There are several forms of weather modification, most notable probably being cloud seeding.
But another form of weather modification that some of you may have never heard of is the hail cannon.
We have several here in Colorado, esp. in the San Luis Valley growing region of south-central Colorado.
A hail cannon shoots off incredible sound into an approaching thunderstorm, and the shock wave is thought to disrupt the formation of hail, causing what would be hail to fall as slush or rain.
The device fires about every 4 seconds during the approach of a thunderstorm.
Hail cannons date back to the 18th century.
There is a lot of debate as to rather they really work or not, but farmers who have hail cannons swear by them.
I ran across the following article about hail cannons in Vermont, and how they have neighbors irritated.