What s happening is that the rafters keep the warm air from migrating much.
Frost in attic rafters.
During winter conditions attic frost is a problem associated with attic bypasses.
Left unchecked frost in an attic can lead to serious mold and decay a patch of frost here and there on the tips of roofing nails poking through the sheathing for instance probably isn t a cause for alarm.
When the outside temperature rises again the frost thaws water collects and can leak into your house.
The frost itself doesn t do any damage but once it melts things get wet which is when the damage.
Near the ridge the frost free area extends over the garage a bit because warm air rises and it ends up near the ridge.
What causes this type of moisture problem in the attic.
Some of it can cross the first rafter there but it doesn t seem to make it across the second rafter.
Notice though that there is some migration.
When the weather warms up the frost will become water droplets and evaporate harmlessly.
When the moisture gets into the attic it condenses on the roof sheathing in the form of frost.
When warm air from inside the house escapes traveling up through the bypasses the moisture condenses on the roof boards and rafters where the frost can form.
The worst frost was on the other side though.
Sticking my head into hundreds of attics every year i see lots where.