frost line

noun3

1. •
   The depth to which the water present in the soil is expected to freeze in winter.

   • The uninsulated water pipes must be installed below the frost line to prevent them from freezing.
2. •
   The depth to which the water present in the soil is actually frozen in a given place at a given point in time.

   • Due to the thin layer of snow on the ground the frost line has penetrated unusually deep this year.
3. •
   In a solar system, the distance from the protosun beyond which hydrogen-containing gas compounds freeze.

   • In our Solar System the frost line lies between Mars and Jupiter at about five astronomical units from the Sun.