I have typical Tennessee humidity; rather humid in the summer into mid-October. This year we've had unprecedented rain. Yet my firewood, covered only on top and placed under large Leland Cypress trees, has remained fairly dry.
Sun exposure would be very minimal. Nothing grows under the deck except a little moss and plantains at the fringe-line where the ground meets the deck edge.
I could have room for at two racks leaving air space at bottom, top, and sides. I have access to oak, hickory, poplar, and the occasional dogwood, holly, hackberry, elm, and sugar and red maple. I can use pressure treated posts and lumber for the racks.
Makes sense to take advantage of the space under the deck. I prefer to not bring such wood into my climate-controlled shop (60-70 degrees) until a couple of months before processing.
"What can be asserted without proof can be dismissed without proof." Christopher Hitchens