I wonder if the snow blanket theory works with greenhouses. What seems to happen to me is an inch of snow freezes to the top of the greenhouse and is unmoveable, as a result the sun which often accompanies cold weather doesn't get in, and the greenhouse gets colder and colder.
It certainy does not apply. A snow blanket on top of the ground will insultate so that the heat stored in the ground does not disappear quickly. The ground is a bad insulator but has a great thermal capacity. This means that there is a lot of low temperature heat available in lower layers that is conducted up to the bottom of the snow. Here the opposite happens. The snow has very little thermal inertia so it does not "steal" much heat as long as the temperature is below thawing. It further inslutes so the heat does not dissipate quickly.
The result is that the temperature difference between deeper layers of the ground and the bottom of the snow is small whereas the temperature difference between he snow bottom and top can be considerable.
The situation on top of the greenhouse is different since there is no soil in contact with the snow bottom. There is only cold air. The snow will of course stop warming from the sun just as described.
The problems in my climate lies in ice formation. If the ground is insulated by early snow and the snow starts melting from heat from below, the soil is obviously not frozen and the resulting water is drained away. (And the temperature will be 0C as described from Sapporo) A heated frame or on a greenhouse, will be unevenly heated and we get ice formation on the colder parts. The same problem can occur in a mild spell if the top layer of the soil is frozen and the snow starts to melt. This situation is worse in pots standing outside since they are less well heated from below and the rim will keep the water in.
I have found it next to impossible to have heated frames outside because of this ice formation. This is why i keep my heated frames inside the greenhouse where there is no snow.
