----------------------------Original message----------------------------
Thanks for your reply, Charley.  I suspected that IR film might not be
sensitive enough to pick up subsurface heat sources if the temperature
differential was too low.  I guess we're going to be doing a lot of ridge
walking this winter.
Many cave do "exhale", an effect which is particularly noticeable in winter
because it is similar to steam rising from the ground.  Breathing Cave, which
you mentioned in your message exhibits an "in-out" air circulation cycle
which reverses several times daily (a neat cave, BTW).
 
Frank