  We investigate the imprint of nonlinear matter condensations on the Cosmic
Microwave Background (CMB) in $\Omega_{0}<1$ cold dark matter (CDM) model
universes. We consider simulation domains ranging from $120h^{-1}$ Mpc to
$360h^{-1}$ Mpc in size. We concentrate on the secondary temperature
anisotropies induced by time varying gravitational potentials occurring after
decoupling. Specifically, we investigate the importance of the Rees-Sciama
effect due to: (1) intrinsic changes in the gravitational potential of forming,
nonlinear structures, (2) proper motion of nonlinear structures, and (3) late
time decay of gravitational potential perturbations in open universes. CMB
temperature anisotropies are obtained by numerically evolving matter
inhomogeneities and CMB photons from an early, linear epoch ($z=100$) to the
present, nonlinear epoch $(z=0)$. We test the dependence and relative
importance of these secondary temperature anisotropies as a function of the
scale of the underlying matter (voids, superclusters) and as a function of
$\Omega_{0}$. The results of the $\Omega_{0}<1$ models are compared to a
similarly executed $\Omega_{0}=1.0$ simulation. We find that in low density
models all three sources of anisotropy could be relevant and reach levels of
$\Delta T/T \sim 10^{-6}$. In particular, we find that for $\Omega_{0}<1$ at
large scales, secondary temperature anisotropies are dominated by the decaying
potential.
