In the book 'Cat's Cradle' by Kurt Vonnegut (great book, btw.) a seed crystal of "ice-nine" causes worldwide devastation as it makes water turn to a solid at room temperature.
Amazingly, it was Kurt Vonnegut's older brother, Bernard Vonnegut, who discovered that silver iodide could be used to seed clouds by serving as the nuclei for ice crystals.
So yes, in rare instances, humans can make rain happen by seeding clouds. They spread silver iodide (AgI) crystals into the clouds. Silver iodide is not soluble in water and one of it's crystal forms is hexagonal and mimics ice. The water molecules fit right in. The silver iodide crystals serve as "seed" nucleation centers and the water molecules glom onto them in the form of ice. They get bigger, heavier, and drop to make rain.