degrees of moisture levels in a stack

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.

 

Tops.
I have my stacks top covered.

Bottoms of the stacks tend to stay wet longer from wind driven rain than closer to the cover that stays relatively drier.

There may be a bit less wind closer to the ground, too.
Plus the ground has to evaporate after every rain, too.



I know from sailing there can be quite a bit more wind higher aloft than at sea level.