## What-If 0012: "Raindrop"

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ryzvonusef (1151717)
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### What-If 0012: "Raindrop"

rhomboidal
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### Re: What-if 0012: Raindrop

It might be a little too whimsical to hope the drop freezes and try to catch it on your tongue.

Jamaican Castle
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### Re: What-if 0012: Raindrop

It worries me a little how many What-Ifs end in a gigantic explosion of some form or another.

zyzyzyryxy
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### Re: What-if 0012: Raindrop

What else, except water, makes a cloud visible?
If we take out all water from the cloud, will it still be visible at all? (Actually a valid what-if question on its own).

Spoiler:
Also, 100km^2 <> (100km)^2 as far as I know?
Altough result seems to be calculated correctly.

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### Re: What-if 0012: Raindrop

The cloud is water. If you took all the water out of the cloud, there wouldn't be a cloud (though if you took only the liquid water, you'd end up with some invisible vapor until that started condensing again).

Jamaican Castle wrote:It worries me a little how many What-Ifs end in a gigantic explosion of some form or another.

Two out of twelve so far?

Considering that the What-Ifs are applying the laws of the universe to some of the weirdest and most extreme conditions, and exploding is what the universe does best (I mean, look up at night; every single star is a gigantic explosion) I'm surprised it isn't more.
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### Re: What-if 0012: Raindrop

Arancaytar wrote:exploding is what the universe does best (I mean, look up at night; every single star is a gigantic explosion) I'm surprised it isn't more.

Or look up in the day and, oh sorry you can't see anything past the glare of the closest giant explosion.
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### Re: What-if 0012: Raindrop

... nvm
Last edited by _Paddy_ on Tue Sep 18, 2012 1:57 pm UTC, edited 1 time in total.

Vroomfundel
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### Re: What-if 0012: Raindrop

I wonder, isn't the raindrop going to assume, eh, raindrop shape, at least to a certain extent? It's portrayed as a sphere all the way down.

Another thing that bothers me a bit is the top speed - is it really going to get that fast? A skydiver normally reaches 200km/h - they can get up to [url="http://en.wikipedia.org/wiki/Michel_Fournier_(adventurer)"]1,600 in theory[/url] but that's in the upper atmosphere where the air is thinner. Normal raindrops reach the ground at a much slower speed - about 30km/h. I'll be interested to see the formula used to calculate top speed - indeed air resistance should be proportional to the great circle surface area (r to the 2) while mass is proportional to r to the 3, but still - how did he come up with the numbers?
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### Re: What-if 0012: Raindrop

Um, .6 km^3? Shouldn't it be .006km^3?
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### Re: What-if 0012: Raindrop

*** wrote:Um, .6 km^3? Shouldn't it be .006km^3?

I believe so

zyzyzyryxy
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### Re: What-if 0012: Raindrop

*** wrote:Um, .6 km^3? Shouldn't it be .006km^3?

Well, 100km^2 * 6cm is 0,006km^3, but the text above suggests he meant (and calculated) (100km)^2 * 6cm.

ttnarg
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### Re: What-if 0012: Raindrop

Vroomfundel wrote:I wonder, isn't the raindrop going to assume, eh, raindrop shape, at least to a certain extent? It's portrayed as a sphere all the way down.

Another thing that bothers me a bit is the top speed - is it really going to get that fast? A skydiver normally reaches 200km/h - they can get up to [url="http://en.wikipedia.org/wiki/Michel_Fournier_(adventurer)"]1,600 in theory[/url] but that's in the upper atmosphere where the air is thinner. Normal raindrops reach the ground at a much slower speed - about 30km/h. I'll be interested to see the formula used to calculate top speed - indeed air resistance should be proportional to the great circle surface area (r to the 2) while mass is proportional to r to the 3, but still - how did he come up with the numbers?

Intressing ideas. but 'raindrop shape' is might not be what you think it is. A falling drop of water is infact more a flatten sphere. A large rain drop form a donnut shape just be for splitting apart. http://www.sailingissues.com/rain-teardrop-shaped.html
How will this effect our big raindrop? Not sure I think it would take a quite some time for it to brack up maybe more then it has.

Top speed is beliveable as when you incress an object in size the volume goes up quicker then the area meening is the mass to air restance ratio is higher so it is not going to be slowed as much as a skydiver. But there will be an air cussion as it reaches the ground as the air cant get out the way quickly. Which will slow the raid drop and help it back apart.

As for what will happen. I dont know. maybe till will hit the ground as hard as said but I think it will be slowed at the end and maybe even brocken apart.

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### Re: What-if 0012: Raindrop

Vroomfundel wrote:Another thing that bothers me a bit is the top speed - is it really going to get that fast?

If I did the numbers right (using http://www.calctool.org/CALC/eng/aerospace/terminal ), I get a terminal velocity of 9500 m/s. I suspect this calculator isn't accurate above the speed of sound, but either way, it's going to be a big number. For the first 20s, air resistance can probably be ignored and it really does reach a speed of 20s * 10m/s/s = 200m/s.

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### Re: What-if 0012: Raindrop

"For five or six seconds, nothing is visible. Then, the base of the cloud begins to bulge downward."

I thought all of the water had condensed into this droplet. Why is there still a cloud?
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### Re: What-if 0012: Raindrop

Now we know what caused the bloop

http://en.wikipedia.org/wiki/Bloop
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### Re: What-if 0012: Raindrop

Is it more likely to fall on land or at sea?

And what would the effects of a water impact be?

One way of thinking about the super-raindrop is as an incredibly low speed meteorite - it's big enough that what it's made of is less important than its momentum, so the first-order effects of the impact are the same as a similar chunk of rock (or of hot fudge sundae!)

twcarlson
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### Re: What-if 0012: Raindrop

I calculated the terminal velocity of such a raindrop as 4490 m/s, which is 10,000 mph or mach 13.5.

In a vacuum, it would take 7.6 minutes of falling to reach this speed, but in air it would be slower and would approach terminal velocity asymptotically.

I got the equation from Wikipedia: t.v. = sqrt ( 2 m g / [rho] A Cd )

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### Re: What-if 0012: Raindrop

*** wrote:Um, .6 km^3? Shouldn't it be .006km^3?
I believe so
No, the equation was written wrong (or at least lacking sufficient parentheses), but the result is correct.

To compare with the other massive explosion what-if, the potential energy of that much water at 2km altitude is about 2.8 megatons, while the baseball had iirc about 4mt of energy.
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### Re: What-if 0012: Raindrop

Yay! Back to hugely catastrophic what-ifs I liked this one.

Also: the punchline was so terrible, and so good at the same time

tobiasgies
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### Re: What-if 0012: Raindrop

Oktalist wrote:I thought all of the water had condensed into this droplet. Why is there still a cloud?

All the water that was supposed to fall as rain condensed into this droplet. While that is a lot of water, it's not all the water in the cloud. At least in my experience, clouds don't suddenly disappear even if it has rained from them for a while.

The punchline of this what-if had me literally laughing out loud for a minute (and groaning at the same time).

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### Re: What-if 0012: Raindrop

That water would weigh 600 million tons (which happens to be about the current weight of our species).

Sounds like someone's using Wolfram Alpha.

jigawatt
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### Re: What-if 0012: Raindrop

That water would weigh 600 million tons (which happens to be about the current weight of our species).

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### Re: What-if 0012: Raindrop

endolith wrote:
That water would weigh 600 million tons (which happens to be about the current weight of our species).
Sounds like someone's using Wolfram Alpha.
More likely, someone remembers the what-if he himself wrote before, wherein the mass of the human race was calculated.
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Vroomfundel
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### Re: What-if 0012: Raindrop

alien wrote:If I did the numbers right (using http://www.calctool.org/CALC/eng/aerospace/terminal ), I get a terminal velocity of 9500 m/s. I suspect this calculator isn't accurate above the speed of sound, but either way, it's going to be a big number. For the first 20s, air resistance can probably be ignored and it really does reach a speed of 20s * 10m/s/s = 200m/s.

I think in our case the terminal velocity is nowhere near that - this mega-drop is going to split into countlets normal drops at some point, which will get slowed down to the speed of ordinary rain. It's a tricky business to calculate this I guess, so maybe Randall took an educated guess and that's why we don't have a formula. Against a 200m/s terminal velocity for a solid body with the similar properties 90m/s sounds believable though.
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Vroomfundel
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### Re: What-if 0012: Raindrop

tobiasgies wrote:
Oktalist wrote:I thought all of the water had condensed into this droplet. Why is there still a cloud?

All the water that was supposed to fall as rain condensed into this droplet. While that is a lot of water, it's not all the water in the cloud.

Randall used the total precipitable water, which he defined as all the water in the column. So no, there should be no cloud - remember that the what-if setups happen by magic
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### Re: What-if 0012: Raindrop

Vroomfundel wrote:I think in our case the terminal velocity is nowhere near that - this mega-drop is going to split into countlets normal drops at some point, which will get slowed down to the speed of ordinary rain.
It's a sphere of water more than a kilometer across. I really don't think it's going to break up significantly over the course of falling less than twice its own diameter.
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RogueCynic
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### Re: What-if 0012: Raindrop

I live in Massachusetts. I've seen rain fall on one side of the street, and the sun shine while it rained, so one big rain drop would not surprise me.
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### Re: What-if 0012: Raindrop

RogueCynic wrote:I live in Massachusetts. I've seen rain fall on one side of the street, and the sun shine while it rained, so one big rain drop would not surprise me.

All those moments will be lost in time, like tears in rain ...

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### Re: What-if 0012: Raindrop

There's absolutely no doubt in my mind that Randall started with the Skrillex punchline and worked backwards to pick the question.

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### Re: What-if 0012: Raindrop

Though the argument is a bit simplified for the sake of humor (obviously...it's xkcd), there are a few meteorologically significant factors involving the dynamics created by the raindrop as it falls.

The extreme amount of friction generated by the winds as they howled past the edge of the rain drop would induce waves, gradually building in size until multiple things happened. First off, just as the tops of sea-waves will generate large amounts of airborne spray during gale force winds, the edges of our gigantic rain drop would undergo a similar process, with a massive amount of water spraying off of the edge of the drop.

Additionally, some sort of induced wave action would create large surface waves on this massive rain drop that would be generated in concentric rings perpendicular to the vertical motion of the rain drop. As these waves converged, they would amplify via constructive interference, and cause the raindrop itself to fragment at the skyward end of the drop. This would surely continue until the drop had fragmented into a generalized deluge of massive precipitation, all hitting the ground around the same time and causing the flooding you speak of. But its lifetime as a single raindrop would be highly limited.

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### Re: What-if 0012: Raindrop

RogueCynic wrote:I live in Massachusetts. I've seen rain fall on one side of the street, and the sun shine while it rained, so one big rain drop would not surprise me.

It should. The situation you're describing is not uncommon at all, regardless of where you live.
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### Re: What-if 0012: Raindrop

geniekid wrote:There's absolutely no doubt in my mind that Randall started with the Skrillex punchline and worked backwards to pick the question.

I must live in a cave, because I didn't get it. For anyone else wondering, Google found me this

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### Re: What-if 0012: Raindrop

sleepysilverdoor wrote:This would surely continue until the drop had fragmented into a generalized deluge of massive precipitation, all hitting the ground around the same time and causing the flooding you speak of. But its lifetime as a single raindrop would be highly limited.
It's only dropping 2km, though, and is more than one kilometer in diameter when it starts. Is it really going to have time to break up noticeably in that short a distance?
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sleepysilverdoor
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### Re: What-if 0012: Raindrop

gmalivuk wrote:It's only dropping 2km, though, and is more than one kilometer in diameter when it starts. Is it really going to have time to break up noticeably in that short a distance?
You know, you have a good point on that one. I didn't take that into consideration. There would probably start to be some sort of deformation in the shape of the drop at that time. I'll still say that there would be considerable amounts of spray around the periphery though.

Wave velocities are usually about half of the ambient wind speed, but I have no idea as to whether or not that relationship holds when the body is experiencing net wind speeds near the speed of sound. If it does, waves starting on the earthward pole of the sphere would not have time to reach the opposite pole, though there would be time for those generated at the 'raindrop equator' to reach the top and break off. that said, it would be nearly impossible to account for the effects of the turbulent eddies generated as the winds are deflected inwards towards the backside of the raindrop by the bizarre pressure gradients locally created by such a fast moving body.

it would be interesting to use a mathematical model to calculate this...

tomintx
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### Re: What-if 0012: Raindrop

We’ll imagine our storm measures 100 kilometers on each side and has a high TPW content of 6 centimeters.

Umm, that is one big storm.

If you drop 6cm of water over 10,000 km2; even if you rain it down over the course of an hour in a nice, gentle, but persistent, rain; you're STILL going to get some major flash flooding.

One of the methods used to state the capacities of surface reservoirs (Lakes) is to express it in terms of "Drained area" and "inches of rain" for the entire drained area. It is not unusual to see a reservoir capable of storing 1 inch (2.54cm) of rain over its drainage area.

Lakes are also measured in "acre-feet" of water, or enough water to cover an acre (43560 ft2) a foot deep.

This water, 0.6km3, is ~486,400 Acre-Feet. This amount of water is more than the capacity of Lake Ray Hubbard, near Dallas. Fortunately for Dallas, the drainage area of Lake Ray Hubbard, at ~2770 km2 is less than 1/3 of the size of this storm, so much of the water would fall somewhere else.

However, it was one big storm.

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### Re: What-if 0012: Raindrop

rmsgrey wrote:Is it more likely to fall on land or at sea?

And what would the effects of a water impact be?

One way of thinking about the super-raindrop is as an incredibly low speed meteorite - it's big enough that what it's made of is less important than its momentum, so the first-order effects of the impact are the same as a similar chunk of rock (or of hot fudge sundae!)

It's probably 70% more likely to land at sea--but there is no such thing as a average event when it is singular.

A water impact might cause some sort of tsunami-like wave, I'm guessing.
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keithl
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### Re: What-if 0012: Raindrop

The problem with magic is the boundary conditions ... how did we get here?

The heat of vaporization of water is 2260 kJ/kg . Conversely, the heat emitted by condensing a kilogram of water is 2260 kJ. 0.6km^3 of water is 6E11 kilograms, so the condensation of that big drop produces 1.4e18 joules. A megaton bomb is 4.2e15 joules. So the instantaneous condensation of that giant water sphere produces energy equivalent to 320 megatons, six Tsar Bombe .

If half that energy (7e17 J) illuminates the ground under the 1e10 m2 cloud, that is 70MJ/m^2, more than 3 times the thermal intensity directly under the Hiroshima fireball. If it is deposited in one spot above the drop, it will send enormous shockwaves through it, spalling off large globs of water moving many times the speed of sound in all directions, as well as a huge burst of hard X-rays.

There are other effects: Where did the air go when the drop appeared? Where did the air come from to fill the space previously occupied by vapor? How much do these cancel out? But it seems clear that the two old timers get xrayed, torched, then hammered by hypervelocity water long before the drop hits the ground. Serves the toothless hillbillies right.
Last edited by keithl on Wed Sep 19, 2012 6:50 am UTC, edited 3 times in total.

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### Re: What-if 0012: Raindrop

SerMufasa wrote:
RogueCynic wrote:I live in Massachusetts. I've seen rain fall on one side of the street, and the sun shine while it rained, so one big rain drop would not surprise me.

It should. The situation you're describing is not uncommon at all, regardless of where you live.

Heck, I remember driving along a straight road that was the rain boundary once, with my back to the sun--producing an excellent half double rainbow. But I have yet to see a huge raindrop.

One problem is that, as it grows, it tends to fall out of the rain-forming regions, so there are definite limits to the size of drops that can form.
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### Re: What-if 0012: Raindrop

In regards to the diameter of 10,000 square kilometers being way too large of a storm, that's actually not unheard of by any means.

http://en.wikipedia.org/wiki/Mesoscale_convective_complex

These systems are actually quite common, especially during the spring and summer months. They tend to form at night and die during the morning hours, and usually form in synoptic scale convergence zones. I actually watched one dissipate on radar this morning over Oklahoma and north Texas. Additionally, they're at least 5x larger than 10,000 square km by definition alone, so that's logical.

Regardless, the entirety of a storms precipitable water NEVER precipitates out. That said, the entire premise of the what-if article is based on that actually happening and somehow all as one giant raindrop to boot, so that's a moot point.

Anyway, a PWAT value of 5 cm is not unheard of at all. It is actually pretty indicative of the sort of environment that an MCC of the size described would form in. The highest PWAT value I've personally seen was measured during the New Orleans RAOB sounding taken while Hurricane Isaac was making landfall, at 2.89 inches (a little over 7 cm). I'm sure that values could get higher than that, but it'd be hard to exceed those found in a hurricane, what with the whole "massive latent heat engine" deal and whatnot.

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### Re: What-if 0012: Raindrop

PolakoVoador wrote:Yay! Back to hugely catastrophic what-ifs I liked this one.

Also: the punchline was so terrible, and so good at the same time

No, it was just terrible, like dubstep.
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