A reader called me with this question last week, and it's a really good one. You might think since almost all our weather in the Northwest comes off the Pacific Ocean, the storms would be picking up salt water from the ocean as they generate. We get similar questions during hurricanes -- why don't they dump 20" of saltwater rain from the Gulf of Mexico?
But no, all the rain -- no matter where in the world it is -- is freshwater (mostly. See below).
Why? It has to do with the evaporating process. When water evaporates from the ocean, only the pure H2O molecules are involved -- it's basically energy turning the water from liquid state to vapor state. The salt particles are, in essence, left behind.
I'm not an oceanographer, but I would assume that means when you've got a really big storm, like a hurricane, taking in a lot of moisture from the warm Gulf/ocean waters, the salt content of the water in its wake is probably a miniscule higher, but negligible compared to the vast volume of the water.
Now that said, there are a few ways to get a salty rain, but it doesn't involve evaporation.
Communities along the coast deal with salt and rust from wind blowing in spray off the ocean. In that case, it is salt water because you are transporting water droplets directly from the water.
Also, there have been noted cases where a waterspout -- a tornado over water -- has sucked up sea water into the sky and carried it over land where it "rained" salt water.
Other random notes:
* Rain water isn't 100% clean. To form, a raindrop needs some sort of particle to cling to -- usually this can be a speck of dust or dirt or soot or whatever. (That's how you can get acid rain, if the drops cling to sulfur particles or other pollutants). But it can also be a particle of salt, so you could technically get a raindrop that has a tiny amount of salt in it, but it's negligible.
* Around the Northwest -- and especially during the mornings in the late spring through summer -- you can get a milky, whitish haze in the sky on otherwise clear days. That's when we have a weak to moderate flow from the west off the ocean blowing in salt particles from sea spray, which can give the sky a little tinge of white, and it's different than the smoggy brown tinge the city areas see during inversions.