In Kurt Vonnegut’s classic novel, Cat’s Cradle, the major plot device is a substance known as ice-nine. It is a polymorph of water that remains in a solid crystalline structure at room temperature. When it comes into contact with liquid water, the structure rapidly spreads through the water converting it all to crystal. In the novel, it’s a dangerous doomsday weapon capable of freezing the entire planet. However, silver iodide, which could be considered to be a real-world analogue to ice-nine, is actually quite useful for weather modification.
Weather modification may sound like science fiction, but humans have been using a method called “cloud seeding” since the 1940s – of which, silver iodide is an important ingredient. As the name implies, silver iodide is a compound created from the reaction between an iodide solution and a solution of silver ions. Silver iodide has a crystalline structure that is very similar to ice. Similar to ice-nine, if silver iodide is introduced to super cooled water, it will induce freezing via heterogeneous nucleation. In other words, the water molecules copy the crystal structure of silver iodide and become ice.
At higher altitudes, the water vapor of clouds is usually a super cool (less than 0 °C). By “seeding” the clouds with silver iodide via plane, one can increase precipitation, reduce fog conditions, or decrease the size and formation of hailstones. This makes cloud seeding useful for a variety of applications – drought areas can have increased rainfall and airports can maintain clear skies. Beijing even fired silver iodide rockets to clear the skies in time for the 2008 Olympics.
And who do you think discovered the method for seeding clouds using silver iodide? None other than atmospheric scientist Dr. Bernard Vonnegut – Kurt’s brother.