## Hey Dude, the hailstones are as big as lemons!!!!!

March 9, 2010

Melbourne was hit with a big mama of a hailstorm on Saturday.  (6/3/10) There was flash flooding, hailstones up to the size of lemons (Pic Left 774 ABC listener at Scorseby and lower right Herald Sun ) and winds up to 100 kph. The lemon sized hailstones left holes in rooves in the central path of the storm. Flood and hail damage is estimated at \$A200 million.

You can see some of the action on Incredible Melbourne Storm on YouTube

How fast do hailstones fall?

Can a hailstone kill you?

Working out the speed of hailstones is tricky because hail falls when it is too heavy for the updraft supporting it. Hailstones bump around inside the thunderstorm hitting raindrops and other hailstones. This bumping slows down their fall. Turbulence, drag and air resistance also effect the speed.

Fear not mathspigs.

Someone somewhere always has an equation. In this case, the National Severe Storms Laboratory USA has come up with an equation for hailstone freefall speed (Density 0.92 g/cc).

Year 11 & 12 mathspigs you can do these calcs and you will find an absolutely wonderful scientific calculator at WEB2.0Calc

It’s raining, um, fruit out here?

The bureau of meteorology runs into problems trying to name the sizes of hailstones. In the major 1999 hailstorm in Sydney the following descriptions were used: “tennis ball” size (6.3 cm), “cricket ball” (7 cm) or “oranges” (8 cm) “grapefruits” (10 cm), “half-bricks” (about 11.5 cm) and “rockmelons” (about 13 cm). More details @ Natural Hazards Quarterly.

Mathspig got caught in this hailstorm. I was flying from Melbourne to Sydney to Armadale. Arriving just after the storm I found all flights temporarily cancelled as many of the small planes were damaged. I was eventually put on a flight… I didn’t know where I was going. I landed in Tamworth and caught a bus to Armadale. As we flew out we could see the storm damaged rooves.

How fast do hailstones fall? Take a look.

As I want this blog to be accessible for mathspigs of high school standard. I’ve done some calcs for you.

You can do the volume calcs. But as you can see a lemon-sized hailstone would be coming at you at 200 kph and it would weigh about half a kilo.  You can work out the exact weight using the density volume calcs and density above @ 0.92 g/cc . (Note: This hailstone speed equation is for terminal velocity in a free fall. If there was a side wind the hailstone would be going even faster!!)

So run for it!!!!

Can a hailstone kill you? Yes!!! There are not that many recorded deaths. According to the Dallas Observer News (13th April 2000) Juan Carlos Osegueraa, 19 year old youth of Fort Worth Texas died trying to save his familiy’s Mazda pickup truck when a softball size hailstone smashed through the window into his temple. He was 2 1/2 m from safety.

Take care, mathspigs.  Don’t be tempted to save a car in a hailstorm.