Jumbo Deluxe Hail

Blog topic: 
I tried to bring some order to the zillion or so pictures I have, and stumbled upon pictures from a fantastic hail storm we had west of Jerusalem in October 2004. The largest stones were recorded in Kibbutz Tzuba (where incidentally a cave was recently discovered, which is said to have been used by John the Baptist, but that is totally irrelevant I guess). The hailstones were up to 5 cm in diameter. Three cars belonging to the Kibbutz had their windshields shattered. Ours which was there at the time wasn't shattered but it did crack. As the pictures demonstrate, there were an amazing number of dents. Luckily for us, we had total insurance coverage, so many of the sheet metal parts were simply replaced, as was the front windshield. Moreover, given that it was a "force majeure" event, our insurance premium did not change. ;-)

4 cm in diameter hail. Fell over Mevasseret Zion (western suburb of Jerusalem) on Oct 29, 2004. In nearby Kibbutz Tzuba, hailstones measuring 5 cm in diameter were measured. 3 car windshields were shattered.

Dents in the sheet metal of a Renault Clio, caused by 5 cm hail.

The hail pictures were taken in Mevasseret, a few miles north east of Tzuba. In Mevasseret, the stones were "only" 4 cms in diameters or less. Doesn't sound like much, but it does make a significant difference in the energy of the hail stones.

Here is a physicists take on it.

Speed of Falling Hail and the necessary updraft in the thunderstorm

The terminal speed of a falling hailstone is obtained by finding the equilibrium between the gravitational pull downwards and the air drag upwards.

If the hailstone has a radius r then the gravitational force down is given by:
$ \displaystyle F_{grav} = {4 \pi \over 3} \rho_{ice} r^3 g $
The air drag upwards is given by (e.g., see here):
$ \displaystyle F_{drag} = {1 \over 2} C_d \rho_{air} v^2 A = {\pi \over 2} C_d \rho_{air} v^2 r^2 $
Where the drag coefficient Cd is about 0.5 for hailstones.

By equating the forces, we find the terminal velocity:
$ \displaystyle F_{grav} = F_{drag} ~~~\Rightarrow ~~~ v_{term} = \sqrt{ 8 g \rho_{ice} r \over 3 C_d \rho_{air}} $
Thus, a 5 cm in diameter hail reaches a terminal velocity of:
$ \displaystyle v_{term}(r=2.5cm)~ \approx~ 30.5~ m/s ~\approx~ 110 ~km/s ~\approx~ 70~ mi/hr $
To obtain such a large hail stone, we need updrafts which are at least that strong!

Energy in the hailstones

The kinetic energy in the hailstone will be:
$ \displaystyle E_{kin} = {1 \over 2} m v_{term}^2 = {16 \pi g \rho_{ice}^2 \over 9 C_d \rho_{air}} r^4 $
For a 5 cm in diameter hailstone, we obtain about 2100 J. This a little bit more than the energy of a 5.56 bullet (used in M-16's for example).

Because of the high dependency in radius, the 5 cm hailstones have an energy which is about (5/4)4 ~ 2.5 times larger than the 4 cm in diameter hail in the picture, even though they are moving only (5/4)1/2 ~ 1.11 times faster.

For comparison, the largest hailstone ever measured had a radius of about 5 inches. Thus, it was moving at about 155 mi/hr and had about 40 times the energy of the hailstones which shattered the wind shields in this middle eastern storm.


Comments (2)

  • anon

    jesus christ! this can kill someone!

    Jul 10, 2006
  • anon

    I remember a similar Storm further south in Beersheva, at around 97...

    The city suffered expensive damages because of the golf-ball hailstones...
    Almost every one in the city had to replace their window shutters and solar heating panels... I do not think anyone was struck badly by a hailstone

    Feb 22, 2007