I know that the OP wanted general info, but one other item is worth mentioning: Some powders are more hygroscopic than others. For instance, my favorite handgun powder, SR-4756, is highly hygroscopic. Even though I live in a normally very dry climate, I never leave 4756 in a powder measure even overnight. Viht handgun powders, I've learned, are not very hygroscopic at all. I've left N-350 in a measure with a fairly loose cap for weeks, with no noticeable absorption of moisture, even though I don't recommend it.