There's a lot of variables in that scenario with the dew and roof, David, I agree. I'm not suggesting it is proof, but it is what got me thinking about the what the difference was. Some roof tiles can also be slightly porous, which simply stops the dew of running off before its evaporated.
Going way back, there was an excellent thread here on IIS that involved myself and Bird in particular discussing cooling of scopes and different materials when exposed to the sky. I started off challenging some of Bird's ideas using my engineering experience, but he eventually won me over... I was assuming atmospheric temperature was the governing factor not knowing about the "infrared window". Now I know!
Its good to learn things...
Al.