  {We study in some details the statistical properties of the turbulent 2-phase
interstellar atomic gas.{We present high resolution bidimensional numerical
simulations of the interstellar atomic hydrogen which describe it over 3 to 4
orders of magnitude in spatial scales.}{The simulations produce naturally small
scale structures having either large or small column density. It is tempting to
propose that the former are connected to the tiny small scale structures
observed in the ISM. We compute the mass spectrum of CNM structures and find
that ${\cal N}(M) dM \propto M ^{-1.7} dM$, which is remarkably similar to the
mass spectrum inferred for the CO clumps. We propose a theoretical explanation
based on a formalism inspired from the Press & Schecter (1974) approach and
used the fact that the turbulence within WNM is subsonic. This theory predicts
${\cal N}(M) \propto M ^{-5/3}$ in 2D and ${\cal N}(M) \propto M ^{-16/9}$ in
3D. We compute the velocity and the density power-spectra and conclude that,
although the latter is rather flat, as observed in supersonic isothermal
simulations, the former follows the Kolmogorov prediction and is dominated by
its solenoidal component. This is due to the bistable nature of the flow which
produces large density fluctuations even when the rms Mach number (of WNM) is
not large. We also find that, whereas the energy at large scales is mainly in
the WNM, at smaller scales, it is dominated by the kinetic energy of the CNM
fragments.}
