Aeolian Transport of Sand and Dust
Importance of Dust Transport by Storms
Dust storms have occurred throughout history. But as regions
of land degradation expand, storms become worse with many important influences.
Dust Storms Influence Weather And Climate By Changing
Albedo Of Earth
This image of a major dust storm blowing dust from the Sahara out over
the Atlantic. The dust increases the amount of solar radiation reflected
to space (it is bright). Thus, dust reduces the amount of solar
radiation reaching the surface, reducing surface temperature. Dust
can remain in the atmosphere for weeks and influence regional weather
From NASA Earth
They Disrupt Day-to-Day Living
The high levels of dust in storms keep people indoors and damages
internal combustion engines. Storms shut down airports, down
power lines, and curtail other activity.
A haboob (dust storm) rolls in from the Gila River Indian reservation
northward towards the south side of South Mountain in Phoenix, Arizona. The
Erosion Multimedia Archive has many photos of dust storms.
From Clayton Esterson.
They [dust storms] disrupt traffic,
coat our cars with a veil of desert dirt, deposit leaves and branches
into our swimming pools and blow empty trash barrels around in the
streets ... If we’re at home,
we usually have to run out to the back yard and rescue the patio chairs
from certain doom: Blowing into our swimming pool. Blinking back the
dirt from our eyes and attempting to ignore the grittiness in our teeth,
we rush to make sure anything that isn’t nailed down is either
brought inside or tethered. Potted plants overturn, hummingbird feeders
whip in the wind and I’ve even seen the patio umbrellas take
off from neighbors’ back yards like kites.
From Desert Drivel Blog
Four people died and 42 others were injured in a series
of chain-reaction interstate highway accidents during a blinding dust
storm, authorities said.
From Public Health
Applications in Remote Sensing.
Dust Storms Erode Soil
Wind erosion physically removes the lighter, less dense
soil constituents such as organic matter, clays, and silts. Thus it
removes the most fertile part of the soil and lowers soil productivity.
Lyles estimated that top soil loss from wind erosion causes
annual yield reductions of 339,000 bushels of wheat and 543,000 bushels
of grain sorghum on 0.5 million hectares (1.2 million acres) of sandy
soils in southwestern Kansas.
From Wind Erosion Research Unit
Wind erosion at Grand Forks County, North Dakota; 1 mile east, 1 1/2
mile north Larimore, North Dakota photographed on 11 may 1955. The
condition of these fields was brought about by high velocity winds
up to 89 mph as recorded at Grand Forks, North Dakota (5 miles away).
The fence corner picture showing one woven wire fence on top of another
indicates that wind erosion has occurred on these fields before. Excessive
grazing by sheep has exposed some areas of the pasture to wind erosion.
From Natural Resources Conservation Service, Wind & Water
Dust Storms Carry Bacteria To Distant Places
Sahara dust storms carry heavy metals, bacteria, and other pathogens to
the Caribbean, causing coral death.
These two images from NASA’s Total Ozone
Mapping Spectrometer (TOMS) instrument show dust blowing off the Sahara
in Africa and crossing the tropical Atlantic.
The Total Ozone Mapping Spectrometer instrument aboard the Earthprobe
TOMS satellite, captured these images of the dust event on June 17, 1999
as dust leaves Africa, and on July 2, 1999 (Right)
as the dust approaches North America.
From Laboratory for Atmospheres TOMS Project, NASA Goddard Space Flight
Center, as reported in Dust
Deals Droughts, Deluges by Forum für
Wissenschaft, Industrie und Wirtschaft, reporting on work by Natalie
Dust concentrations in the air at Barbados in the Caribbean and onset
of clral disease in Barbados waters. The increasing drought in the
Sahel, beginning in the early 1970 and continuing to 2000 or later,
has led to an increase in Saharan dust in the air over the Caribbean,
an increase in pathogens in the air, at the same time coral diseases
started to increase.
Dr. Joe Prospero, University of Miami as discussed on US Geological Survey
web site on Coral
Mortality and African Dust.
This half of an air filter represents 40 liters of air (roughly the amount
it would take to fill a 10 gallon aquarium) sampled during a dust storm
in Mali, Africa. The filter is placed on nutrient media for 48 hours
so the viable microbes can grow. The shiny, colorful circles (indicated
by the black arrows) are bacterial colonies.
The fuzzy patches (indicated by the red arrows) are fungi.
From U.S. Geological Survey Open-File Report 03–028, January 2003 African
Dust Carries Microbes Across the Ocean:
Are They Affecting Human and Ecosystem Health? a 396-KB pdf file.
Work by the U.S Geological Survey finds that:
- As of August 2005, preliminary identification
has been made of >300
kinds of microorganisms cultured from air samples collected on St.
Croix, St. John and Trinidad during dust and non-dust conditions. Air
samples collected during dust events in the USVI and Trinidad contain
approximately 2-3 times as many culturable microorganisms per volume
as do air samples collected during non-dust conditions. Of those microorganisms
identified to date, 25% are known plant pathogens and 10% are known
opportunistic pathogens of humans. (Griffin, Ramsubhag, Smith, Gray
in preparation; publication - Griffin et al. 2001, 2003)
- Air in Mali contains orders of magnitude
more microorganisms per volume than air sampled in the downwind areas
(USVI and Trinidad) and more species. Of the hundreds of microorganisms
cultured and isolated from Sahara and Sahel (Mali, West Africa) air
samples, DNA sequencing has been used to identify 50 types of bacteria
(and 3 genera of fungi) and preliminary identifications have been
made on >100 additional
kinds of bacteria. Of the cultureable bacteria identified thus far,
10% are known animal pathogens, 5% are plant pathogens, and 27% are
opportunistic human pathogens. (Kellogg, Smith, Coulibaly, Gray in
preparation; publication - Kellogg et al., 2004)
- A pilot study found that dust collected in the USVI during African
dust conditions was highly toxic to gametes and embryos of some marine
organisms (Nipper, Carr, Garrison in preparation).
Mortality and African Dust: Summary of Findings.
Dust Fertilizes The Oceans
In the chapter on Carbon Cycle, The Ocean, and
the Iron Hypothesis we
learned that many large oceanic areas do not have enough dissolved iron
necessary to support large populations of phytoplankton. Dust carries iron
from the land to the open ocean, supplying needed iron, leading to phytoplankton
blooms covering large areas of the North Atlantic and Caribbean.
Causes of Blowing Dust
As wind speed increases over soil with no vegetation, small
sand particles (0.1–0.5 mm in diameter) begin to move.
At first, particles on the surface creep foreward. As wind speed continues
to increase, small particles fly through the air for a few centimeters
before falling back to the surface. This is called saltation. When the
saltating particles hit the ground, they may dislodge other particles,
especially smaller particles. Finally, if the wind speed is high enough,
small particles become suspended in the air. Suspended particles are
carried high into the air where they become the dust of a dust storm.
Creep, saltation, and suspension of particles by wind.
From Wind Erosion Research Unit, Wind
Erosion Simulation Models.
The size of particle moved by the wind depends on the size
of the particle and the speed of the wind. The relation between the size
of the particle moved by a moving fluid and the speed of the fluid above
the surface was determined by Filip Hjulstrom in his thesis at Uppsala
University in 1935.
The the Hjulstrom Diagram of sediment transport by water. Similar results
apply to transport by wind, but wind speed must be faster than water
speed to move particles of the same size. Very small and very large
particles are the hardest to entrain. Click on the image for a zoom.
Transport by Jack Morelock and Wilson Ramirez, Geological
Oceanography Program, Department of Geology at the University of Puerto
Rico at Mayagüez.
The Hjulstrom diagram is the result of several processes:
- Wind speed goes to zero very close to the ground. Sand particles
extend into the wind and are pushed by the wind. Very small particles
do not extend high enough to be pushed.
small particles tend to stick together.
- As a result, small particles are eroded less easily than sand particles.
- Larger particles and pebbles are too heavy to be moved by the wind.
- As a result, sand particles are moved by wind more easily than clay
particles of pebbles.
Wind erosion is influenced by many factors:
- Vegetation. Vegetation slows
the wind at the soil level, retarding erosion. Roots bind the soil,
further retarding erosion. Lack of vegetation (ground cover) enhances
wind erosion. Windbreaks of trees and shrubs reduce wind speed near
- Soil moisture. Surface tension by water in moist soil exceeds the
wind force on surface particle. Wind cannot erode moist soil.
- Structure of the surface. Organic material, iron, and free aluminum
at the surface reduces erosion. Sodium or salt at the surface leads
to dust at the surface, enhancing erosion.
- State of the soil surface. Desert pavement, a layer of pebbles covering
the surface, plus desert varnish on the pebbles, a thin layer of
clay with iron and manganese oxides produced by bacteria at the surface,
strongly inhibits erosion. Driving on the surfce destroys the protection
leading to rapid erosion.
Desert pavement in Egypt's Western Desert. The pavement is a gravel layer
that protects the fine-grained soil beneath from wind erosion. Photo
courtesy of Boston University Center for Remote Sensing.
From Iraq Desert and Dust.
- Climate. Erosion requires arid climate in regions with strong winds.
Hjulström, F. 1935: Studies of the morphological activity
of rivers as illustrated by the river Fyris, University of Uppsala Geological
Institute Bulletin, 25, 221–557.
5 January, 2009