  A recent survey of the fine-structure excitation of neutral carbon reveals
that the interstellar medium in the Galactic plane exhibits a thermal pressure,
nT/k, that ranges from about 10^3 to 10^4 cm{-3}K from one location to the
next, with occasional excursions in excess of about 10^5 cm^{-3}K. The large
excitations for small amounts of gas indicate that some regions are either
subjected to shocks or must be pressurized within time scales much shorter than
the time needed to reach thermal equilibrium. These rapid fluctuations probably
arise from the cascade of macroscopic mechanical energy to small scales through
a turbulent cascade. One consequence of this effect is that changes in gas
temperature can arise from near adiabatic compressions and expansions, and this
may explain why investigations of 21-cm emission and absorption reveal the
presence of hydrogen at temperatures well below the expected values derived
from the balance of various known heating and cooling processes.
