Found this web page, it tells how to measure frost depth using a frost tube. Might have to try it next winter.
http://ine.uaf.edu/werc/projects/permafrost/frost_tube.htm

 It also tells how to estimate the frost depth using the average daily soil surface temperature. Start reading under the heading teacher support.
 I am going to try it using air temp and temp under the snow that i have recorded to see what the estimated difference would be. Stay tuned.