We present an analysis of the large-scale molecular cloud structure and of the stability of clumpy structures in nearby molecular clouds .
In our recent work , we identified a structural transition in molecular clouds by studying the probability distributions of gas column densities in them .
In this paper , we further examine the nature of this transition .
The transition takes place at the visual extinction of A _ { \mathrm { V } } ^ { \mathrm { tail } } = 2 - 4 mag , or equivalently , at \Sigma ^ { \mathrm { tail } } \approx 40 - 80 M _ { \odot } pc ^ { -2 } .
The clumps identified above this limit have wide ranges of masses and sizes , but a remarkably constant mean volume density of \overline { n } \approx 10 ^ { 3 } cm ^ { -3 } .
This is 5 - 10 times larger than the density of the medium surrounding the clumps .
By examining the stability of the clumps , we show that they are gravitationally unbound entities , and that the external pressure from the parental molecular cloud is a significant source of confining pressure for them .
Then , the structural transition at A _ { \mathrm { V } } ^ { \mathrm { tail } } may be linked to a transition between this population and the surrounding medium .
The star formation rates in the clouds correlate strongly with the total mass in the clumps , i.e , with the mass above A _ { \mathrm { V } } ^ { \mathrm { tail } } , dropping abruptly below that threshold .
These results imply that the formation of pressure confined clumps introduces a prerequisite for star formation .
Furthermore , they give a physically motivated explanation for the recently reported relation between the star formation rates and the amount of dense material in molecular clouds .
Likewise , they give rise to a natural threshold for star formation at A _ { \mathrm { V } } ^ { \mathrm { tail } } .