Depends on the area but I find that there is about a two foot blanket of moisture gradient on the ground. The closer to the ground, the more the moisture. In reality there is a much larger gradient of moisture that you can often see on tall metal structures like lattice towers but I doubt you will stack your woodpile over 50 feet.
After today's 40+mph gusts during the torrential rain we got blew part of my well weighted tarp off, the top 2 feet of my stack is the wettest. Aside from that, I agree with LLigetfa. Normally, the tops and outer edges are the driest.