I had never heard of a hail cannon. But last week I randomly saw on some group, I follow on Facebook, someone asking about loud booms heard near Greeley. Someone chimed in and said in was the "hail cannons". What? Thanks to Google I found out "hail cannons" are a real device that farmers use hoping to destroy hail before it destroys their crop. Quite controversial, the guys at Mythbusters were even called in to test the device.

 

Wikipedia defines a hail cannon like this:

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 while a storm is approaching and until it has passed through the area, yet there is little or no scientific evidence for their effectiveness.