  A distribution function approach is applied to describe the dynamics of the
laser beam in the Earth atmosphere. Using a formal solution of the kinetic
equation for the distribution function, we have developed an iterative scheme
for calculation of the scintillation index ($\sigma^2$). The problem reduces to
obtaining the photon trajectories and their correlations. Bringing together
theoretical calculations and many-fold computer integrations, the value of
$\sigma^2$ is obtained. It is shown that a considerable growth of $\sigma^2$ in
the range of a moderate turbulence is due to the correlations of different
trajectories. The criteria of applicability of our approach for both the
coherent and partially coherent light are derived.
