Cloud seeding introduces tiny particles, often silver iodide, into clouds that already contain moisture, giving water vapor a surface to condense around more readily than it would on its own. The method cannot create rain from a clear sky; it can only coax a bit more precipitation from clouds already primed to produce some. Studies estimate the gain at perhaps ten to fifteen percent above what would have fallen anyway, a modest number that is nonetheless significant to a region managing a reservoir during a multi-year drought, where every additional percentage point of runoff has a measurable effect on water supply.