The Frost/Thaw line refers to the current allowable weights in place for a specific location in the Province of Alberta. Depending on the time of year, different parts of the Province may be at different allowable weights.

The Province of Alberta monitors the frost and thaw depth through a network of thermistors located at 68 different locations in the province. Based on the readings at each location, the Province then makes a decision to determine the start of Spring/Period 1 season as well as the Winter/Period 4 season.

To see the allowable weights in the different locations throughout the Province, open the following map:
http://www.transportation.alberta.ca/content/doctype260/production/tr-lhp-frost-thaw-contour-map.pdf

You can view the historical records for the Frost/Thaw changes by opening this document:
http://www.transportation.alberta.ca/content/doctype260/production/trans-historical-seasonal-weight-changes.pdf