Corey, isn't that what we've been saying all along? Wood is not a sponge....unless it is punky. It just doesn't soak up the rain and as soon as the rain stops the remaining moisture that fell in the form of rain just evaporates right off. Hence, we don't cover our wood until just before snow falls. Then we cover the tops only.
It also says how silly it is to cover a wood pile when rain is coming and then uncover it, then cover it, etc., etc. There is a lot of work involved with wood burning; why make it any harder?
I'm sorry, but this fantasy of wood not absorbing much water sitting out in the rain for sometimes days, then shedding all of the little water it did take as soon as the sun hits is it flat out rubbish/folklore/BS with no basis in fact or shred of proof of any kind. Want proof that it's wrong? What are paper towels made from???
I've spent the bulk of my life as a professional woodworker and building inspector (ie. I see a little wood rot here and there...). I made it a point to STUDY wood and the effects moisture has on it from many different angles and perspectives. I've read several books written by scientists about wood in general, wood movement and the effects of moisture, and the right and wrong ways to dry it, not to mention how to manipulate wood using water as a tool. I also have a very strong background in Physics and more then a few years burning wood for my primary source of heat.
First, while Corey's information is interesting, there are several facets that make it not very reliable for the uncovered argument. First and foremost, the bulk of his dried wood seems to be Hedge, aka, Osage Orange. If you've never seen Osage Orange it has a grain density not dissimilar from hard plastic. You can hardly see the pores with a good strong magnifying glass. In fact it's one of the few woods that has such a high specific gravity that it doesn't float. So I think it is fair to say it is a little on the uncommon side of the spectrum and not a good example. Second, where in the stack was the sample wood pulled from? Very top, side, middle? Essentially the wood in the middle is, to some degree covered by the wood over it.
So that aside. What makes wood, or any other material dry or not? Same thing that makes your house loose heat or gain it, a difference between the inside and the outside. If you have a split of wood that is 40% MC at it's core and say 30% around it's surface, that is surrounded by relatively dry air (even 95% humidity air has 5% of it's molecules able to take on water vapor) the moisture on the woods surface will move into the air, faster or slower depending on the humidity, but it will move. Since the outer surface of the wood has less moisture then the core, the moisture in the core will continue to migrate to the outer surface in a continuing attempt to reach equilibrium. This in and of itself proves that moisture moves through wood. So now it starts raining on all those piles of uncovered wood and the splits get soaked with water. Water, I think even we all can agree water is 100% MC... So water is sitting on top of something that was engineered by nature to be at 50-60% moisture content, which is now at 30% and the water, 100%, soaks in. Remember the paper towels? The longer the water sits on the wood, the deeper it soaks in, the diffusion from the core to the surface, to the air has completely stopped and reversed. The moisture content is going back up all through the wood. Want to see proof? Take a piece of clean, green wood, the whiter the better, and bring it in the house to dry, then put it in the oven on a nice low setting maybe 125 degrees. (I'll not be responsible for any house fires, if your too stupid for your own good, it's your problem) and dry it for several hours. Cut it in half and see what the inside of the end grain looks like then take half and put it out in the rain, or simulated rain, for a while. Cut it open again, across the grain, and look at the stains and how far they've penetrated. It absorbed water. Even better, use an accurate scale that weights in grams and weigh both pieces, put one out in the rain for a while and then weight it again, any weight over the dry weight is water. So, sure, if the amount of dry weather you have overtakes the amount of wet, eventually your wood will dry, to some extent, but if you expose it to only dry and never wet, it WILL dry faster. And unless you have your wood layed out only one layer deep across your yard, and even if you do, the sun is not going to make up for the rain.
Think I'm wrong? Ok, tell me this, as a building inspector, I continually see window sills, door frames, decking, roof trim and the like that is sitting out in the sun & wind all day long, but when it is uncovered, as in not painted or otherwise treated to repel water, it rots! Why do you think that is? Wood does not rot without water. By the thinking of the "the sun makes up for it" crowd, the sun should dry it back to it's kiln dried state and it should be as fine as your dinning room table for centuries... but it isn't.
Now I'm not saying that having your wood exposed to the sun doesn't help, but having it exposed to a soaking rain sure as hell doesn't either. I designed my sheds to have a permanent roof, which sits over, not on top of, my wood and it is open to the wind on all six sides (I keep it up off the ground at least eight inches and have plastic over the soil under the shed) and it dries a hell of a lot faster then it ever did leaving it exposed to the elements. Even if it will dry eventually, I think anyone can agree that 3 steps forward, two steps back is not preferable over 3 steps forward and no steps back. (Unless your dancing or just stepped on a hot coal or something.)