The characteristic lifetimes of molecular clouds remain uncertain and subject to debate , with arguments having recently been advanced in support of short-lived clouds , with lifetimes of only a few Myr , and in support of much longer-lived clouds , with lifetimes of 10 Myr or more .
One argument that has been advanced in favour of long cloud lifetimes is the apparent difficulty involved in converting sufficient atomic hydrogen to molecular hydrogen within the short timescale required by the rapid cloud formation scenario .
However , previous estimates of the time required for this conversion to occur have not taken into account the effects of the supersonic turbulence which is inferred to be present in the atomic gas .

In this paper , we present results from a large set of numerical simulations that demonstrate that H _ { 2 } formation occurs rapidly in turbulent gas .
Starting with purely atomic hydrogen , large quantities of molecular hydrogen can be produced on timescales of 1–2 Myr , given turbulent velocity dispersions and magnetic field strengths consistent with observations .
Moreover , as our simulations underestimate the effectiveness of H _ { 2 } self-shielding and dust absorption , we can be confident that the molecular fractions that we compute are strong lower limits on the true values .
The formation of large quantities of molecular gas on the timescale required by rapid cloud formation models therefore appears to be entirely plausible .

We also investigate the density and temperature distributions of gas in our model clouds .
We show that the density probability distribution function is approximately log-normal , with a dispersion that agrees well with the prediction of Padoan , Nordlund & Jones ( 1997 ) .
The temperature distribution is similar to that of a polytrope , with an effective polytropic index \gamma _ { eff } \simeq 0.8 , although at low gas densities , the scatter of the actual gas temperature around this mean value is considerable , and the polytropic approximation does not capture the full range of behaviour of the gas .