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.
And the dust carries many pollutants introduced into the soil by the
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 weather and climate over
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 Wind
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.
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
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
(Left) 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
Mahowald and Lisa Kiehl.
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)
From U.S. Geological Survey Open-File Report 03–028, January
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 culturable 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 forward. 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
The transport of grains summarized in the Hjulstrom diagram
results 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.
- Very 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.
Factors Influencing Wind Erosion
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 the
ground. Small changes in vegetation lead to large changes in erosion by the
the vegetation covers more than 20% of
- Soil moisture. Surface tension by water
in moist soil exceeds the wind force on surface particle. Wind cannot
erode moist soil. Soil with a moisture content of only 1% is very difficult
- 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
surface 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
- 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.
27 July, 2009