MONTHLY
WEATHER
REVIEW
VOLUME 92, NUMBER 8
AUGUST 1964
SOME PRELIMINARY DUST DEVIL MEASUREMENTS"
PETER C. SlNCLAlR
Institute of Atmospheric Physics, The University of Arizona, Tucson, Ariz.
ABSTRACT
The development of a mobile recording system for measuring temperature, pressure, and wind velocity in dust
devils is discussed. Four dust devil penetrations are presentcd in which the above three quantities were measured
simultaneously. All four cases show quantitatively for the first time the warm, low-pressure core of the dust devil.
Whilc the instrumentation in some cases lacked proper response and damping, the results were of sufficient accuracy
to guide the devclopment of a largcr multi-level system.
1. INTRODUCTION
One of the most interesting features of arid regions dur-
ing the summer is the frequent occurrence of dust devils
(fig. 1). These relatively small-scale atmospheric vortices
may range in size from a few inches to hundreds of feet in
diameter and from a few feet to thousands of feet in height.
They are a part of the general classification of atmospheric
thermals in which the character of their structure is more
closely related to the chimney than to the bubble type
thermal.
Near the ground, the primary visible feature is the
usually intense rotary motion which may be either clock-
wise or counterclockwise. It is believed that this rotary
motion is maintained by the tendency of the air to con-
serve its angular momentum as it spirals in toward the
dust devil's center. Superimposed on this motion is a
strong vertical motion which results in a combined flow
pattern similar to that of a helical vortex. There is also
a radial velocity component which near the ground sup-
plies the vortex with warm boundary-layer air and thereby
acts to sustain the dust devil's vertical motion. Knowl-
edge of these features is largely the result of visual ob-
servations of dust and other debris picked up by the dust
devil as it moves along with the general environmental
wind flow. (It should not be inferred that dust particle
'Work supported by Oficc of Il'aval Research.
or other debris trajectories correspond to air parcel
trajectories.)
A t higher levels very little is known since the concentra-
FIGURE 1.-A typical desert dust dcvil. Thc cnlarged and more
diffuse region just above the ground may be due to particles
which have bcen centrifuged, because of their density, out of thc
main vortex.
363
364 MONTHLY WEATHER REVIEW Vol. 92, No. 8
tion of dust and debris makes visual observations ex-
tremely difficult. Numerous sailplane flights in the upper
portions of the dust devil have shown, however, that
strong vertical motions may exist to very great heights.
The author has made a number of flights in the upper por-
tions of dust devils to over 15,000 f t . MSL (ground eleva-
tion approximately 2,500 ft.). The vertical motions ex-
perienced above dust devils of this size may range from a
few hundred feet per minute to 2,000 ft./min., and extend
over a horizontal distance of the order of a mile.
As a result of the dust devil’s intensity and/or size, i t
may have a significant role in the heat transfer processes
that take place in desert regions during the summer
months. I n addition, a detailed study of dust devils may
lead to new knowledge of thermals in general; also new in-
sights concerning other rotational systems such as the
tornado may become apparent. To provide a firm founda-
tion for subsequent theoretical analysis, a quantitative
description based on actual measurements near and within
dust devils seemed desirable. The measurement program
was accomplished in two phases: (I) measurements were
made of temperature, pressure, and wind velocity using
off -the-shelf or very easily modified off-the-shelf equip-
ment; (2) the results of phase 1 measurements were then
used as a guide to design more sophisticated instrunienta-
tion for the study of horizontal as well as vertical varia-
tions of pressure, temperature, and wind velocity. The
purpose of this paper is to discuss the instruments briefly
and present the measurements of phase 1.
2. INSTRUMENTATION
Instruments were designed or selected with simplicity,
ruggedness, mobility, and nlinimuin construction time in
mind. The primary purpose of this mobile recording
system (MRS) was to obtain some insight as to the magni-
tude of the pressure, temperature, and horizontal wind
velocity variations in some selected dust devils during the
summer of 1960. The MRS (fig. 2) is built around a pole
(vaulting pole) which has an extremely high strength-to-
weight ratio. The entire unit weighs approximately 50 lb.
and can be quickly transported by one man through t,he
usual desert undergrowth.
The temperature sensor is mounted on a cross-bar near
the top of the pole and consists of a standard-type resist-
ance thermometer whose signal is amplified sufficiently to
be displayed on a microammeter mounted in the photo
box (fig. 3) near the middle of the pole. The amplifier,
batteries, calibration and balancing controls, and a “slave”
microammeter are mounted on top of the photo box.
Prior to each dust devil penetration the temperature range
selector is adjusted until the “slave” microammeter indi-
cates the maximum usable range. The response of the
temperature unit is approximately 0.5 sec. and is primarily
limited by the response of the microammeter.
From the dimensions of some large dust devils and an
estimate of their tangential wind velocities, the most
simplified analysis yields a total pressure drop of a few
millibars. For such pressure measurements, a standard
aircraft altimeter was modified by the addition of a larger
more sensitive aneroid unit (fig. 4). This resulted in
almost a 20-fold magnification for the 100-ft. altitude
needle of the altimeter; i.e., after modification one com-
plete revolution of the 100-ft. needle corresponded to a
3.5-mb. pressure change.
To minimize the friction inherent in the pressure unit,
a small electric buzzer was used as a high frequency
vibrator. The frequency was such that the vibration did
not affect the pressure reading during the dust devil pene-
tration. However, the unit is sealed ih a metal box and
hence the heat dissipated by the buzzer acts to warm the
aneroid unit causing a slight drift (0.04 mb./min.) in
indicated pressure. As the sampling period is only about
20 sec., this error is not significant. There is an addi-
tional instrumental pressure drift due to the external
heatsing of the metal box which is a result of the high desert
temperatures. This error can initially be approximately
0.1 mb. per 20 sec. fot an instantaneous 10” C. increase
in outside air temperature and hence can also be consid-
ered insignificant during the sampling period.
Because of the relatively large aneroid mass, and
deflection with pressure change, the aneroid, and hence
the pressure reading, is susceptible to change in orientation
of the aneroid axis or, equivalently, to angular displace-
August 1964 Peter C. Sinclair 365
FIGURE 3.-View of photo panel and c:tmcra with photo box sidc
Tcmpcrnture range selector, cnlibratioii controls, and removed.
"slave" micronmmctcr arc locntcd abovc thc camcra.
ments froin tlie verticd of the h4RS pole. For slow or
constant angulnr displtminents of the pole the error was
approximately 0.1 mb./deg. through a rurige of 45 degrees
from the vertical. These errors were compensthted for by
simultaneously p1iotogr:iphing t i ctdibm ted mercury-in-
alcohol pitch-level on the photo panel. However, for
oscillation frequencies near the 1-sec. time consttsnt of the
instrument, only partitil conipelwtion t i t most could be
accomplished because of the inertia inherent in the
pitch-level.
The high wind speeds near the visible dust devil
vortex make the design of the pressure antenna (pressure
sensing port) of critical importance. The simplest fised
static pressure antenna [I] lias been suggested by Prandtl
and Tietjens [2]. This design was used liere and consists
of a metal disk (0.125 in. thick, 4 in. ditmieter) supported
by a narrow (0.250 in. diameter) metal tube. The disk
has a 0.125-in. hole (static port) in the center which
allows an air connection from the environment through
the metal supporting tube to the metal box containing tlie
modified altimeter. ‘I’he pressure antenna is mounted
with the static port directed downward a t the end of the
cross-arm near the top of the h4RS (see fig. 2 ). The
inverted position was selected for a number of reasons:
FIGURE 4.-Iiitcrior view of pressure instrument showing the
ancroid connection t o the modified altimeter. Thc vibrator is
mountcd on top of the aneroid.
(I) Smoke tests indicated that the vertical wind component
a t tlie antenna height (6.5 ft. above the ground) would
probably be less than 1,000 ft. min.-’, and hence dynamic
pressure rariations would be less than 0.1 nib. (2) Wind
tunnel calibration tests of the antenna showed that when
the static port was directed into the wind, variations of
the wind with respect to the circular plate of the antenna
produced n rtither uniform dynamic pressure increment ;
however, when the static port was directed with the wind
(downstrenni), the flow over the circular plate created a
low pressure region on the static port side resulting in
much larger dynmnic errors whose form was complicated
by subsequent flow separation. (3 ) With the static port
opening directed downward, clogging of the narrow inlet
tube by sand and other foreign particles would be mini-
mized. For flow parallel to the metal disk and over the
0.125-in. opening, there is a small suction effect which in
this case ctm be neglected. It has been shown to be
approximately 0.01 nib. for this type of antenna in a 45
m.p.1). wind (cf. 111, p. 5 ).
A Bendis-Friez Windial was used to measure wind
velocity (see figure 2). Prior knowledge indicated that
because of the Windial’s response time (both speed and
direction) i t was poorly suited for dust devil investigations.
But lack of time necessitated use of a stock item. The
wind direction and speed dials were removed from the
Bendis-Friez displny box and mounted liorizontally on
the AIRS photo panel (fig. 3). The unit was converted
from 115 v.a.c. to 6 1y.d.c. by a simple circuit rearrange-
inent suggested by Bendis-Friez.
The data recording was accoinplished by simply photo-
phphing the instrument panel with a 16-mm. Bolex movie
camera (1o-mm. lens) a t 32 frames per second. The
photo box is completely enclosed to protect the camera
366 MONTHLY WEATHER REVIEW Vol. 92, No. 8
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and to ensure good photography. The box is covered
with a semi-transparent plastic sheet which transmits
sufficient light (even within the dust devil) for photo
recording.
3. THE MEASUREMENTS
’
The field operations involved the rapid transport of the
MRS by truck to the general dust devil area. The MRS
was then carried by hand to n point downwind of the dust
devil and placed in such a position that the dust devil
would pass directly over the MRS. The results of four
dust devil penetrations which occurred in the desert a
few iiiiles west of Tucson, Ariz. are shown in figures 5, 6,
7, and S. The various scales (excluding the pressure and
tilt angle) should be self-explanatory. The pressure units
can be converted to millibars by use of 1 “pressure
unit”=5X10-’ nib. The tilt angle refers to the number
of degrees that the MRS was tilted away from plumb
vertical and indicates where corrections ha]-e been
applied and/or where the data may have a tilt angle error.
All four dust devil penetrations show the anticipated
warm core, low pressure center. The temperziture rise
varied from almost 9’ C. for the larger dust devil (fig. 5)
to 3.5’ C. for the sinaller one (fig. 8). An interesting
temperature feature of the two most intense dust devils,
shown in figures 5 and 6, is that the tempernture gradient
is less st,eep in the rear than in the front of the dust devil
(front refers to that portion of the dust devil which
passed over the MRS first). This may be related to the
difference in wind fields in these two regions but further
measurements are needed before this can be determined.
The dashed portion of the temperature trace in figure 7
indicates a partial loss of data as a result of a temperature
range change during the dust devil penetration.
The total pressure drop (Ap) varied in the same manner
from 2.3 mb. (fig. 5) for the larger dust devil to 0.6 nib.
(fig. 8) for the smaller dust devil. The largest pressure
drop is shown in figure 6 where Ap=3.5 nib. Because the
pressure instrument is underdaniped, these total pressure
drops are soniewhat in error due to excessive “overshoot-
ing”. By impressing a continuous sinusoidal pressure
oscillation on the instrument with a pressure generator
it \vas found that a t 0.5 c.p.s. the insti~ument had a niaxi-
muni “overshoot” of 30 percent. Consequently, the
pressure pulse shown in figure 6 should be reduced by
about 1/3, i.e. A p =2.4 nib. (simi1:Lr corrections sliould be
made in the other figui-es). For continuous correction of
the pressure trace, the pressure generator data showed that
smoothing the trace using a running me:in of At=O.9 sec.
resulted in a smoothed pressure trace that wzis a good
approsiniation to the actual pressure trace.
Because of the relatively slow response of the Windinl,
the wind data are probtibly most accurate in the time
interval preceding the passage of the dust devil center.
The dust devil center is assumed to be near the pressure
niininiuiii and tli e closely rela ted tern pertiture m axim uni .
4
WIND DIRECTION
I 2 3 4 5 6 7 0 9 1 0 1 1 1 2 1 3 1 4 L 5 1 6 1 7
TIME (seconds)
FIGURE 5.-Tempcrature, pressure, and horizontal wind velocity
measurements through a dust devil on August 2, 1960, 1230
RIST.
--? .
I
I
230-
% 190- -
150-
Y . z 110-
0 1 2 3 4 5 6 7 8 3
WIND DIRECTION
\P-
TIME (seconds)
FIGURE 6.-Tempcrature, pressure, and horizontal mind vclocity
mcasurcrnents through a dust devil 011 July 19, 1960, 1305
MST.
on either side of the low pressure center. The exception
shown in figure 5 may be the result of an off-center pene-
tration of an eccentric vortex (note that the wind direction
chwnge was only 90 degrees). In the other cases, the
I n general, the wind speed traces show two maxima, one change in wind direction seems indicative of n center
August 1964 Peter C. Sinclair 367
T I L T (r Z 0 1 1
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2 300-g
Y 3 4 0 -L o D O
5 0 -z
z 4 0 -1
G 80-
g 1 2 0 -
6 1 6 0 -
3 2 4 0 -
2 2 0 0 -
t
WIND DIRECTION:
I 2 3 4 5 6 7 8 9 IO I I 12 13 14 1 5 16 17 $8 19 2 0
TIME (seconds)
FIGURE 7.-Temperature, pressure, and horizontal wind velocity
mcasurernents through a dust devil on July 14, 1960, 1330
MST.
penetration of a concentric vortex. The dashed portion
of the wind speed trace in figure 6 is the result of the slow
response of the Windial. Apparently, after passing
through the dust devil center, the Windial did not respond
to the almost step-function change in wind direction (note
the large overshoot after t =3 sec.), with the result that
the propellor turned backwards a t a rate of more than
20-m.p.h. The best estimate from the recorded data of
what actually happened is represented by the dashed
curve.
4. CONCLUSIONS
While the instrumentation in some cases lacked the
proper response and damping, the results were of sufficient
ANGLE( O p - I
PRESSURE
TEMPERATURE
3 7 - 1
- WIND SPEED
L.
4 . 2 0 .
a 2 6 0 - -
K 1 8 0 ‘ .
2 i 1 4 0 - WIND DIRECTION
3
:
, , ,-, , , , , ,
I 2 3 4 5 6 7 8 0 10 I 1 I2 I3 14 15 16 17 18
TIME (seconds)
FIGURE 8.-Tempcrature, pressure, and horizontal wind velocity
measurements through a dust devil on August 30, 1960, 1435
MST.
accuracy to guide the development of more sophisticated
iastrumentation. This was true of the quantitative
results shown in figures 5-8, and equally important were
the data gathered on the mobility requirements for a
larger multi-level system.
REFERENCES
1. Iowa State College, Physics Department, “Investigation of
Seismic and Sonic Disturbances,” Final Report, ONR Project
2. L. Prandtl and 0. G. Tietjcns, Applied Hydro- and Aerome-
chanics, Dover Publications, Inc., New YOTK, 1957, pp. 226-
227.
NO. NR 350-017, 1950, 90 pp.
[Received March 13, 1964; revised May 5, 196,$]