We had a BBQ a few days ago and left our pack of aluminum foil on the table out side. It rained that night and the following night. I went out to take the trash out and heard a odd noise. It was the pack of foil boiling hot. The water from the rain was steaming out of it. When I picked it up you could feel the warmth coming from the center of it.It was 2 days after the BBQ, was never near the heat of that. It was on a separate table.The table is shaded all day, so it didn't heat from the sun. Plus aluminum does not transfer heat.It just don't make since to me. I unrolled some of it and it is dark grey from being heated so hot. I never heard of it and tried to find info, but to no avail. So out of curiosity I am asking.
Aluminium is one of the best conductors of heat so the effect may have been a rare entropic incident where the water rapidly evaporated due to the warmth of the air. What's more likely is that the aluminium was exposed to the water and the rain was slightly penetrative. Aluminium usually has aluminium oxide protection to protect against corrosion and reaction but something may have depleted that layer causing a reaction and the gas you saw steaming wasn't water but hydrogen gas.
Aluminum does not go wel in heat, in fact if left under extreme heat + pressure (A.K.A.) microwave, or toast oven, it will explode, there will be nothing left but tiny fragments of aluminum tinfoil.
