32.881678,-111.854982,Francisco Grande, Casa Grande, Arizona,
USA

Gary Osoba writes:

Here's a technical piece predicting frequency of occurrence,
scaling, types of geographic distribution, and other helpful information
relative to dust devils. The data presented was gathered in the Santa Cruz Flats
region.

http://ozreport.com/docs/Renno.et.al.JAS.98DD.pdf

Based on the heat engine framework, a simple scaling theory for
dust devils is proposed and compared to observations. This theory provides a
simple physical interpretation for many of the observed characteristics of dust
devils. In particular, it predicts the potential intensity and the diurnal
variation of dust devil occurrence. It also predicts that the intensity of dust
devils depends on the product of two thermodynamic efficiencies, corresponding
respectively to vertical and horizontal temperature gradients.

Dust devils are low pressure, warm-core vortices with typical surface diameters
between 1 and 50 m. Since they receive their vorticity from local wind shears
that can be either due to the convective circulation itself or due to
larger-scale phenomena, they rotate either cyclonically or anticyclonically with
equal probability (Williams 1948; Sinclair 1966; Carroll and Ryan 1970).

To a first approximation, a dust devil moves with the speed of the ambient wind,
typically at about 5 m s21. In general, dust devils slope with height in the
wind shear direction. In environments of high wind speed (*10 m s21), dust devil
diameters are biased toward large values. About 55% of the dust devils observed
around Tucson, Arizona, have diameters between 3 and 15 m, and 15% have
diameters larger than 15 m (Sinclair 1966, 1969, 1973).