We have used the Arecibo telescope to carry out an survey of 31 dark clouds in the Taurus/Perseus region for narrow absorption features in HI ( \lambda 21cm ) and OH ( 1667 and 1665 MHz ) emission .
We detected HI narrow self–absorption ( HINSA ) in 77 \% of the clouds that we observed .
HINSA and OH emission , observed simultaneously are remarkably well correlated .
Spectrally , they have the same nonthermal line width and the same line centroid velocity .
Spatially , they both peak at the optically–selected central position of each cloud , and both fall off toward the cloud edges .
Sources with clear HINSA feature have also been observed in transitions of CO , ^ { 13 } CO , C ^ { 18 } O , and CI .
HINSA exhibits better correlation with molecular tracers than with CI .

The line width of the absorption feature , together with analyses of the relevant radiative transfer provide upper limits to the kinetic temperature of the gas producing the HINSA .
Some sources must have a temperature close to or lower than 10 K. The correlation of column densities and line widths of HINSA with those characteristics of molecular tracers suggest that a significant fraction of the atomic hydrogen is located in the cold , well–shielded portions of molecular clouds , and is mixed with the molecular gas .

The average number density ratio [ HI ] / [ H _ { 2 } ] is 1.5 \times 10 ^ { -3 } .
The inferred HI density appears consistent with but is slightly higher than the value expected in steady state equilibrium between formation of HI via cosmic ray destruction of H _ { 2 } and destruction via formation of H _ { 2 } on grain surfaces .
The distribution and abundance of atomic hydrogen in molecular clouds is a critical test of dark cloud chemistry and structure , including the issues of grain surface reaction rates , PDRs , circulation , and turbulent diffusion .