I use to think that once the soil was frozen the temperature of it would not change much. I thought freezing was freezing. A few years ago we installed some remote soil probes that monitor the soil, and it proved I was very wrong in my thoughts. Last year we saw very little fluctuation in the soil temperatures at the freezing mark because it did not get very cold at all. This year once the snow blanket had melted we started seeing the soil temperatures react to the ambient temperatures. As you can see in the following chart if the air warms up the soil follows suit, as it gets colder so does the soil.
We really like seeing the frost get into soil and act as a natural way to break up the soil. This freezing and thawing helps to expand and contract the soil to help it loosen up and become more mellow in the spring. So below freezing can mean a lot of different things to different people, for us this winter it is a good thing to have the natural aerification at work to help our soils. Now some more precipitation would be a great thing!