I might have to take a refresher course in thermal dynamics....did they issue some revised laws. Could of sworn the attic Artic use to be classified as a desert..
From Wikipedia (I know, I know, but for this type of thing, it's usually accurate):
"Humid air is less dense than dry air because a molecule of water (M ≈ 18 u) is less massive than either a molecule of nitrogen (M ≈ 28) or a molecule of oxygen (M ≈ 32)"
Now, the increased power of cold, dry air is most pronounced when talking of internal combustion engines, since the more O2 the engine can work with, the more power it can generate. I remember trying to climb to 10,000 feet in my father's Cessna 150 as a kid one hot, humid summer day. Wasn't going to happen, I don't think we got much over 8,000 before giving up. On a winter day, no problem at all.
How much this effects electric aircraft is up for debate, as plowing the props through the denser air takes more power, with the power system not benefiting from it. But given that, like most quadcopters, the Solo is fairly over powered, my money would be on greater performance at the cost of flight duration.
But specifically for the density question, cold, dry air is more dense than hot humid air.