If you live anywhere in a warm, humid coastal area, you’re no doubt familiar with wet concrete in winter. Some days you walk outside and find the carport slab is soaking wet. How did it happen? Did rain blow into the carport? If it’s not rain, is it moisture from the ground that came up through the concrete? Could it be condensation from the water vapor in the air? Let’s take a look.
I took the photo at right on a February day in New Orleans a few years ago and it perfectly demonstrates the physics of this phenomenon. What you’re looking at are the stone steps and pavers near Jackson Square. They were mostly wet — but the water didn’t get there from rain or any other liquid water source above the stone.
Could the water have come from below? The photo above doesn’t eliminate that possibility. Notice the nose of the steps. That part of the stone was dry. It wasn’t directly in contact with the ground, having a bit of air beneath, so maybe the water did come from below.
But no, that’s not the answer either. Photo #2 (below) was taken nearby at about the same time. The sidewalk and steps were completely dry. The critical difference between the two locations was shade. The dry area was open to the sky. That kept the stone warmer than the other stone, which was under tree cover.
Let’s go back to the lead photo. The stone there was wet because of condensation. The nose of the steps was dry because it was warmer. That part of the stone had air on three sides. That warmed it up enough that it could be above the dew point.