The frost line is the maximum depth at which soil is frozen, or the lower limit of permafrost. The depth of the frost depends on the local climate, the heat transfer properties of the soil, and the adjacent materials and nearby heat sources.
The frost line can vary by latitude, and it is deeper closer to the poles. In fact, in the United States, the frost line ranges anywhere from zero to six feet. This is due to the fact that snow cover and asphalt insulate the ground, and homes can heat the ground. Alternatively, in Arctic and Antarctic areas, the frost line is so deep that the soil remains permafrost throughout the entire year.