I'm not recommending the practice, but I kept all components, including powder and primers in an un-airconditioned garage in Texas for decades with no apparent ill effects. The last few years, I accumulated more powder and ran out of storage space. I also began reading that my storage practice was detrimental to powder and primers, but I think much of this may have come from the Internet, so may not have been accurate information. Anyway, I began keeping most stuff inside. In more than fifty years of handloading, I don't recall having powder deteriorate or primers that didn't go bang.

Whether or not it really makes a difference, I don't know. I doubt my garage temperature exceeded ninety degrees in the summer or twenty-five in the winter. I would guess the humidity level could be called low to moderate.