  High ambient interstellar pressure is suggested as a possible factor to
explain the ubiquitous observed growth-rate discrepancy for supernova-driven
superbubbles and stellar wind bubbles. Pressures of P/k ~ 1e5 cm-3 K are
plausible for regions with high star formation rates, and these values are
intermediate between the estimated Galactic mid-plane pressure and those
observed in starburst galaxies. High-pressure components also are commonly seen
in Galactic ISM localizations. We demonstrate the sensitivity of shell growth
to the ambient pressure, and suggest that superbubbles ultimately might serve
as ISM barometers.
