  Volcanic ash clouds often become multilayered and thin with distance from the
vent. We explore one mechanism for development of this layered structure. We
review data on the characteristics of turbulence layering in the free
atmosphere, as well as examples of observations of layered clouds both
near-vent and distally. We then explore dispersion models that explicitly use
the observed layered structure of atmospheric turbulence. The results suggest
that the alternation of turbulent and quiescent atmospheric layers provides one
mechanism for development of multilayered ash clouds by modulating vertical
particle motion. The largest particles, generally $> 100 \mu$m, are little
affected by turbulence. For particles in which both settling and turbulent
diffusion are important to vertical motion, mostly in the range of 10-100
$\mu$m, the greater turbulence intensity and more rapid turbulent diffusion in
some layers causes these particles to spend greater time in the more turbulent
layers, leading to a layering of concentration. For smaller particles, mostly
in the submicron range, the more rapid diffusion in the turbulent layers causes
these particles to ``wash out'' quickly.
