A model is proposed for determining the distances to falling interstellar clouds in the galactic halo by measuring the cloud velocity and column density and assuming a model for the vertical density distribution of the Galactic interstellar medium .
It is shown that falling clouds with N ( H~ { } I ) ~ { } ^ { < } _ { \sim } 10 ^ { 19 } ~ { } { cm ^ { -2 } } may be decelerated to a terminal velocity which increases with increasing height above the Galactic plane .
This terminal velocity model correctly predicts the distance to high velocity cloud Complex M and several other interstellar structures of previously determined distance .
It is demonstrated how interstellar absorption spectra alone may be used to predict the distances of the clouds producing the absorption .

If the distance , velocities , and column densities of enough interstellar clouds are known independently , the procedure can be reversed , and the terminal velocity model can be used to estimate the vertical density structure ( both the mean density and the porosity ) of the interstellar medium .
Using the data of Danly and assuming a drag coefficient of C _ { D } \cong 1 , the derived density distribution is consistent with the expected density distribution of the warm ionized medium , characterized by Reynolds .
There is also evidence that for z~ { } ^ { > } _ { \sim } ~ { } 0.4 ~ { } { kpc } one or more of the following occurs : ( 1 ) the neutral fraction of the cloud decreases to \sim 31 \pm 14 \% , ( 2 ) the density drops off faster than characterized by Reynolds , or ( 3 ) there is a systematic decrease in C _ { D } with increasing z .
Current data do not place strong constraints on the porosity of the interstellar medium .