Raindrops rarely grow larger than 5 mm. two reasons were given on p. 169. can you think of a third? lead iodide is an effective ice-forming nucleus. why do you think it has not been used for that purpose?

As the size of the drop increases, the velocity of falling increase also, as this happens, the shape of the drop turns into a ring until it breaks and split in two or more small drops.

Lead iodide is insoluble in water.

Explanation:

Lead iodide is an effective ice - forming nucleus. Remember that the properties of water are quite different in solid or liquid state. Lead iodide besides is toxic, that means, it can not be used otherwise the pollution by lead in living things will be enormous plus the bioaccumulation that will represent.

Also Lead iodide is insoluble in water and denser that it, so if it used to increase the drop size, what is going to happened is that first the rain is going to be of Lead iodide and then is going to be out of water.

Hope this info is useful.