  This paper presents a possible way to understand dissolution of OB
associations (or groups). Assuming rapid escape of parental cloud gas from
associations, we show that the shadow of the formation environment for
associations can be partially imprinted on the velocity dispersion at their
dissolution. This conclusion is not surprising as long as associations are
formed in a multiphase interstellar medium, because the external pressure
should suppress expansion caused by the internal motion of the parental clouds.
Our model predicts a few km s$^{-1}$ as the internal velocity dispersion.
Observationally, the internal velocity dispersion is $\sim 1$ km s$^{-1}$ which
is smaller than our prediction. This suggests that the dissipation of internal
energy happens before the formation of OB associations.
