Hail Cannon
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 then travels at the speed of sound through the cloud formations above, a disturbance which manufacturers claim disrupts the growth phase of the hailstones.
Conflicts between farmers and surrounding residents have been reported in newspapers in Colorado and Vermont, due to the cannons' booming sound.
The device is repeatedly fired every 4 seconds over the period when the storm is approaching and until it has passed through the area. It is claimed that what would otherwise have fallen as hail stones then falls as slush or rain.
It is said to be critical that the machine is running during the approach of the storm in order to affect the developing hail stone. These machines can not alter 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 30 years with most development in the last 10 years.
The protected area for an individual machine is said to be approximately a 500 meter radius with a lower level of effectiveness as distance from the device increases.
Radar controlled systems are available to replace human operation of the unit which is particularly important in areas subject to hail storms at night.
Scientific Evidence
There is very little empirical evidence in favor of the effectiveness of these devices.
A 2006 review by Jon Wieringa and Iwan Holleman in the journal Meteorologische Zeitschrift summarized a variety of negative and inconclusive scientific measurements, concluding that "the use of cannons or explosive rockets is waste of money and effort."
From a theoretical perspective there is reason to doubt that hail cannons are effective. For example, thunder is a much more powerful sonic wave, and is usually found in the same storm that generates hail, yet doesn't seem to disturb the growth of hailstones. Charles Knight, a cloud physicist at the National Center for Atmospheric Research in Boulder, Colorado was quoted in a newspaper article of July 10, 2008 as saying, "I don't find anyone in the scientific community who would validate hail cannons, but there are believers in all sorts of things.
It would be very hard to prove they don't work, weather being as unpredictable as it is."
Use of rockets
Though hail cannons may not be effective, the use of rockets to disperse storms is a widely accepted possibility.
In the largest rain dispersal operation on record in China, the Beijing Municipal Meteorological Bureau fired a total of 1,104 "rain dispersal rockets" within an eight-hour period prior to and during the opening ceremonies of the Games of the XXIX Olympiad on August 8, 2008. The rockets, launched from twenty-one separate sites, "successfully intercepted a stretch of rain belt from moving towards the stadium..." said Guo Hu, head of the Beijing Municipal Meteorological Bureau.
and Iwan Holleman, If cannons cannot fight hail, what else?, Meteorologische Zeitschrift, 15, issue 3, June 2006.
'Hail cannon' opponents decry its use, Bennington Banner, Neal P.