  It is generally recognized that the interstellar medium has a vast range of
densities and temperatures. While these two properties are usually
anticorrelated with each other, there are nevertheless variations in their
product, i.e., the thermal gas pressure divided by the Boltzmann constant k. In
neutral gas, the relative populations of neutral carbon atoms in the excited
fine-structure states can give a direct measure of a local thermal pressure. A
picture of the distribution function for thermal pressures in H I regions is
now arising from a survey of interstellar C I absorption features in the UV
spectra of 21 early-type stars, observed with a wavelength resolving power of
200,000 by the STIS instrument on the Hubble Space Telescope. Most of the gas
is within the range 1000 < p/k < 10,000 cm^{-3}K, but there is also evidence
for some of the material being at much higher pressures, i.e., p/k > 10^5
cm^{-3}K. While the fraction of gas at these elevated pressures is quite small,
it seems nearly ubiquitous. This phenomenon may arise from small-scale,
short-lived density enhancements that are produced by converging flows of
material in supersonic turbulence.
