighs less than the air itself ...


I don't understand that statement at all. He's talking about the drops of liquid water, not water vapor, right? If they weighed less than the air, why would they fall?

There may be more air than water by weight, but liquid water is still denser than air, thus it falls
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Re: What-If 0093: "Windshield Raindrops"

Postby brenok » Wed Apr 23, 2014 10:57 am UTC

Tualha wrote:Randall saith:
Even in the heaviest rainstorms, the water in the air weighs less than the air itself ...
I don't understand that statement at all. He's talking about the drops of liquid water, not water vapor, right? If they weighed less than the air, why would they fall?

I think he means in a given volume of air during a rainstorm, the mass of the water drops on the volume is less than the air inside it.

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cellocgw
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Re: What-If 0093: "Windshield Raindrops"

Postby cellocgw » Wed Apr 23, 2014 11:31 am UTC

This strip reminds me of a couple things.

Over at YouGoogleTube, there are some videos of people who fill a beer bottle about half-way full with water, then give a sharp vertical slap to the top of the bottle. The water rises, a small vaccuum forms at the bottom, the water is pulled back wicked awwsum fast, and the bottle shatters.

His drawing of a raindrop & its shockwave looks suspiciously like a nuke's initial shockwave. This clearly means we need to resurrect the Star Wars Missile Defense System only use it to stop raindrops!

And just because I feel like trolling a bit: do you get wetter walking or running thru the rain? :twisted:
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Re: What-If 0093: "Windshield Raindrops"

Postby FLHerne » Wed Apr 23, 2014 12:17 pm UTC

cellocgw wrote:And just because I feel like trolling a bit: do you get wetter walking or running thru the rain? :twisted:

Depends. If you have a large hat or a crazy hairstyle, and there's no wind, walking might be the best option. Otherwise your shoulders and thighs will definitely get wet, and of course wind will just get all of you soaked.

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Re: What-If 0093: "Windshield Raindrops"

Postby karhell » Wed Apr 23, 2014 12:38 pm UTC

cellocgw wrote:And just because I feel like trolling a bit: do you get wetter walking or running thru the rain? :twisted:

Depends whether the wetter weather wear the weather's wetter weather wetter whether wetter weather wetter weather weather wetter...
ahem... "the wetter the better" is what I wanted to say
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Re: What-If 0093: "Windshield Raindrops"

Postby ramblinjd » Wed Apr 23, 2014 1:27 pm UTC

I feel like Randall must be a bit busy this week. I was expecting, a "but assume the car CAN go Mach = 3..." and then drawings of raindrop force profiles until they were powerful enough to go not only through most production windscreens, but also the driver, the seat, the chassis, and on and on.

willglynn
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Re: What-If 0093: "Windshield Raindrops"

Postby willglynn » Wed Apr 23, 2014 1:52 pm UTC

Raindrops are tiny. ... Even at very high speeds, they can't break a windshield via their momentum alone.

They do, however, sting pretty badly if you should happen upon them while skydiving. That's what happens when you fall into the pointy side of the raindrops...

(Yes, I know, raindrops flatten out horizontally while falling. This is why weather radars have long focused on horizontal polarization, and are adding vertical polarization to better distinguish large raindrops from other types of precipitation. I still prefer to tell less technical skydivers that they're hitting the pointy part.)

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Red Hal
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Re: What-If 0093: "Windshield Raindrops"

Postby Red Hal » Wed Apr 23, 2014 3:57 pm UTC

So ... what about a Skrillex storm?
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Re: What-If 0093: "Windshield Raindrops"

Postby airdrik » Wed Apr 23, 2014 8:26 pm UTC

ramblinjd wrote:I feel like Randall must be a bit busy this week. I was expecting, a "but assume the car CAN go Mach = 3..." and then drawings of raindrop force profiles until they were powerful enough to go not only through most production windscreens, but also the driver, the seat, the chassis, and on and on.

Or, in other words:
Image

We definitely need more BHG

I also rather appreciate the last title text (and corresponding xkcd comic referenced by Djehutynakht). Rain on windows and windshields and associated light diffraction patterns are truly mesmerizing.

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Re: What-If 0093: "Windshield Raindrops"

Postby rmsgrey » Thu Apr 24, 2014 1:08 am UTC

Normal road cars can't go at mach one because they leave the ground at a lower speed when the lift forces they generate exceed their weight (actually, the reduced traction from the lift might stop the car from accelerating before it achieves take-off speed - if you accelerate slowly enough to lets things stay close to equilibrium then it wouldn't)

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mojacardave
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Re: What-If 0093: "Windshield Raindrops"

Postby mojacardave » Thu Apr 24, 2014 12:28 pm UTC

I was sure that Randall had already addressed the walking/running through rain argument, but I went back to have a look and couldn't find anything. I know I've read a web article about it somewhere, and I have no idea where else it could be. Any ideas?

Aside: Is the What-If archive new for this week, or have I just not noticed it before?

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karhell
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Re: What-If 0093: "Windshield Raindrops"

Postby karhell » Thu Apr 24, 2014 12:32 pm UTC

answer to aside : the what-if archive has been there for quite a while, at least a year if my memory still works properly.
AluisioASG wrote:191 years ago, the great D. Pedro I drew his sword and said: "Indent thy code or die!"
lmjb1964 wrote:We're weird but it's okay.
ColletArrow, katakissa, iskinner, thunk, GnomeAnne, Quantized, and any other Blitzers, have fun on your journey!

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Re: What-If 0093: "Windshield Raindrops"

Postby alsodanlowe » Thu Apr 24, 2014 6:41 pm UTC

This entry made me want to go finish Capillarity and Wetting Phenomena (something I checked out once to follow a citation from the team that isolated the mechanism of ballistospore ejection). I found it's on Google Books as one of their 'mostly complete' previews of books, if anyone else is interested.

I see I can't link. Didn't see that in the rules. Anyway, the ISBN is 978-0387005928.

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Re: What-If 0093: "Windshield Raindrops"

Postby SecondTalon » Thu Apr 24, 2014 7:23 pm UTC

alsodanlowe wrote:I see I can't link. Didn't see that in the rules.

#3 to be exact.
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Re: What-If 0093: "Windshield Raindrops"

Postby squall_line » Tue Apr 29, 2014 7:23 pm UTC

cellocgw wrote:This strip reminds me of a couple things.

Over at YouGoogleTube, there are some videos of people who fill a beer bottle about half-way full with water, then give a sharp vertical slap to the top of the bottle. The water rises, a small vaccuum forms at the bottom, the water is pulled back wicked awwsum fast, and the bottle shatters.


Really? This one does? I usually think of Glass Half Empty when I come across those videos (and not just because those videos are linked to by that What If).

cellocgw wrote:And just because I feel like trolling a bit: do you get wetter walking or running thru the rain? :twisted:


mojacardave wrote:I was sure that Randall had already addressed the walking/running through rain argument, but I went back to have a look and couldn't find anything. I know I've read a web article about it somewhere, and I have no idea where else it could be. Any ideas?


You may have read it, read about it, or saw it demonstrated on MythBusters. They did it a few years ago, IIRC (though I don't care enough to look up the episode number right now).

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Re: What-If 0093: "Windshield Raindrops"

Postby WhoopsieDoo » Tue Apr 29, 2014 10:11 pm UTC

I'm also not sure I understand:
On the other hand, if you're driving under a thunderstorm with strong updrafts ... ... the precipitation can smash your windshield at any speed.


Isn't the whole article about how that's not the case?

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Re: What-If 0093: "Windshield Raindrops"

Postby gmalivuk » Wed Apr 30, 2014 1:56 am UTC

It's a reference to the conditions in which hail forms.
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