  We evaluate the effective number of atoms in experiments where a probe laser
beam with a Gaussian profile passes through an atomic medium consisting of a
cold atom cloud released from a magneto-optical trap. Considering the case
where the initial distribution is a Gaussian function of position and velocity,
we give a quantitative description of the time variation of the effective atom
number while the cloud is exploding and falling. We discuss the two cases where
the effective number is defined from the linear and nonlinear phase shifts,
respectively. We also evaluate the fluctuations of the effective atom number by
calculating their correlation functions and the associated noise spectra.
Finally we estimate the effect of these fluctuations on experiments where the
probe beam passes through a cavity containing the atomic cloud.
