  We report on the anisotropic expansion of ultracold bosonic dysprosium gases
at temperatures above quantum degeneracy and develop a quantitative theory to
describe this behavior. The theory expresses the post-expansion aspect ratio in
terms of temperature and microscopic collisional properties by incorporating
Hartree-Fock mean-field interactions, hydrodynamic effects, and
Bose-enhancement factors. Our results extend the utility of expansion imaging
by providing accurate thermometry for dipolar thermal Bose gases, reducing
error in expansion thermometry from tens of percent to only a few percent.
Furthermore, we present a simple method to determine scattering lengths in
dipolar gases, including near a Feshbach resonance, through observation of
thermal gas expansion.
