Hail cannon
A Hail cannon is a pseudoscientific device that acts as a shock wave generator alleged 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 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. Most manufacturers do not touch on the fact that the action of thunder itself would be the equivalent of thousands of such devices yet has no effect on hail.
These devices can engender conflict between farmers and neighbors when used, as those who have purchased such an expensive device defend the pseudoscience behind its function at risk of being taken for a fool by snake oil merchants.[citation needed] The cause of the conflict arises from the fact that the device[citation needed] is repeatedly fired every 1 to 10 seconds over the period when the storm is approaching and until it has passed through the area. 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. Dating back to the 18th century, modern 'radar controlled systems' are nowadays available to replace human operation of the unit.
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.[1] 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."
See also
References
External links
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