The far-infrared [ C ii ] 158 \mu m fine structure transition is considered to be a dominant coolant in the interstellar medium .
For this reason , under the assumption of a thermal steady state , it may be used to infer the heating rate and , in turn , the star formation rate in local , as well as in high redshift systems .
In this work , radio and ultraviolet observations of the Galactic interstellar medium are used to understand whether C ii is indeed a good tracer of the star formation rate .
For a sample of high Galactic latitude sightlines , direct measurements of the temperature indicate the presence of C ii in both the cold and the warm phases of the diffuse interstellar gas .
The cold gas fraction ( \sim 10 - 50 \% of the total neutral gas column density ) is not negligible even at high Galactic latitude .
It is shown that , to correctly estimate the star formation rate , C ii cooling in both the phases should hence be considered .
The simple assumption , that the [ C ii ] line originates only from either the cold or the warm phase , significantly underpredicts or overpredicts the star formation rate , respectively .
These results are particularly important in the context of the Damped Lyman- \alpha systems for which a similar method is often used to estimate the star formation rate .
The derived star formation rates in such cases may not be reliable if the temperature of the gas under consideration is not constrained independently .