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Roger Weller, geology instructor
by Ilse Severson
There is something eerie about driving through clear skies while
on the freeway and seeing these tornado-like shapes slowly make their way across
roaming farm fields. Arizona is home to the tornado’s less destructive cousins,
the dust devils. They are called different names all over the world including:
genies, whirlwinds, spirits, and willy willies. Dust devils are most active
between ten in the morning and one in the afternoon. They are referred to as
meteorological curiosities because there is little known about them. Recently,
dust devils have also been discovered on Mars!
Dust Devil on Mars
Credit Source: NASA JPL
Here on earth, they form at the peak of the afternoon during
ideal clear skies. The sun heats up a particular spot as light wind intensifies
surface heating. Surrounding hot air meets with the hot air near the particular
hot spot. With the help of moving air, the convergence of hot air turns into a
swirling column called a convective vortex. As the hot air rises, it feeds the
dust devil’s energy. The hot air then rises pushing cooler and denser air
towards the top. The vortex’s vertical position can be triggered from anything
as simple as a moving car, animal or a mild breeze. As it moves, it picks up
dust, sand, and debris which allows the dust devil to become visible to the
human eye even during far away distances. The intensity of the dust devil
depends on the depth of the convective plume and the existence of wind speed.
Cold terrain or change in topography causes the column of air to be cut off
causing the convective plume to quickly dissipate.
Dust devils are affected by cloud covering due to the slowing
of surface heating. Their rotation is random and they can turn on either
direction. They also serve as boundary layer transports and pollution
dispensers. Interestingly enough, they also create static electricity through
swirling particles that rub against one another. As the particles swirl, they
gain a static electrical charge that remains in one place. The amount of voltage
carried by an oversized dust devil can be as much as the voltage carried by
power lines. Dust devils can create ten thousand volts per meter. On Mars, their
voltage intensity is so vast that they glow in the dark and toss lightning bolts
as they travel. The temperature inside a dust devil is hard to measure because
they are constantly moving and hostile.
Credit Source:©2006 Paul Naton
Dust devils are classified by different sizes ranging from small, moderate, large, and gigantic. The small dust devils have a diameter less than one meter. They are the most common and last less than a minute. Moderate dust devils are up to ten meters in diameter and have a duration of up to four minutes. Large dust devils are typically a hundred meters in diameters; they can last up to twenty minutes and are visible from miles away. Lastly, gigantic dust devils have a diameter exceeding one hundred meters and can be composed of two or more dust devils. The last category is common in Mars but extremely rare here. Dust devils can also be very destructive; it's no wonder that they are referred to as the tornado’s cousins. Dust devils are invisible unless they pick up debris so the rocking motion sometimes experienced by motorist is often a mischievous dust devil at work. Dust devils have also been known to knock cyclist off their bikes and can appear in groups.
Credit Source: California Department of Public Health and
Centers for Disease Control and Prevention
Valley Fever Statistics Desease Diagram
Credit Source: Arizona Department of Health Services
U.S. Centers for Disease Control, and The Sacramento Bee
Aside from the dust devils mischievous acts, the biggest threat
they pose to human health is the risk of Coccidioidomycosis, otherwise known as
Valley Fever. “Cocci” fungus grows in alkaline soil which is found in the South
East of the United States and can be found in Arizona. The fungus lies dormant
on the soil and is picked up by dust devils as they travel across contaminated
terrain. Valley fever causes pneumonia when it is inhaled by inconspicuous
victims including animals that are in the way of a traveling dust devil.
According to the study from the University of Arizona Climatology, valley fever
can weaken immune systems with certain groups at greater risk of severe
infection. Among those groups are HIV, AIDS, diabetic or organ transplant
patients, African Americans, Filipinos, and pregnant women. The study also
showed that symptoms of valley fever include: fever, fatigue, cough, shortness
of breath, headaches, night sweats, muscle aches or joint pains, and rashes on
upper body or legs. Valley fever can be treated but if left untreated, it can
ultimately lead to death.
Dust devils are still a mystery to climatologist and their study
on this planet is being developed in order to help the exploration of astronauts
on Mars. A deeper understanding here on Earth will help with the understanding
of their effect on pollution and electric capabilities. One day they could help
with more than the erosion to the paint on cars! With additional knowledge, we
could possibly learn to harness them as additional sources of renewal energy.
Huang JY, et al. (2012). Coccidioidomycosis-associated deaths, United States, 1990-2008.
Emerging Infectious Diseases, 18(11): 1723-1728.
Raasch, S., and T. Franke. "Structure and Formation of Dust Devillike Vortices in the Atmospheric
Boundary Layer: A High-Resolution Numerical Study." Journal of Geophysical Research.Atmospheres, vol. 116, no. 16, 2011, Research Library: Science & Technology, https://cochise.idm.oclc.org/login?url=http://search.proquest.com.cochise.idm.oclc.org/docview/993140906?accountid=7278, doi:http://dx.doi.org.cochise.idm.oclc.org/10.1029/2011JD016010.
University of Alabama Huntsville. "Dust devil and the details: Spinning up a storm on Mars."
ScienceDaily. ScienceDaily, 18 December 2014. <www.sciencedaily.com/releases/2014/12/141218154454.htm>.
Westrup, Hugh. "Red Menace?" Current science, vol. 88, no. 12, Feb 07, 2003, pp. 4-5, Research
Library: Science & Technology, https://cochise.idm.oclc.org/login?url=http://search.proquest.com.cochise.idm.oclc.org/docview/195952826?accountid=7278.
Writers, Staff. "Chasing Dust Devils." UPI Space Daily, Aug 11, 2008, Research Library: Science &