Wet vs dry timber for outside use
I think you're being confused by the terms wet and dry. These terms are usually used to describe the condition of wood after it's cut from a tree or pressure treated, not the climate changes of the season. In your case the wood you'll be using will be pressure treated. The P.T. wood you get in home stores has been recently pressure treated and needs to dry out before it will be stable to use. Otherwise, it will warp, twist and split and ruin your project. As Harper indicated, it takes months to dry out. Once they do dry out, you can stain/ seal them and build your project and the climate changes will have little effect on them.
When I built my deck I had 25 - 5/4&quot; x 6&quot; x 14' planks that had just been pressure treated and were very wet. I laid them out in position on my deck joists for 6 months before screwing them down. I flipped them over often. By the time I was ready to screw them down, I needed three additional planks to make up for the shrinkage due to the drying.
One final note. You can buy wood that has been kiln dried so you don't have to dry it yourself. You just have to find a dealer in your area and be prepared for sticker shock because it's expensive.