I think this more of an ammo can storage question than ammunition.
Cool and dry is the best storage conditions for gunpowder, and then hence, ammunition. Extremely dry might not be the best for match 22lr has that stuff is coated in grease, and I have noticed, the boxes for Eley match are actually sealed in such a way to prevent drying of the grease coating. But, for a general statement, because gunpowder deteriorated exponentially with respect to temperature
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cool is much better than hot.
Something else, inspect your ammunition. Even with cool, and dessiccant, the gunpowder inside the case is still deteriorating. So look for signs of NOx outgassing from the gun powder.
On this round, corrosion around the primer was a give away, that the powder had deteriorated internally.
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One of the NOx molecules created by the deterioration of gun powder is nitrogen dioxide. That stuff is horribly reactive, and when a NO2 molecule bumps into water (call it humidity), it changes into nitric acid gas. Which is horribly corrosive. These compounds will eat pin holes in brass, as can be seen in these photos
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You see green tarnish on the outside of loaded ammunition, it could be from NOx releases when gunpowder breaks down.
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In terms of storing ammunition cans, if the floor is wet, the bottom of the ammunition can will rust. Ammunition cans are made from cheap steel, painted for corrosion protection, but I have had them rust on the bottom in the garage. So, if you are worried about your ammunition cans rusting on the bottom, I think elevating them off a basement floor is a good idea. Won't do anything to slow the deterioration of the gunpowder in the cases, but it will keep the bottoms of your cans from rusting out.!