Parameter Tree:
EARTH SCIENCE > CRYOSPHERE > FROZEN GROUND > ACTIVE LAYER

Definition:
Active layer, also called frost zone or mollisol, is part of the soil included
with the suprapermafrost layer (i.e., existing above permafrost) that usually
freezes in winter and thaws in summer.
Its bottom surface is the frost table, beneath which may lie permafrost or
talik. The depth of the active layer varies anywhere from a few inches to
several feet.


Reference:
American Meteorological Society Glossary of Meteorology Online
http://amsglossary.allenpress.com/glossary