  We aim to better understand how the spatial structure of molecular clouds is
governed by turbulence. For that, we study the large-scale spatial distribution
of low density molecular gas and search for characteristic length scales. We
employ a 35 square degrees 13CO 1-0 molecular line survey of Cygnus X and
visual extinction (A_V) maps of 17 Galactic clouds to analyse the spatial
structure using the Delta-variance method. This sample contains a large variety
of different molecular cloud types with different star forming activity. The
Delta-variance spectra obtained from the A_V maps show differences between
low-mass star-forming (SF) clouds and massive giant molecular clouds (GMC) in
terms of shape of the spectrum and its power-law exponent beta. Low-mass SF
clouds have a double-peak structure with characteristic size scales around 1 pc
(though with a large scatter around this value) and 4 pc. GMCs show no
characteristic scale in the A_V-maps, which can partly be ascribed to a
distance effect due to a larger line-of-sight (LOS) confusion. The
Delta-variance for Cygnus, determined from the 13CO survey, shows
characteristic scales at 4 pc and 40 pc, either reflecting the filament
structure and large-scale turbulence forcing or - for the 4 pc scale - the
scale below which the 13CO 1-0 line becomes optically thick. Though there are
different processes that can introduce characteristic scales, i.e. geometry,
decaying turbulence the transition scale from supersonic to subsonic turbulence
(the sonic scale), line-of-sight effects and energy injection due to expanding
supernova shells, outflows, HII-regions, and the relative contribution of these
effects strongly varies from cloud to cloud, it is remarkable that the
resulting turbulent structure of molecular clouds shows similar
characteristics.
