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A Hail cannon is a shock wave generator used to disrupt the formation of hailstones in the atmosphere in their growing phase. An explosive charge of acetylene gas and air is fired in the lower chamber of the machine. As the resulting energy passes through the neck and into the cone it develops into a force that becomes a shockwave.This shockwave, clearly audible as a large whistling sound, thentravels at the speed of sound through the cloud formations above,disrupting the growth phase of the hailstones.

The deviceis repeatedly fired every 4 seconds over the period when the storm isapproaching and until it has passed through the area. What wouldotherwise have fallen as hail stones then falls as slush or rain. It iscritical that the machine is running during the approach of the stormin order to affect the developing hail stone. These machines can notalter the form of an already developed solidified hailstone.

While the history of Hail cannons date back into the 18th century,the modern Hail Cannon has been developed extensively over the last 30years with most development in the last 10 years.

The protected area for an individual machine is approximately a 500meter radius with a lower level of effectiveness as distance from thedevice increases.

Radar controlled systems are available to replace human operation of the unit which is particularly important in areas subject to hails storms at night.

There is little scientific evidence of the effectiveness of thesedevices. For example, thunder is a much more powerful sonic wave, andis usually found in the same storm that generates hail, yet doesn'tseem to disturb the growth of hailstones.

Charles Knight, a cloud physicist at the National Center for Atmospheric Research in Boulder, Coloradosays, "I don't find anyone in the scientific community who wouldvalidate hail cannons, but there are believers in all sorts of things.It would be very hard to prove they don't work, weather being asunpredictable as it is."