We present an alternative explanation for the nature of turbulence in molecular clouds .
Often associated with classical models of turbulence , we instead interpret the observed gas dynamics as random motions , induced when clumpy gas is subject to a shock .
From simulations of shocks , we show that a supersonic velocity dispersion occurs in the shocked gas provided the initial distribution of gas is sufficiently non-uniform .
We investigate the velocity size-scale relation \sigma \propto r ^ { \alpha } for simulations of clumpy and fractal gas , and show that clumpy shocks can produce realistic velocity size-scale relations with mean \alpha \thicksim 0.5 .
For a fractal distribution , with a fractal dimension of 2.2 similar to what is observed in the ISM , we find \sigma \propto r ^ { 0.4 } .
The form of the velocity size-scale relation can be understood as due to mass loading , i.e .
the post-shock velocity of the gas is determined by the amount of mass encountered as the gas enters the shock .
We support this hypothesis with analytical calculations of the velocity dispersion relation for different initial distributions .
A prediction of this model is that the line-of sight velocity dispersion should depend on the angle at which the shocked gas is viewed .