  Halide pervoskites are an important class of semiconducting materials which
hold great promise for optoelectronic applications. In this work we investigate
the relationship between vibrational anharmonicity and dynamic disorder in this
class of solids. Via a multi-scale model parameterized from first-principles
calculations, we demonstrate that the non-Gaussian lattice motion in halide
perovskites is microscopically connected to the dynamic disorder of overlap
fluctuations among electronic states. This connection allows us to rationalize
the emergent differences in temperature-dependent mobilities of prototypical
MAPbI$_3$ and MAPbBr$_3$ compounds across structural phase-transitions, in
agreement with experimental findings. Our analysis suggests that the details of
vibrational anharmonicity and dynamic disorder can complement known predictors
of electronic conductivity and can provide structure-property guidelines for
the tuning of carrier transport characteristics in anharmonic semiconductors.
