  We introduce a model for the large-scale, global 3D structure of molecular
clouds. Motivated by the morphological appearance of clouds in surface density
maps, we model clouds as cylinders, with the aim of backing out information
about the volume density distribution of gas and its relationship to star
formation. We test our model by applying it to surface density maps for a
sample of nearby clouds and find solutions that fit each of the observed radial
surface density profiles remarkably well. Our most salient findings are that
clouds with higher central volume densities are more compact and also have
lower total mass. These same lower-mass clouds tend to have shorter gas
depletion times, regardless of whether we consider their total mass or dense
mass. Our analyses lead us to conclude that cylindrical clouds can be
characterized by a universal structure that sets the timescale on which they
form stars.
