The frost level by code is just the maximum depth of unproyected soil that is needed to not cause structural damage to a building over a long period - 30 to 50 years annual occurrence period. Snow is a great insulator. I have dug down through 12" of snow to set surveying pins and there was only 4" of frost after a month of -0F to -30F lows for more than a month previously.
Here, some utility contractors will lay a strip of charcoal down and keep it going for a day or two and then cover with blankets to keep in the heat and use the lower thermal inertia/mass of the soil to soften things up a bit. If they are lucky, they will get some snow for additional insulation. the biggest problem is having no snow early in the winter since we have "brown Christmases" about 25% of the time.
When it comes to frost heaving for a post/cylindrical foundation, the ground freezes from the top down and pulls the post or foundation upward. A smooth surface (like PVC or a Sonotube) gives little for the frozen soil to grab on to. Rougher surfaces like had dug or drilled holes without and perviouse drainage material material will heave much quicker, especially if hand dug.
