  Reliably predicting the geomorphology and climate of planetary bodies
requires knowledge of the dynamic threshold wind shear velocity below which
saltation transport ceases. Here we measure this threshold in a wind tunnel for
four well-sorted and two poorly sorted sand beds by visual means and by a
method that exploits a regime shift in the behavior of the surface roughness
caused by momentum transfer from the wind to the saltating particles. For our
poorly sorted sands, we find that these measurement methods yield different
threshold values because, at the smaller visual threshold, relatively coarse
particles do not participate in saltation. We further find that both methods
yield threshold values that are much larger (60--250\%) for our poorly sorted
sands than for our well-sorted sands with similar median particle diameter. In
particular, even a rescaling of the dynamic saltation threshold based on the
90th percentile particle diameter rather than the median diameter cannot fully
capture this difference, suggesting that relatively very coarse particles have
a considerable control on the dynamic threshold. Similar findings were
previously reported for water-driven sediment transport. Our findings have
important implications for quantitative predictions of saltation
transport-related geophysical processes, such as dust aerosol emission.
