These devices frequently engender conflict between farmers and neighbors when used, because they are repeatedly fired every 1 to 10 seconds while a storm is approaching and until it has passed through the area, yet there is little or no scientific evidence for their effectiveness.
A mixture of acetylene and oxygen is ignited in the lower chamber of the machine. As the resulting blast passes through the neck and into the cone, it develops into 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 hailstones.
Manufacturers claim that what would otherwise have fallen as hailstones 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 hailstones, 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. 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 Hail cannons at Wikimedia Commons
- Hail Storms on a google map - non commercial
- Wieringa, J. 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. Goswami, July 15, 2008.
- Cannons both hailed and blasted, Rocky Mountain News, July 10, 2006.
- History Repeated: The Forgotten Hail Cannons of Europe (pdf), Bulletin of the American Meteorological Society, Volume 62, Issue 3 (March 1981)
- NPR story on hail cannons
- Shock waves generator animation
- Weather radar for hail storms detection