Data points are a hobby of mine.

BTW; I hadn't seen any updated hardiness zone maps. Helps explain the persistance of my gladeoli, which ought to have died according to the old map.

It's a good point though, about the snow cover. The snow helps keep the ground from getting too far below 0C, and serves as insulation. We don't get enough cold for it to make too much of a difference here, but I would imagine it would make a big, big difference up north.