The structure of molecular clouds and their global emission properties

Molecular clouds show complex structure, often characterized as fractal. This review summarizes the recent attempts to quantify the observed structure of interstellar clouds, resulting in the complementary scenarios of a clump ensemble with a power law mass spectrum and a power law mass size relation on one hand, and a fractional Brownian motion structure on the other. Due to the broken up structure of interstellar clouds, the interaction between the ambient external radiation field and the molecular cloud, as described within the framework of photodissociation regions (PDRs), is crucial and in fact makes it possible to understand several aspects of the global emission properties of Milky Way clouds in the main tracers of the dense ISM, the [CII], [CI] and CO far-IR and mm-wave lines. These results are relevant for the proper interpretation of extragalactic far-IR and submm line observations which will become available in large quantity within the next years.