Some questions raised by today's nature walk

Today I noticed a tree that had snow on the branches, similar to the trees I photographed January 8, after a recent snowfall.  Now, snowfall is not so recent, and most of the trees no longer have snow on the branches.  The one that did seemed to be in a somewhat sheltered spot, near a house and some other trees.  Seeing it made me wonder, what is it that causes the snow to stop being on the branches of trees? Does the sun melt the snow? Or does the wind blow it off? Or is it both?

Today's walk lasted about an hour, the last hour before sunset.  I noticed the trees silhouetted against the sky.  What is it about winter dusk that draws our attention to trees silhouetted against the sky? Are trees not against the sky in summer and in daylight?