Ever seen an hailstone like this?

[Chris Lott]

I read about the Aurora, Nebraska storm in a book I have. It stated that the tops on this particular storm reached 70,000 ft. and the 7 inch hail in the storm was referred to as "volleyball size" hail. It said it tore big chunks of ground up like it was nothing, and it also stated I believe, that this was a slow moving storm and it had quite a bit of flooding as well.

 

[John Diel]

I don't suppose anyone would know if you could get a cast of the hailstones in question?

I do several Severe Weather Safety classes every year and having something like that for the kids (or adults for that matter) would be great!

 

[Jayson Prentice]

I know I saw a couple posts wondering what speed a hailstone like that would fall at...

Well I may have the answer!

I'm just going to put all the math on here, just so if I make a mistake somebody can notice it. :?

Hailstone was 7 inches in diameter, that would make a 3.5 inch radius, that has to be put into cm for the equation so, radius = 8.89 cm.

So, terminal velocity of the hailstone would be: 59.63 meters/sec. Translate that to mph, 59.63 x ~2.24= 133.4 mph!

There ya have it, terminal velocity of a hailstone that size would be a lil' over 133 mph!

Also, added to this, you would need to take i nto consideration the downdrafts/RFD going against the hail...which could add more speed to it's decent.

Actually, you wouldn't. The formula used is the terminal velocity formula, so this is the fastest speed that a hailstone could reach at that size. Anything going that fast is going to hurt and damage something, especially when I'm sure that thing weighed a good amount. Would be interesting if anybody can pull up some radar of that cell.

 

[Mike Hollingshead]

The link Dave posted has a radar image of it. You can also download level II data from the ncdc site on it(if you have a viewer). Well I guess if they have it. It seems some of the better events radar data is often missing. The southern cell produced the most flooding as well as a killer tornado(Nebraska's first in like 10+ years or something....only to have another killer tornado in ne NE the very next day!.....crazy statistic).

John, that is a GREAT idea. That would be a great conversation piece I'd think. I'd buy one.

 

[Greg Campbell]

I know I saw a couple posts wondering what speed a hailstone like that would fall at...

Well I may have the answer!

I'm just going to put all the math on here, just so if I make a mistake somebody can notice it. :?

Hailstone was 7 inches in diameter, that would make a 3.5 inch radius, that has to be put into cm for the equation so, radius = 8.89 cm.

So, terminal velocity of the hailstone would be: 59.63 meters/sec. Translate that to mph, 59.63 x ~2.24= 133.4 mph!

There ya have it, terminal velocity of a hailstone that size would be a lil' over 133 mph!

Wait a minute! What equation?

Terminal velocity is a matter of weight vs. drag. If the hailstone was a fused conglomerate, is would be full of voids, making it lighter than a solid sphere of ice. Drag would be difficult to calculate for such a lumpy, irregular object, and would probably be higher than it's size indicates.

On the other hand, drag is roughly a 'speed squared' value. Even if the hailstone was extra rough and 30% air, it would still be falling at over 100MPH. (Just a wild assed guess )

-Greg

 

[GPhillips]

Would be interesting if anybody can pull up some radar of that cell.

Below are links to the 0.5, 4.3, and 12.0 slides through the storm. The radar images are at 0003 UTC and Storm Data had the hailstone at 0005 UTC.

http://members.cox.net/geophi/0.5.jpg
http://members.cox.net/geophi/4.3.jpg
http://members.cox.net/geophi/12.0.jpg

 

[Jayson Prentice]

I guess I never really posted the equation that I used, its' listed in a meteorology textbook that I have and I'm assuming that this would be for roughly circular hailstones, so as you said with it being so 'lumpy' this is a pretty tough thing to figure out. We could probably just leave it at it was traveling pretty darn fast! lol... But, as the rough estimate it was probably somewhere around 120-130 mph..

The equation that is given is this: (Square root: 1/3p divided by k) x (Square root: r) With r being in cm, the answer is then given in meters/sec. The p and k values are not direcly stated, but it shows that that equation is equal to: 20 x (Square root: r). I'm sure there is a better explanation of values, etc somewhere that I just don't know about. I was simply doing my best to try to get a rough estimate out there of how fast this thing was likely traveling. Hope it helped...

 

[Andrew Khan]

I know I saw a couple posts wondering what speed a hailstone like that would fall at...

Well I may have the answer!

I'm just going to put all the math on here, just so if I make a mistake somebody can notice it. :?

Hailstone was 7 inches in diameter, that would make a 3.5 inch radius, that has to be put into cm for the equation so, radius = 8.89 cm.

So, terminal velocity of the hailstone would be: 59.63 meters/sec. Translate that to mph, 59.63 x ~2.24= 133.4 mph!

There ya have it, terminal velocity of a hailstone that size would be a lil' over 133 mph!

Also, added to this, you would need to take i nto consideration the downdrafts/RFD going against the hail...which could add more speed to it's decent.

Actually, you wouldn't. The formula used is the terminal velocity formula, so this is the fastest speed that a hailstone could reach at that size. Anything going that fast is going to hurt and damage something, especially when I'm sure that thing weighed a good amount. Would be interesting if anybody can pull up some radar of that cell.

Yes, but this is without any other external influencing on the hailstone itself, besides gravity. That said, there were likely to be some sort of strong RFD/downdrafts affecting the falling motion of the hail stone, so to accuratly get an approximate speed, you would need to incorporate this into as one of the factors.

 

[Robert Dewey]

Yeah, Andrew is right. Terminal velocity only factors in gravity as the main vector (it's usually calculated in a vaccuum). If you have other external components acting on the object with great force in the direction of the objects mean motion (typically "down"), the object can easily exceed terminal velocity.

For example, that hailstone in a downburst of 200MPH will exceed it's gravitational terminal velocity of 120MPH.

But in the Aurora case, if the downdraft winds weren't exceeding the terminal velocity of the actual hailstone, then the hailstone itself wasn't exceeding terminal velocity.

 

[Greg Campbell]

Yeah, Andrew is right. Terminal velocity only factors in gravity as the main vector (it's usually calculated in a vaccuum).

???
There is/can_be no 'TV' in a vacuum . If the moon were to loose all angular motion, its impact speed would be limited only by the time available to accelerate before it hit Earth.

If you have other external components acting on the object with great force in the direction of the objects mean motion (typically "down"), the object can easily exceed terminal velocity.

For example, that hailstone in a downburst of 200MPH will exceed it's gravitational terminal velocity of 120MPH.

But in the Aurora case, if the downdraft winds weren't exceeding the terminal velocity of the actual hailstone, then the hailstone itself wasn't exceeding terminal velocity.

I guess it depends on your point of perspective. Relative to the air, it will still be falling @ 130MPH. If embeded in a 100 MPH (Aurora?) downdraft, it will be moving at 230MPH towards to the ground. If the downdraft is 200, it will fall at 330.

As the downdraft nears earth, it will spread out and the winds will shift to a horizontal vector. The hailstone's vertical velocity will also slow, although any lateral motion will increase it's ground relative speed. 130 down + 100 sideways = 164 diagonal MPH. Ouch!!

 

[Jayson Prentice]

Yea, there are several factors that would change the speed that it was falling at, but this is a 'decent' estimate I would say. Without doing more research into it using that simple formula is likely to work pretty good...

 

[Greg Campbell]

Here's a neat way to estimate it.

From http://en.wikipedia.org/wiki/Terminal_velocity

Approximating terminal velocity is much more easily done than calculating the terminal velocity because of the difficulty in finding the value of Cd. One simple small scale method is to hang an object out a car window by a thin string. The terminal velocity of the object is the speed of the car when the object hangs at a 45° angle. This can be easily proven mathematically because it is when the atmospheric drag (in the horizontal direction) is equal to the force of gravity.

Mental image: Driving down the road at 100+MPH with a 10lb hailstone swinging from a fishing pole stuck out the window. When pulled over, tell the police you're doing a "very important science experiment."

 

[Jeff Snyder]

Terminal velocity only factors in gravity as the main vector (it's usually calculated in a vaccuum).

Gravity is a force... F = m*a, so A = F / m. So, an applied force acts to accelerate. In a vaccum, a hailstone (or anythign else) will accelerate indefinately assuming there aren't other forces acting against gravity. In a non-vacuum, such as the atmosphere, air acts as a drag force that acts opposite gravity. So, without friction or things like turbulent eddies (both of which would be absent in a vacuum), the hailstone would have no terminal velocity -- it would accelerate until something stopped it. The terminal velocity is, therefore, the speed at which the downward acceleration/force from gravity is balanced by friction/drag (which act opposite the motion, or act upwards in this case).

Terminal velocity formulae are very complex, particularly for irregularly-shaped objects like giant hailstones. I wouldn't try to be too precise in your numbers given that the formula used is an approximation.

 

[Robert Dewey]

Well... I guess I can take the worlds biggest idiot award now...

How could I have forgotten one of the worlds best known experiments? The feather and the bowling ball / brick / whatever-heavy-object-you-choose in a vacuum?

Anyway, I guess a downburst would act against the drag vector... Allowing the object to accelerate until it reaches the drag force / weight force equililbrium. Wouldn't that mean an increase in ground-relative speed?

If the downburst suddenly stopped, then the drag vector would increase, and the object would begin to deccelerate as the drag / weight force evens out.

 

[Chuck Vlcek]

Well, to drag this topic out further, why not make a cast of that stone, then a replica of the same shape and density, then drop it from an airplane over (or near) where the original stone fell and measure its fall speed or at least the force of its impact. The unknown would be the difference in vertical velocity of the air column it falls through (original vs replica) but it would at least be a start. I recall reading about research involving impact sensors measuring the kinetic energy of impacting hailstones.

An updraft can "support" a higher than theoretical weight of hailstone in two ways: 1) irregular shape increasing drag and 2) simply slowing the fall thru a hail growth zone from top to bottom, so that it continues to grow even while falling thru the updraft. After falling below the freezing level, it might then exit the updraft and get caught in a downdraft to minimize melting.