What Causes a Circular Hole in Snow on an Attic Skylight?

[propicas]

http://mail-attachment.googleusercontent.com/attachment/u/0/?ui=2&ik=9243a84e58&view=att&th=13cca00ffc021b0c&attid=0.1&disp=inline&safe=1&zw&saduie=AG9B_P-JSIKWxP2SILKnAZDSPntW&sadet=1360600148882&sads=XMagKLjppUEvSoI9GidqQVsXpcU

This is in the skylight of my attic. The snow piled up in such a way that there is a circular hole in the center. My theory is that the hot air from the room (obviously it is on the top floor) was hot enough to melt some of the snow, but the cold roof outside prevented melting close to the edges.

What else could it be?

 

[mfb]

I cannot see the image, and I think this is a general problem - the URL looks like some private email attachment.

With some directed flow of air, this might be possible. As alternative, the walls around the window could be colder than the window itself, and cool the outer parts.

 

[russ_watters]

If the skylight is domed, the snow will tend to pile up more on the sides than on top.