  Recent hydrodynamical models of supernova remnants (SNRs) demonstrate that
their evolution depends heavily on the inhomogeneities of the surrounding
medium. As SNRs expand, their morphologies are influenced by the non-uniform
and turbulent structure of their environments, as reflected in their radio
continuum emission. In this paper, we measure the asymmetries of 96 SNRs in
radio continuum images from three surveys of the Galactic plane and compare
these results to the SNRs' radii, which we use as a proxy for their age. We
find that larger (older) SNRs are more elliptical/elongated and more mirror
asymmetric than smaller (younger) SNRs, though the latter vary in their degrees
of asymmetry. This result suggests that SNR shells become more asymmetric as
they sweep up the interstellar medium (ISM), as predicted in hydrodynamical
models of SNRs expanding in a multi-phase or turbulent ISM.
