frost line

frost line (plural frost lines)

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. (astronomy) 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.

• (expected depth of soil freezing): frost depth, freezing depth
• (actual depth of soil freezing): frost front
• (distance from the protosun): snow line, ice line