Are compounds which are used to seed clouds like silver(I) iodide hygroscopic?
Silver iodide is not hygroscopic. It is used to seed clouds, because when the atmosphere is saturated with water vapor, liquid water should start condensing. But to do this, it needs a seed, a place when the first molecules will start to condense. No water molecule feels that it should be the first one to show other molecules where to be condensed. But, as silver iodide has the same hexagonal shape and same dimensions as snow, it may make believe it is an ice crystal. And the neighboring water molecules will be mistaken, follow the example and deposit on it. The common salt cannot do this. It does not look like ice.