Tornadoes - Severe Convective Storms and Tornadoes: Observations and Dynamics - page 317

Figure 6.11. Cycloidal marks (two of which are indicated by arrows) in a field created by small-
scale vortices in a tornado rotating around an axis of rotation associated with a larger vortex
(photograph by Ted Fujita, circa mid-1960s; courtesy of History of Science Collections,
University of Oklahoma Libraries).

leads to superadiabatic lapse rates near the ground. Fire whirls owe their existence
entirely to intense surface heat sources. Volcanic eruptions are sometimes
responsible for producing tornadoes and waterspouts, which are associated with
''pyro''-cumulonimbus clouds. Steam devils occur when very cold air is advected
over a relatively warm water surface. Dust devils are driven by solar insolation at
the ground, but may depend on frictionally induced vertical shear near the surface,
or clear air cellular convection and the tilting of horizontal shear due to the
vertical circulation of cells, or baroclinically generated horizontal vorticity. Dust
devils typically occur in an unsaturated atmosphere and may not be visible when
there is no dust to act as tracers. While the causes of each of these vortices are
somewhat different, they all involve intense vortices making contact with the

ground.
There are also funnel clouds that do not become tornadoes and are pendant
from high-based cumuliform clouds—not cumulonimbus clouds—which most
likely are not fed by boundary-layer air because their bases are so high ( Figure
6.17 ). The dynamics of these vortices are not well known, though funnel clouds
are observed relatively frequently, particularly underneath ragged cloud bases and
when the cumuliform tower above begins to dissipate.