Can three inches of rain kill you?

[Vonie]

In my (oh-so boring) physics class were were taking about rain and what if 3 inches of rain felll all at once, like a big wet layer of...rain. So there's a huge, falling 3-inch thick layer of rain falling from normal cloud height and you're standing in a field. Assuming the rain would stay together until the very moment it hits your head. Would it kill you?

What says ye?

 

[Dave A]

I doubt it, your only getting hit by a small ammount of that rain that falls directly onto you. I know you can be killed by water, at certain speeds it could be as good as hitting a solid but that I guess depends on how the water can disperse, but I think 3 inches would disperese the second it touches you would.

 

[Kylehnat]

"Normal" cloud height can be anywhere from 1,000 to 50,000 feet. The type of clouds from which most rain falls is 3,000-10,000 feet. The type of storm that could instantaneously dump 3 inches of rain would have cloudtops in the 20,000-50,000 foot range (and it would be frozen at that altitude).

A rough estimate would be that a 3-inch-think chunk of rain (the part that hits your head) weighs about 3 lbs. Go buy a little 3-lb. dumbell, wrap in something soft, and have a friend drop it on your head while standing on the kitchen counter. Then do it off the top of your house. Then do it off the top of the Sears Tower. Then drop it out of a plane. Then drop it out of a B-2.

For perspective, a 3-inch chunk of rain falling from 3,000 feet would have enough energy to power a 100 watt lightbulb for 2 minutes . From 50,000 feet? Half an hour.

 

[Dave A]

Poor analogy, the difference between a 3lb dumbell and 3lbs of water is that when it hits you it is a solid. When the water hits you, it disperses upon impact. You can liken that to diving of a diving board into a swimming pool, if you replace the water with bricks you will get bandly hurt on impact or even killed, why, because the bricks are solid, the water is not. It all depends on how it disperses upon impact. i don't think you will feel the full weight of the water hitting you either, because as the bottom layers of water disperse the water above it will also begin to disperse and any that doesn't will hit you after the water below has dispersed. As I said, it all depends on how much the water disperses. I don't know how to work it out, but my guess would be that no, it wouldn't kill you.

As for the water being frozen because of the altitude, that is not part of the question.

 

[ExigeEvan]

"Normal" cloud height can be anywhere from 1,000 to 50,000 feet. The type of clouds from which most rain falls is 3,000-10,000 feet. The type of storm that could instantaneously dump 3 inches of rain would have cloudtops in the 20,000-50,000 foot range (and it would be frozen at that altitude).

A rough estimate would be that a 3-inch-think chunk of rain (the part that hits your head) weighs about 3 lbs. Go buy a little 3-lb. dumbell, wrap in something soft, and have a friend drop it on your head while standing on the kitchen counter. Then do it off the top of your house. Then do it off the top of the Sears Tower. Then drop it out of a plane. Then drop it out of a B-2.

For perspective, a 3-inch chunk of rain falling from 3,000 feet would have enough energy to power a 100 watt lightbulb for 2 minutes . From 50,000 feet? Half an hour.

Yes but you're not looking at the whole picture.

Sure it will have a large amount of Potential Energy but as it is rain it's a liquid, so can't really be compared to dropping a dumbell on your head. It's like saying a bucket of water has the same effect as say a bucket of marbles. The marbles won't deform on impact, but the water will, reducing the force of the impact.

The other thing to remember is that is theoretically impossible. If the rain fell as a sheet, where would all the air it was displacing go?

[EDIT] tree'd.

 

[Dave A]

It would be pushed out to the sides, unless the sheet of rain was global and had no holes in it. Then it would fall so far, and stop falling once the air beneath has been compressed as much as it can be.

Bear in mind that the sheet of rain in the question is stuck together until the moment it hits your head so it can't break up.

 

[ExigeEvan]

It would be pushed out to the sides, unless the sheet of rain was global and had no holes in it. Then it would fall so far, and stop falling once the air beneath has been compressed as much as it can be.

Bear in mind that the sheet of rain in the question is stuck together until the moment it hits your head so it can't break up.

Actually I was thinking it would be more like dropping a bed sheet from the sky. Unless it tangled and twisted it would probably float elegantly to the ground.

 

[Dave A]

Oh right, yeah ofcourse you'd have to take into consideration it wouldn't be falling like a missile, unless it was a very focused sheet that was only covering the exact area above you, then it would fall pretty quick. But if we're talking abou a sheet of any notable size then the air underneath would slow it down notably. It shouldn't be as slow as a bed sheet though, a bed sheet is a hell of a lot lighter.

 

[Vonie]

Ok, ignore the fact that the air would be stuck, the rain is just falling until it hits you, say from 7,000 feet.

I sided with no as well.

 

[Dave A]

Then it will break up as it falls and it will hit you in droplets.

Still no.

 

[Barracuda]

you also have to consider speed. The water based on its size and weight will fall until a certain speed and remain at that speed until the moment of impact. I highly doubt a 3inch sheet of water will hurt you in any way, Maybe a 3ft sheet but not just 3 inch.

case closed, this myth is busted , lol.

 

[Kylehnat]

Poor analogy, the difference between a 3lb dumbell and 3lbs of water is that when it hits you it is a solid. When the water hits you, it disperses upon impact.

Most liquids are not particularly compressible. At a high enough velocity, liquid impacting a flat surface will behave more like a solid. Keep in mind that falling into a river, lake, or ocean from high altitude (even as little as 150 feet) is often fatal. Water is not always warm and fluffy.

As for the water being frozen because of the altitude, that is not part of the question.

You're not having very much fun with this, are you? Free your mind, man! *takes hit from bong*

What if the water is supercooled, and freezes upon impact? What happens then?

 

[Perfect Balance]

It shouldn't be as slow as a bed sheet though, a bed sheet is a hell of a lot lighter.

Weight doesn't have anything to do with how fast something falls.

 

[Vonie]

Most liquids are not particularly compressible. At a high enough velocity, liquid impacting a flat surface will behave more like a solid. Keep in mind that falling into a river, lake, or ocean from high altitude (even as little as 150 feet) is often fatal. Water is not always warm and fluffy.

Right, bu thte water you diven into (or bellyflop into) has nowhere to easily go. The water falling on you can just slow down or go to the side.

 

[wfooshee]

Water will not fall as a solid object. I don't mean solid as in a brick, but say you contained a large volume of water, no voids, and let the container fall from a high altitude, then magically removed the container during the fall, the water would then break up immediately into raindrops; it would not adhere to itself. There is a limit to the volume that can be contained by surface tension, and it will break into that smaller volume nearly instantly. Any falling water is not a continuous stream, it can't be done. Don't argue waterfalls, why do you think they have a white appearance? Drops falling in an air volume.

Real physics aside, if the water could adhere to itself well enough to fall in a single volume equivalent to 3 inches of rain, it would be fairly massive, I would think, and the fact that it would dissipate on impact would not significantly lessen that impact. 15 pounds of water or 15 pounds of gravel, doesn't matter, it's going to hurt. Briefly.

(I have no idea what a 3-inch thick slab of water weighs, I just used 15 pounds as an example. I don't want that on my head!)

 

[ExigeEvan]

Well define the size of the sheet.

Okay, 1kg = 1litre.

So if the water is 7.5cm (~3 inch) by 26cm by 27cm (approximate dimensions taken from anthropometric data, no acound for curvature of head that gives you a volume of 5265 cm cubed. Which is 5265ml which is approx 5.3l thus 5.3 kilos.

So now all we need is someone to load a bucket with 5.3Kg of water and dump it on someones head. Any offers?

 

[Dave A]

Weight doesn't have anything to do with how fast something falls.

That is true to a certain extent, but things under a certain weight will get carried by the wind more, and thermals will catch them and slow them down more than something heavier. If you made a kite out of lead, would it still fly? Though shape often has far more to do with it.

Most liquids are not particularly compressible. At a high enough velocity, liquid impacting a flat surface will behave more like a solid. Keep in mind that falling into a river, lake, or ocean from high altitude (even as little as 150 feet) is often fatal. Water is not always warm and fluffy.

There's a difference between you falling into water and water falling onto you. If you fall into a river, lake, the sea ect then as you fall into it, the water underneath you will compress and be pushed out to the sides, the water being pushed can only go as far as the water next to it will allow and eventually it will reach a limit. Here we're talking about the water hitting you, as soon as water hits you it can move somewhere else and there's nothing surrounding you preventing it from being pushed away as far as the force of the impact allows.

What if the water is supercooled, and freezes upon impact? What happens then?

Then it would be like being hit by a block of ice, that would kill you.

 

[Kylehnat]

Here we're talking about the water hitting you, as soon as water hits you it can move somewhere else and there's nothing surrounding you preventing it from being pushed away as far as the force of the impact allows.

Hydrodemolition is used to tunnel through solid rock and break apart concrete. Aim a pressure washer at your window and see what happens. That water has plenty of space to just move out to the side, right?

Then it would be like being hit by a block of ice, that would kill you.

But at least you'd die with an awesome icecube hat!

 

[Evolution.]

Weight doesn't have anything to do with how fast something falls.

I believe Dave tree'd me here, but this is only a true statement in an undisturbed vacuum.

 

[Dave A]

Hydrodemolition is used to tunnel through solid rock and break apart concrete. Aim a pressure washer at your window and see what happens. That water has plenty of space to just move out to the side, right?

The key word, "pressure". The speed water falls is not going be generating that much force so it can kill you from the impact alone. It would take much more than 3 inches of it to be heavy enough to crush or even overwhelm you.

But at least you'd die with an awesome icecube hat!

I doubt you'd have much of a head left to put a hat onto .

 

[ExigeEvan]

Hydrodemolition is used to tunnel through solid rock and break apart concrete. Aim a pressure washer at your window and see what happens. That water has plenty of space to just move out to the side, right?

But at least you'd die with an awesome icecube hat!

You can't compare an impact force to a continuous force. It's like comparing a slap to a push.

 

[Dave A]

It's like comparing a car rolling down a shallow hill into a wall in neutral and then someone flooring the gas and hammering it down the hill into the wall. Increasing the force of the impact can dramatically alter the resulting impact. You can't compare one example where something is forced, and one where it's not.

 

[Keef]

Rain falls pretty slowly. The biggest drops only get up to about 20 mph. There isn't much force behind it at all. I think if a 3 inch thick layer of water fell from the sky just as rain does, the most it would do is make you go "Damn! That was a lot of water!" People stand under water falls, people get hit with firehoses, and people jump into water, falling at probably 20 miles per hour, without incident.

 

[3-Wheel Drive]

Weight doesn't have anything to do with how fast something falls.

I believe Dave tree'd me here, but this is only a true statement in an undisturbed vacuum.

Incorrect. Perfect Balance is right, weight has nothing to do with falling, vacuum or not. What you are thinking of is air resistance. For example, if two 1ft radius falls are dropped, and one weighs 1lb, and the other weighs 10lbs, both will fall at the same rate, whether in air or a vacuum. Now, if they both weighed 1lb and one was the size of a house, the big one would fall more slowly due to friction with the air, and would only fall at equal rates if in a vacuum.

 

[Dave A]

That's not 100% true either, take feathers for example, you take a real feather and then an exact replica of that feather, but made of metal. Thus it's heavier, the feather might float down, or evevn float upwards. Why is that if the weight is not a factor. It's because the feather is light enough to escape gravity with little force pushing it upwards.

 

[wfooshee]

It's still air resistance, not any other force. Without the air, they behave identically. The acceleration imparted by gravity is the same regardless of the object's weight. When we say weight is not a factor, we mean ideally, without the influence of a containing medium. Witness Buzz Aldrin's hammer/feather drop on the moon during Apollo 11. If it is moving through some medium (air, water, ice) then that meduim's friction on the object affects it, and will affect a less massive object to a greater degree.

Put another way, weight is not a factor in how much gravity affects an object. The feather has the same gravitational acceleration as the hammer, or your metal feather.

 

[Sharky.]

I believe Dave tree'd me here, but this is only a true statement in an undisturbed vacuum.

You've been "tree'd" (well, kinda) again: Basic physics.

When something falls, it's acceleration is g - acceleration due to gravity = 9.81ms-2.

g is not affected by weight - g is what gives objects weight, so cannot be affected by itself.

Drag (and some other stuff... FAMINE!) affects the speed that something falls.

 

[niky]

This thread makes my head hurt. Simply because the idea itself is absurd, and the terminal velocity of water in air just isn't enough to really hurt or kill you.

Nobody has been killed by standing under a waterfall... except maybe by being dumb enough to drown in it. And that's a larger volume than three inches of rain. Or three feet.

But then, I may be a ghost... and my memory of having survived tons of water cascading down on me twenty years ago are the only true memories I still have left...

 

[Sakiale]

It's Mass and shape (aka, I guess, drag). If an object weighs say 1 lb, but is only a cubic cm, then it will fall at the normal speed--[accelerating at] 9.8m/s/s until terminal velocity. If it weighs 1 lb, but covers a 6'x6' flat square, then it will fall like a smooth sheet of paper or feather, it will be slowed down by its shape/drag. 👍

Gravity is not affected by weight, but the force applied IS affected by weight-- terminal velocity is when the object falling stops accelerating--in other words, the upwards force of its air resistance and gravity cancel each other out, to zero. But, if an object weighs 1 lb and has the force of 1 lb of gravity acting on it, then it will fall. However, an object weighing 9 lbs must have more than 9 lbs force of gravity to fall.

So, yeah, the main factors are Air Resistance (related to surface area and drag)/mass & shape.👍

 

[Boundary Layer]

Drag (and some other stuff... FAMINE!) affects the speed that something falls.

Terminal velocity = (2W/CDAp)^0.5

Just going back to ExigeExcel's example where he provided a few measurements:

W = 5.3 * 9.81 = 51.993 N
For a square prism with b = h, CD = 1 for Reynold's number > 1000. 26 x 27 cm is close enough....
A = 0.0702 m^2
p = 1.2 kg/m^3 for atmospheric air

So, this hypothetical sheet of water will hit you with a velocity of 35.134 m/s provided it's falling from a high enough altitude to attain terminal velocity (which it undoubtedly is).

The kinetic energy of the body of falling water is then 3271.15 J


...I think it could hurt, but I don't think it's lethal force though. Unless you're wearing a basin on your head you aren't going to recieve the water's full energy as blunt trauma. But I don't know enough about how water behaves under direct central impact to make any sort of guess at how much energy you do take from the collision.





It'll muss up your hair.