Hi Jim: What a good question!

The following is from the C&H web site: (I hope someone else has more encouraging information)

Moisture makes granulated sugar hard and lumpy. Once this happens, there is no way to adequately restore it. Always store granulated sugar in a covered container in a cool, dry area.

Store powdered sugar in a cool, dry location (not the refrigerator). When it gets moist, it develops lumps. And because of its physical properties, it tends to absorb strong odors – it can even absorb odors through the package.