Explain this mystery

[blown motor]

In the picture, the raised circle is ice on a glass top patio table. Disregard the green topped can. How did the ice form in such a perfect circle? It's about 1/4" thick and slightly off centre. There is nothing above the table that would have dropped on it.

[jimynick]

Well, I warned ya! Musta been them idgits from over Mitchell way. Keep yer powder dry partner! 

[6bblgt]

the sun kept the metal ring warmer than the glass

[blown motor]

the sun kept the metal ring warmer than the glass

I had wondered about that too but it was lightly snowing all day. No sun.

[MoparLeo]

The table underneath acts as an insulater so the temperature of the glass is colder than the edges than in the center. The glass is probably not perfectly centered over the table.

[7E-Bodies]

Gotta go with @MoparLeo on this one.    

[jimynick]

Ooh, ooh, I know! The centre wasn't Sanforized and you left it out in the rain! Do I get a prize? 

[YellowThumper]

Combination of a few comments about warmer spots. Any breezes will also change location of the cold/warmer spot.