To understand how cloud seeding works, you need to get a basic grasp on how hail forms. It's all about thunderclouds and supercooled water. The first thing you need to know is that thunderclouds have updrafts. The second thing is that when water is supercooled, that means it's still in liquid form even though it's below freezing.
High up in the clouds (and under certain other conditions) water can remain in liquid form below freezing if it can't attach to, and form ice around, something solid.
But say it's summer and a little drop of water freezes in a cloud. Normally it would then fall earthward and melt into rain before hitting the ground. But if that cloud is thunderous, it has a strong updraft, which pushes the little pellet of ice back up to the colder areas where it meets with supercooled water, which instantly freezes around it. The pellet, now bigger, drops again, only to be shoved up again by that updraft until it meets more supercooled water, gets another layer of ice and falls. Depending on the strength of the updraft, this process can repeat for quite a while with the pellet, now a full-fledged hailstone, bouncing up and down inside the thundercloud until it gets so heavy it finally dive-bombs your greenhouse.
Typically hailstones are no larger than peas or marbles. But small fry like these aren't what insurers are worried about when they send cloud seeders into storms to prevent major damage. The idea behind cloud seeding is this: Silver iodide has a similar molecular structure to ice. That means it can be a "nucleus" for a storm cloud's supercooled water. The hope is that the little ice pellets will drop to the ground before they can form large, harmful balls of hail.
The scientific jury remains out as to whether this controversial methodology actually works. Cloud-seeding enthusiasts point to some data that claims major reductions in crop damage where the strategy has been put into practice.
If somebody was cloud seeding over Vivian, South Dakota, on June 23, 2010, it didn't work very well — that's when the largest haistone on record fell out of the sky. So, how big can a hailstone get? This one was 8 inches (20.32 cm) in diameter [source: NSSL]. That's almost as big as a bowling ball!