Anti-hail shock wave generator

To improve the transmission of positive ions from ground level to cloud level by a shock wave generator, a shroud is provided which surrounds the barrel of the generator for guiding a convective air flow vertically along the sides of the barrel to an orifice of the barrel. Positive ions present in the ambient air and also created by the environment surrounding the hot barrel are drawn upwardly by convection and a negative pressure following each explosion. The shroud is higher than the barrel and positive ions are drawn into the area in front of the barrel where shock waves displace the ions upwardly to cloud level for preventing hail nuclei formation.

FIELD OF THE INVENTION 
The present invention relates to an anti-hail shock wave generator 
including a shroud member surrounding a barrel of the generator for 
directing positive ions in front of the barrel. 
BACKGROUND OF THE INVENTION 
An anti-hail shock wave generator or cannon is known in the art from U.S. 
Pat. No. 3,848,801. In such a device, a shock wave is generated by 
detonating an explosive mixture of combustible gas and air in a combustion 
chamber having an upper orifice. A conical barrel is fit over the upper 
orifice and directs the shock wave resulting from the explosion upwardly 
to the sky. By firing the generator at regular intervals (eg. less than 25 
seconds, and usually every 8 to 14 seconds), a succession of shock waves 
are created which disrupts the internal microstructure of the clouds to 
prevent the formation of hail nuclei within a small area (typically a 500 
m radius) over the generator. It is believed that transport of positive 
ions from ground level to cloud level by the succession of shock waves is 
largely responsible for the disruption of the formation of hail nuclei. 
By using the known device, crop damage due to hail has been known to be 
completely eliminated or at least significantly reduced without any 
adverse environmental effects, however, to achieve good results, special 
care must be taken to operate the device properly starting about 15-25 
minutes before a hail storm in order to disrupt sufficiently the hailstone 
formation process. By operating the device at a faster firing rate, the 
combustion chamber and the barrel are subjected to more mechanical and 
temperature stress, and more fuel is consumed. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide an anti-hail shock wave 
generator with an enhanced ability to provide positive ions in front of 
the barrel opening. 
When the generator is operated at a faster rate (less than 8 seconds 
between firings), the temperature of the uninsulated combustion chamber 
and the barrel rises. At these higher temperatures, the barrel is able to 
ionize gas particles (creating positive ions) in the surrounding air, and 
these ionized gas particles located inside the barrel are propelled 
upwardly by the explosion and the resulting shock waves. Positive ions 
neutralize negatively charged particles in hail producing clouds, with the 
result of arresting the hail formation process. 
According to the invention, there is provided an anti-hail shock wave 
generator comprising a combustion chamber having an upper orifice, fuel 
injection means for injecting fuel into the chamber, ignition means for 
igniting the fuel in the chamber, a conical barrel having a small diameter 
lower end connected to the upper orifice and a large diameter upper end, a 
shroud member surrounding the barrel, the shroud having a top extending 
above the upper end, and means for mounting the shroud member over the 
barrel with a separation between the shroud and the barrel being selected 
to aid convective air flow carrying ionized particles from the surface of 
the barrel to the upper end.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
As shown in FIG. 1, cannon (10) comprises a combustion chamber (12) which 
may comprise a substantially cylindrical body with a rounded bottom and a 
rounded top portion which leads into a neck (15). The bottom of combustion 
chamber (12) is solidly mounted to a concrete pad (32) by feet (30). One 
or more air inlet ports (34) are provided with flaps (35) which are seated 
in ports (34) and open inwardly to provide one way valves for air rushing 
into chamber (12) after each ignition. Chamber (12) is provided with a 
fuel injector (14) which may comprise a solenoid valve controlling flow of 
acetylene gas from a an acetylene gas reservoir (not shown) into a central 
portion of chamber (12). Ignition means (16) are provided for igniting the 
acetylene gas injected into chamber (12), and ignition means (16) comprise 
spark gap electrodes and a high voltage generator coil (42). The spark gap 
generator coil (42) and solenoid valve (14) are controlled by control 
means (40). A conical barrel (18).has a large diameter upper end (22) and 
a small diameter lower end (20) which is connected to an upper orifice in 
neck (15) of chamber (12). 
When the shock wave generator cannon (10) is operated, control means (40) 
cause gas to be released through solenoid valve (14) into chamber (12) 
until sufficient gas for a full explosion resulting in a significant shock 
wave is present in chamber (12). Mixing of the acetylene gas with air in 
chamber (12) is automatic and rapid. A short time after solenoid valve 
(14) is closed, control means (40) trigger spark gap coil (42) to create a 
high voltage pulse resulting in a spark across the electrodes of ignition 
means (16). As the gas in chamber (12) rapidly combusts, a shock wave 
results which is directed by conical barrel (18). The momentum of the 
combustion gases is directed upwardly, and once the combustion gases have 
fully expanded, the upward momentum of the gases causes a negative 
pressure to be created in the combustion chamber (12) which results in 
flap (35) being drawn open so that fresh air may be drawn from ambient 
through port (34) to fill combustion chamber (12). 
As shown in FIG. 2, the shroud member (50) comprises an inner sheet member 
(56), a plurality of support ribs (60) and legs (58). Fresh air is drawn 
in at bottom inlet (52) and caused to rise in the space between sheet (56) 
and barrel (18) by convection. Convection causes air to flow towards the 
end (22) of barrel (18), allowing for the shocks waves generated to 
propagate the positive ions naturally occuring in the air upwardly. The 
conditions of high temperature and possibly shock waves in the space 
between sheet (56) and the barrel (18) also help increase the 
concentration of positive ions in the air. Since barrel (18) will 
dissipate less heat and therefore maintain a higher temperature if its 
coefficient of radiant emission is low, the material for barrel (18) is 
selected to have a lower emissivity, such as stainless steel. 
Ribs (60) are preferably between three and six in number and are welded to 
two parts of inside sheet (56) to form the inwardly tapered inlet (52) and 
the outwardly tapered upper portion end with upper outlet (54). While the 
inside sheet (56) may comprise truncated conical parts, it is possible to 
form shroud (50) with a polygonal cross-section, provided air driven by 
convection is able to be guided from the inlet (52) to the outlet (54). 
The material for shroud (50) should be resistant to elevated temperatures 
while able to withstand rain and wind. 
As can be appreciated, wind could clear away any positive ions guided to 
end (22) by shroud (50), and therefore, it is advantageous to provide 
outlet end (54) a little higher than barrel end (22) to provide some 
shelter from the wind. Smaller air currents present in the region in front 
of end (22) help to mix the ion enriched air from shroud (50) with the air 
in front of end (22). Preferably, the shroud (50) stands about 20 cm to 30 
cm higher than end (22) when barrel (18) is 3 m to 6 m high. Lower 
diameter (20) is preferably about 10 cm to 14 cm and upper end (22) 
preferably has a diameter of about 80 cm. The angle of inclination of the 
side of barrel (18) is preferably about 7.degree. to 9.degree., and sheet 
(56) is approximately parallel to barrel (18) in its upper portion. 
Therefore, the upper diameter of shroud (50) is about 90 cm to 120 cm. 
It is important to select a fuel and ignition system which can operate even 
when rain water passes through barrel (18) into chamber (12). It is 
important to select the parameters of fuel, combustion chamber volume to 
upper orifice size as well as barrel (18) dimensions in order that a good 
shock wave is generated and sufficient aspiration through ports (34) takes 
place in order to bring in sufficient fresh air for the next combustion. 
The preferred material for combustion chamber (12) is steel having a wall 
thickness of about 0.6 cm to 1.0 cm. Ports (34) are preferably two in 
number and have a diameter of about 15 cm. The combustion chamber has an 
internal diameter of about 45 cm and a volume of about 160 to 180 liters. 
The volume of acetylene gas injected is about 5 to 10 liters and the 
recommended time period between ignitions is 3.5 to 6 seconds. 
Although the invention has been described above with reference to the 
example of the preferred embodiment, it is to be understood that other 
embodiments are contemplated by the invention as defined in the appended 
claims.