  The influence of dust devils on the martian atmosphere depends on their
capacity to loft dust, which depends on their wind profiles and footprint on
the martian surface, i.e., on their radii, $R$. Previous work suggests the wind
profile depends on a devil's thermodynamic efficiency, which scales with its
height, $h$. However, the precise mechanisms that set a dust devil's radius
have remained unclear. Combining previous work with simple assumptions about
angular momentum conservation in dust devils predicts that $R \propto h^{1/2}$,
and a model fit to observed radii and heights from a survey of martian dust
devils using the Mars Express High Resolution Stereo Camera agrees reasonably
well with this prediction. Other observational tests involving additional,
statistically robust dust devil surveys and field measurements may further
elucidate these relationships.
