A hail cannon is a shock wave

Shock wave

A shock wave is a type of propagating disturbance. Like an ordinary wave, it carries energy and can propagate through a medium or in some cases in the absence of a material medium, through a field such as the electromagnetic field...

 generator claimed to disrupt the formation of hailstones in the atmosphere

Atmosphere

An atmosphere is a layer of gases that may surround a material body of sufficient mass, and that is held in place by the gravity of the body. An atmosphere may be retained for a longer duration, if the gravity is high and the atmosphere's temperature is low...

.

These devices frequently engender conflict between farmers and neighbors when used, 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.

Historical use

In the French wine-growing regions church-bells were traditionally rang in the face of oncoming storms, later replaced by firing rockets or cannons.

Modern systems

An explosive charge of acetylene

Acetylene

Acetylene is the chemical compound with the formula C2H2. It is a hydrocarbon and the simplest alkyne. This colorless gas is widely used as a fuel and a chemical building block. It is unstable in pure form and thus is usually handled as a solution.As an alkyne, acetylene is unsaturated because...

 gas

Gas

Gas is one of the three classical states of matter . Near absolute zero, a substance exists as a solid. As heat is added to this substance it melts into a liquid at its melting point , boils into a gas at its boiling point, and if heated high enough would enter a plasma state in which the electrons...

 and air is fired in the lower chamber of the machine

Machine

A machine manages power to accomplish a task, examples include, a mechanical system, a computing system, an electronic system, and a molecular machine. In common usage, the meaning is that of a device having parts that perform or assist in performing any type of work...

. As the resulting energy

Energy

In physics, energy is an indirectly observed quantity. It is often understood as the ability a physical system has to do work on other physical systems...

 passes through the neck and into the cone it develops into a force

Force

In physics, a force is any influence that causes an object to undergo a change in speed, a change in direction, or a change in shape. In other words, a force is that which can cause an object with mass to change its velocity , i.e., to accelerate, or which can cause a flexible object to deform...

 that becomes a shock wave

Shock wave

A shock wave is a type of propagating disturbance. Like an ordinary wave, it carries energy and can propagate through a medium or in some cases in the absence of a material medium, through a field such as the electromagnetic field...

. 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.

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

National Center for Atmospheric Research

The National Center for Atmospheric Research has multiple facilities, including the I. M. Pei-designed Mesa Laboratory headquarters in Boulder, Colorado. NCAR is managed by the nonprofit University Corporation for Atmospheric Research and sponsored by the National Science Foundation...

 in Boulder, Colorado

Boulder, Colorado

Boulder is the county seat and most populous city of Boulder County and the 11th most populous city in the U.S. state of Colorado. Boulder is located at the base of the foothills of the Rocky Mountains at an elevation of...

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."

External links

• 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