Three recent surveys of 21-cm line emission in the Galactic plane , combining single dish and interferometer observations to achieve resolution of 1′ to 2′ , \sim 1 km s ^ { -1 } , and good brightness sensitivity , have provided some 650 absorption spectra with corresponding emission spectra for study of the distribution of warm and cool phase H I in the interstellar medium .
These emission-absorption spectrum pairs are used to study the temperature of the interstellar neutral hydrogen in the outer disk of the Milky Way , outside the solar circle , to a radius of 25 kpc .

The cool neutral medium is distributed in radius and height above the plane with very similar parameters to the warm neutral medium .
In particular , the ratio of the emission to the absorption , which gives the mean spin temperature of the gas , stays nearly constant with radius to \sim 25 kpc radius .
This suggests that the mixture of cool and warm phases is a robust quantity , and that the changes in the interstellar environment do not force the H I into a regime where there is only one temperature allowed .
The mixture of atomic gas phases in the outer disk is roughly 15 % to 20 % cool ( 40 K to 60 K ) , the rest warm , corresponding to mean spin temperature \sim 250 to 400 K .

The Galactic warp appears clearly in the absorption data , and other features on the familiar longitude-velocity diagram have analogs in absorption with even higher contrast than for 21-cm emission .
In the third and fourth Galactic quadrants the plane is quite flat , in absorption as in emission , in contrast to the strong warp in the first and second quadrants .
The scale height of the cool gas is similar to that of the warm gas , and both increase with Galactic radius in the outer disk .