The

Jorrnral ofMeteorclo

Precipitation in Ohio, USAClassic thunderstonns of 1960

JOURNAL OFMETEOROLOGY"An international magazine for everyone

and in their influence on the human

Vol.29 No.292 October 2004 n5

interested in weather and climate,and physical environment."

THERMAL IMAGING OF ADESERT DUST DEVIL

ByRALPH D. LORENZLunar and Planetary Loboratory, University of Arizona, 1629 E. University

BIvd, Tucson, AZ 85721, USA. Email: rlorenT@lpl.arizona.edu

Abstract: Dust devils are convective vortices rendered visible by the levitation of dust and solar absorption

which may play a significant part in the energy budget of the devil. We present imaging with a thermal

camera of an encounter with a desert dust devil which shows the dust to remain hot long after lifting.

INTRODUCTIONDust devils are small (sub-m to few tens of m diameter) convective vortices rendered

visible by lofted surface particulates. Their strong local winds can present a nuisance (in

extreme cases causing damage to light buildings). Triboelectric charging of the particulates

can cause strong electric fields andradio intedercnce (Houseret a|2ffi3}T}ley areknown

to be a significant influence on the climate on Mars, by raising dust into the atmosphere,

and may affect air quality on Earth for the same reason. Dust devils resemble tornadoes or

hurricanes in that in mechanical terrns they are cyclostrophic or Rankine vortices operating

as a heat engine (Renno e/ a I . , 1998) , but they differ in that dust devils are not driven by the

latent heat of water, but rather are associated with strong surface heating and convective

instability characteristic of early afternoon desert conditions.Invisible convective vortices doubtless occur where dust is not available, although

micrometeorological data on these transient and small-scale features is not abundant and

thus it is not known whether dust-laden vortices are more intense than those without dust.

Small-scale sudace vortices can occur in the absence of strong sudace heating (and are

occasionally rendered visible by lofting auturnn leaves or sno% for example) but these

appear generally to be short-lived, persisting for only a few seconds. Thus a fundamentalquestion is the source of energy for dust devils, which typically remains visible for several

minutes. As engines, they convert heat into work, which is consumed by frictional

dissipation. Their longevity, and the fact they can be observed to increase in intensity,

requires a source of heat. The two possibilities are hot air drawn into the vortex froni a layer

adjacent to the hot surface, and sunlight absorbed by airborne dust itself. It is also not

known whether the airflow into the devil is radially symmenic, orif surface inflow is collimated(Hess and Spillane, i990) along 'legs'.

Dust devils have been studied, both on Mars and Earth, by remote optical

observation (Hess and Spillane 1990; Metzger et a1.,1999; Sinclair.,1969) yielding basic

statistics such as size, lifetime ald location and in-situ measurements of wind, temperature

and pressure, recorded either serendipitously by the passage of a devil over a fixed platform

likeaMarslander(e.g.Renno eta1.,2000),orbythedeploymentofaninstrumentpackagefrom a vehicle into the path of a moving devil (Sinclair, 1973).

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TIfiRMAL IMAGING

Vol.29 No.292 October 2004

The author thermally imaged (Fig.1 ,front cover) a large (-4m diameter) dust devilencountered atI3.MhrsMSTon2TJune2004atasite 32.264 "N 111.273 oWinaregion

(-15km West of Tucson, Arizon4 USA) which was the subject of a previous dust devilstudy (Sinclair, 1973). Several smaller devils had been observed in the preceding hours (wenote that polarizing sunglasses considerably enhance the visibility of devils by suppressingthe background sky brightness) but none close enough to resolve thermally - our thermalcamera (IRISYS 1011) is a 16x16 pixel camer4 with arelatively wide field of view (20").

Thermal images show a convolution of dust opacrty and temperafure whereasoptical images Fig.2, inside front cover) can indicate the dust loading alone. When thedust is thick, the brightness temperafure approximately indicates its physical temperature.Since the brightrress temperature of the sky in clear, dry conditions (typical for the formationof dust devils) can be -30"C, thermal imaging therefore can yield a high contrast and may beeffective inproviding 'early warning' of marginally-visible devils.

Calculations indicated that a levitated 10-100 mm dust particle passes its ownmass of air in <1s, and thus must reach a thermal steady state rapidly. Near desert noon,such a particle may intercept enough sunlight to warm itself by 10-100 K per second, wereit not to transfer this heat to the air. Sunlight absorption by airborne dust must thereforecontribute substantially to the devil's energy budget. Indeed, in the present observation,the optically-thick parts of the devil near the ground remain only slightly below the groundtemperature, even though the dust itself has had time to warm or cool. No evidence on thisoccasion was observed of a cold 'track' or inflow zone or 'legs' on the ground.

CONCLUSIONThe author believes this is the first scientific thermal imaging observation of a

dust devil, although informal reports from US Navy helicopter personnel on duty in haq(Lt. Cdr. Clint Cresap, USN, pers. cornrn, Feb 2004) indicates that dust devils are visible indaytime FLIR (Forward-Looking InfraRed). Our observation supports the idea that dustabsorption may have been important in the energy budget of at least this particular dustdevil, and suggests a more detailed study, supported with radiometrically-calibrated opticalimaging and ideally in-situ meteorological measurements, will be fruitful.

REFERENCESmSS, G D. and K. T. SPILLANE (1990) Characteristics of dust devils in Australia. J. Appl. Meteorology29,498-507.HOUSER, J. G., FARRELL, W. M. and S. M. METZCER (2003) ULF and ELF magnetic activity from aterrestrial dust devil. Geophys. Res. Lett.,30,27.METZGER, S. M., J. R. CARR, J. R. JOHNSON, T. J. PARKER, and M. T. LEMMON (1999) Dust devilvortices seen by the Mars Pathfinder camera. Geophys. Res. Lett.26, 2781.RENNO, N. O., NASH, A.A., LIININE, J. I. and J. MURPHY (2000) Martian and rerresrrial dust devils.Testing. of a scale theory using Pathfinder data. J. Geophys. Res., 105, 1851-1865.RENNO, N. O., M. L. BURKETT and M. P. LARKIN (1998) A simple thermodynamic theory for dustdevils. .L Atmos. Sci.. 55. 3244-3252.SINCLAIR, P. C. (1973) The lower structure of dust devils. J. Atmos. Sci., 30, 1599-1619.

SINCLAIR, P. C. (1969) General characteristics of dust devils. J. Appl. Meteorology,8,32-45.

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