An extensive set of measurements of the scintillation of a
laser propagated over long horizontal paths through atmospheric turbulence,
at altitudes spanning the tropopause was recently obtained.
These measurements were made over sequences of parallel but displaced
paths, like the rungs of a ladder. It is shown here that the intensity
reductions caused by scintillation of two parallel paths separated by 35.6
m are partially correlated. Further, the correlations between paths with
discrete experimental separations are used to construct the correlation
functions for arbitrary path displacement. The variance in continuous
moving averages of the relative intensity is then found in terms of the
correlation functions, parameterized by the distance the propagation
path is swept through the turbulence. An empirical formulation is developed
for use in assessing the expected distribution of intensity reductions
in various laser systems. This analysis recovers the statistics of
atmospheric scintillation for the important regime in between the two
extremes of a snapshot and a long time average.