To take the vapor pressure thing one step further…
The greater the difference in vapor pressure between wood and air, the faster the absorption/desorption process will take place. Temperature has little affect on the vapor pressure of air compared to relative humidity… i.e., there is little difference in the EMC of wood between 55°/65% humidity and 90°/65%, whereas there is a big EMC difference between 90°/65% and 90°/90%. As it releases water vapor, the vapor pressure of air around the wood increases, slowing the desorption (seasoning) rate… unless the air is exchange by wind and/or convection.
I know there will be a bunch of disagreement here, but…
Placing “unseasoned” firewood in a shed, under a roof or under a tarp (even if just top-covering) cannot speed the seasoning process… it can only slow it down. Anything that restricts air flow (natural wind movement or convection) will increase the vapor pressure surrounding the wood, slowing the desorption rate. And don’t forget, the wind ain’t always blowing; that’s when convection is your best friend… and any sort of cover (even a heavy tree canopy) restricts free-flowing convection. Yeah, a “cover” will keep the rain off, but really that means nothing… wood absorbs liquid water very slowly (the vapor pressure inside the wood tries to prevent it), most of it just runs off… and once it stops raining the vapor pressure inside the wood rapidly forces out the tiny amount of absorbed liquid water from the outer surfaces. Even covering “seasoned” firewood can cause it to “stay wetter longer” during extended rainy times… because the vapor pressure inside the shed, or under the cover, remains higher, longer. If you have condensation... water droplets... under your cover you are slowing the desorption (seasoning) rate... that's the truth, and it really is that simple.
Firewood should not be covered or placed in a shed until it is fully seasoned… and then only what you’re planning to use for that heating season. Seriously, until I joined this board I’d never heard of anyone putting “green” or “unseasoned” firewood in a shed or under a cover. Woodsheds are where you put your fully seasoned firewood in October, just before you start using it… simply to keep the snow and ice off it, and normally real close to the house for convenience.
There… now I’ve started another, “to cover, or not to cover” argu…. errrr… discussion.