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 land degradation.

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 large regions.
From NASA Earth Observatory.

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.
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 Erosion Pictures.

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) 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:

1. 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)
2. 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)
3. 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).
   
   From Coral 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.
From Sediment Transport by Jack Morelock and Wilson Ramirez, Geological Oceanography Program, Department of Geology at the University of Puerto Rico at Mayagüez.

The transport of grains summarized in the Hjulstrom diagram results of several processes:

1. 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.
2. Very small particles tend to stick together.
3. As a result, small particles are eroded less easily than sand particles.
4. Larger particles and pebbles are too heavy to be moved by the wind.
5. 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:

1. 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 wind. If the vegetation covers more than 20% of
   
   
2. 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 to erode.
   
   
3. 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.
   
   
4. 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 and Dust.
   
   
5. Climate. Erosion requires arid climate in regions with strong winds.

References

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.

Revised on: 27 July, 2009