(Caution! Nerdy and increasingly OT discussion in progress!)

I'll buy that. Air cools more quickly than water. That would certainly apply to an open container, but would likely apply to a closed container as well, as long as there was an air space at the top.

I can attest to the "pressure seal" effect. When my polyethylene rain barrels start to freeze over in a serious way (2" of ice) I break the ice with a spade or axe and the water squirts upward quite impressively. I suppose some sort of foam "tube" (like the ones you wrap water pipes with) would keep a hard seal from forming. Gotta try that.