  The neutral hydrogen 21 cm line is an excellent tracer of the atomic
interstellar medium in the cold and the warm phases. Combined 21 cm emission
and absorption observations are very useful to study the properties of the gas
over a wide range of density and temperature. In this work, we have used 21 cm
absorption spectra from recent interferometric surveys, along with the
corresponding emission spectra from earlier single dish surveys to study the
properties of the atomic gas in the Milky Way. In particular, we focus on a
comparison of properties between lines of sight through the gas disk in the
Galactic plane and high Galactic latitude lines of sight through more diffuse
gas. As expected, the analysis shows a lower average temperature for the gas in
the Galactic plane compared to that along the high latitude lines of sight. The
gas in the plane also has a higher molecular fraction, showing a sharp
transition and flattening in the dust - gas correlation. On the other hand, the
observed correlation between 21 cm brightness temperature and optical depth
indicates some intrinsic difference in spin temperature distribution and a
fraction of gas in the Galactic plane having intermediate optical depth (for
0.02 < $\tau$ < 0.2) but higher spin temperature, compared to that of the
diffuse gas at high latitude with the same optical depth. This may be due to a
small fraction of cold gas with slightly higher temperature and lower density
present on the Galactic plane.

Astrophysics of Galaxies