A hail cannon is a shock wave generator claimed to disrupt the formation of hailstones in the atmosphere.
These devices frequently engender conflict between farmers and neighbors when used,[1] because they are repeatedly fired every 1 to 10 seconds over the period when a storm is approaching and until it has passed through the area, yet there is little or no scientific evidence for the effectiveness.
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In the French wine-growing regions church-bells were traditionally rang in the face of oncoming storms, [2] later replaced by firing rockets or cannons.[3]
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 shock wave. This shock wave then travels at the speed of sound through the cloud formations above, a disturbance which manufacturers claim disrupts the growth phase of the hailstones.
Manufacturers claim 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 stones, although all manufacturers unanimously agree that the area of effect of their device is only 100 to 200 square meters directly above.
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.[4] 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."
Media related to [//commons.wikimedia.org/wiki/Category:Hail_cannons Hail cannons] at Wikimedia Commons