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I. Overview of Dust Storms

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Impacts of Dust Storms

Impacts of Dust Storms »2009 Dust Storm in Sydney, Australia

If you’ve been in Sydney, Australia in September, you know that the weather is usually clear and pleasant, with average high/low temperatures of 22° C/11°C. It rains about 11 days, making September one of the driest months of the year.

A Typical Image From the Collaroy-Narrabeen Coastal Monitoring Cameras, Sydney, Australia

However, if you were there on 23 September 2009, the city would have looked vastly different. For it was engulfed in millions of tons of fine red dust from the most massive dust storm in nearly a century.

An image from the Collaroy-Narrabeen Coastal Monitoring Cameras showing Sydney, Australia encased in dust

The dust cloud traveled approximately 1500 km from the drought-ravaged interior and was nearly 160 km long and 400 km wide. The mean wind was 65 km/h, with gusts up to 100 km/h.

This ECMWF model analysis shows 10-m winds (in knots) at 0400 local time on 22 September 2009. Notice how the dust was blown from the interior desert areas down to the coast of New South Wales.

ECMWF 10-m Wind Field (kt) for 18 UTC 22 Sep 2009

This plot from NOAA’s HYSPLIT model makes the dust transport even clearer.

NOAA HYSPLIT model backward trajectories ending in Sydney, 250 m above ground, 10 local time 23 Setp 2009

If you’ve never experienced an intense dust storm like this, you might want to watch some of the videos listed below.

Impacts of Dust Storms »Dust Storms and Health

The dust storm put Sydney’s 4.5 million residents at risk for cardiovascular, respiratory, and other health problems.

Dust particles pose health danger due to:

  • Their size; soil particles within a dust cloud get smaller as the cloud travels and the larger particles settle; eventually, the remaining particles become so small that our lungs cannot readily expel them
  • Their ability to carry organisms, such as spores, fungus, bacteria, and viruses; these can lead to diseases such as eye infections, meningitis, and valley fever

Not only does dust directly affect people in the source regions, but it can impact those far downstream.  

Impacts of Dust Storms »Impact on Air Traffic

Sydney’s dust storm had a major impact on its air traffic. On 23 September, visibility at the airport was reduced to 400 meters and flights were delayed for hours, with some diverted to cities hundreds of kilometers away.

Dust can also impact global air traffic management. This MSG visible image shows the reach that a dust storm can have. This dust cloud extended from the coast of Guinea and Guinea-Bissau over the Cape Verde Islands and to the west coast of the Iberia Peninsula.

Artificially colored Meteosat-8 visible 0.6 band image showing the extension of a dust cloud from the coast of Guinea and Guinea-Bissau over the Cape Verde islands and Madeira to the west coast of the Iberia Peninsula.

Dust can severely erode turbine engines that operate in desert areas. For example, helicopters can sustain severe damage after operating from 20 to 100 hours in dusty environments. Note that volcanic ash impacts engines more than dust does.

Photograph of a Helicopter Maintenance Unit crew strapping a tarp to a helicopter to prevent possible damage during a major dust storm in Balad Air Base, Iraq

Impacts of Dust Storms »Visibility

Intense dust storms reduce visibility to near zero in and near source regions, with visibility improving away from the source. From the edge of blowing dust to within 240 km downstream, visibility can range from 800 to 4800 meters.

Typical distribution of visibility within a dust storm

Dust settles when winds drop below the speed necessary to carry the particles, but some level of dust haze will persist for longer periods of time. For example, dust haze may remain at 5000 to 9000 meters downstream for days after a dust storm.

Note that air-to-ground or slant-range visibility is more reduced than surface visibility. This may make it impossible to, for example, pick out an airfield from above, even when the reported horizontal surface visibility is three miles (or about 5 km) or more.

Impacts of Dust Storms » Agriculture and Fertilization

Even when dust is only visible at its source region, it can have environmental impacts in other areas. It’s been estimated that 200 million metric tons of dust are transported from Africa over the Atlantic Ocean every year. Around one fifth of it reaches and fertilizes the Amazon basin, which is more than 7000 kilometers away from its origin, the Bodélé depression in Chad. In fact, the minerals and nutrients transported from Africa help achieve a nutrient balance in various parts of Amazonia.

3 panels of TOMS aerosol data showing a dust plume over the Sahara Desert extending from the Atlantic Ocean and Canary Islands on 26 Feb, 28 Feb, and 1 March 2000

Impacts of Dust Storms »Oceans and Coral Reefs

As dust travels over the oceans, some of it is deposited over the water. This increases the mineralization of the oceans, stimulating phytoplankton growth and changing the food chain. 

Fungal spores carried by dust particles may be killing coral reefs. For example, a soil fungus found in dust air samples caused the Caribbean coral reef death events of 1983 and 1987. These events coincided with prolonged drought in the Sahel region of North Africa.

Graphs showing Barbados mineral dust annual averages and benchmark Caribbean events

For more information about the relationship between African dust, coral reefs, and human health, see the USGS documentary video at http://gallery.usgs.gov/videos/223. It discusses how recent changes in the composition and quantities of African dust transported to the Caribbean and the Americas might provide clues as to why Caribbean coral reef ecosystems are deteriorating and human health is being impacted.

These photographs show how healthy brain coral in Carysfort Reef (Florida, USA) largely died off in a 50-year period. The arrows in the top photograph point to tags used to monitor the coral’s health.

Photographs showing changes to Carysfort Reef brain coral from 1960 to  2009

Impacts of Dust Storms »Dust and Precipitation

If it rained one night and you came out the next morning to find your car looking like this, what would you think had happened? Clearly it wasn’t a normal rainstorm. In fact, it was dust precipitation or mud rain—that is, rain that contains a noticeable concentration of sand or dust particles that originated far away.

Photograph of car with mud rain on it

Mud rain occurs when rain removes dust from the air, a process known as scavenging. Dust may act as cloud condensation nuclei and/or exist below the base of an existing cloud and simply be washed out by falling raindrops.

Schematic of a dust plume settling due to precipitation

Before dust settles with rain, a complex interaction occurs between the dust, clouds, and precipitation.  There’s no clear consensus as to what that interaction is—if the dust increases or decreases precipitation.

Observational studies suggest that dust inhibits or reduces precipitation. Under this scenario, dust nuclei add to the overall number of nuclei, creating many smaller droplets that are too small to collide and coalesce efficiently, thereby preventing or lessening the amount of precipitation.

But studies using cloud models suggest that dust behaves like giant cloud condensation nuclei, increasing precipitation by enhancing the processes of collision and coalescence that occur as droplets grow.

Regardless of which is correct, there’s no doubt that dust has an impact on precipitation. If dust actually decreases it, a positive feedback mechanism is at work in which arid regions contribute to less precipitation, which then increases the extent of those areas.

Scientists are investigating many other dust-related topics, such as the impact of African dust storms on hurricane activity in the north Atlantic Ocean and on the Asian summer monsoon regime.

Physical Processes

Physical Processes »Dust

Physical Processes » Dust » Moving Sediment

Dust moves through several processes:

  • By saltation, where small particles move forward through a series of jumps or skips, like a game of leap-frog. The particles are lifted into the air, drifting approximately four times farther downwind than the height that they attain above ground. If saltating particles return to the ground and hit other particles, they jump up and forward, continuing the process.
  • By creep, where sediment moves along the ground by rolling and sliding. Large particles and/or light winds favor creep.
  • By suspension, where sediment materials are lifted into the air and held aloft by winds. If the particles are sufficiently small and the upward air currents are strong enough to support the weight of the individual grains, they will remain aloft. The larger particles settle more quickly, although increases in wind speed keep progressively larger particles aloft. Note that strong winds can lift suspended dust particles thousands of meters upward and thousands of kilometers downwind, with turbulent eddies and updrafts holding them in suspension.

Note that the latter two processes are integral to the formation of dust storms since they loft dust into the air. Click the Play Button to see the processes in action.

Animation of aeolian sediment transportation by creep, saltation, and suspension

Physical Processes » Dust » Particle Size and Settling Velocity

Dust particles remain suspended in the air when upward currents are greater than the speed at which the particles fall through air. This graphic shows the fall speed, or settling velocity, as a function of particle size.

Settling velocity versus particle size for mineral grains in air

Dust particle size is usually measured in micrometers, which are 1/1000 of a millimeter or 1/1,000,000 of a meter. Particles capable of traveling great distances usually have diameters less than 20 micrometers. (That's much smaller than the width of a human hair.)

Of the following types of particles, which fit this description? (Choose all that apply.)

The correct answers are A and B.

Appropriate source regions for dust storms have fine-grained soils rich in clay and silt.

Returning to the graph, dust particles fall at a speed of about 100 millimeters/second or roughly four inches per second. Particles larger than 20 micrometers in diameter fall disproportionately faster: 50-micrometer particles fall at about 500 mm/s or half a meter per second. Particles smaller than 20 micrometers settle very slowly. Ten-micrometer particles fall at only 30 millimeters/second while 2-micrometer particles fall at only 1 millimeter per second. The finest clay particles settle so slowly that they can be transported across oceans without settling.

Physical Processes » Dust » Source of Dust: Desert Sources

Precipitation binds soil particles together and promotes plant growth. Plant growth, in turn, binds the soil even more and shields the surface from wind. Consequently, dust storms occur in regions with little vegetation and precipitation. These conditions most often occur in deserts—when it hasn't rained recently. The rule of thumb is that dust is unlikely within 24 to 36 hours of a rainstorm.

A thin veneer of stones called desert pavement covers many desert regions. This veneer results from the process of deflation where wind removes the finer-grained material, leaving only stones on the surface, which suppress blowing dust. If the pavement is disrupted by human activities, such as farming or off-road driving, the fine-grained material will be exposed to the wind again, raising the likelihood of dust storms. Studies show that large-scale military operations in the desert increase the likelihood of dust storms at least five-fold. Click Play to view the animation.

Photo of desert pavement and animation of its formation

When seasonal rains occur over desert and near-desert environments, runoff water can create flash floods. The resulting erosion washes soil particles downstream. This continues until the velocity of the water slows to a point where it can no longer carry the load of sediment. The heaviest particles are deposited first, the lightest particles last. Once the water evaporates, the stream bed becomes a prime source for blowing dust.

Photograph of flooding in Ouagadougou, the capital of Burkina-Faso (West Africa)

Physical Processes » Dust » Other Dust Sources

This montage shows other sources of dust. Are you surprised to see ocean sediments and glacial deposits among them? Click each land feature for more information. (The information will appear below the graphic.)

Montage of photographs showing sources of dust storms other than deserts (agricultural areas, coastal areas, dry lake beds, glacial deposits, ocean sediments, and river flood plains)
agriculture coastal flood_plain oceans glacial lake_beds

Agricultural areas: Agricultural land that's fallow, recently tilled, or has a marginal growing climate is a potential source area for dust. The mechanical breaking of soil creates an environment rich in fine-grained soil that is picked up and moved by seasonal winds.

We see this in the grain belts of northern Syria and Iraq, where seasonal rain is relied upon to water the crops. When drought occurs, the area becomes an active dust source region.

The same occurs in Colorado, New Mexico, and western Texas. An extreme example occurred in the American mid-west during the 1930s, known as the Dust Bowl.

Coastal areas: These MODIS images show well-defined dust plumes extending from the coastal area of the United Arab Emirates near Abu Dhabi. The dust plumes were generated by prefrontal southerly winds in advance of a cold front to the north.

River flood plains (alluvial plains): The flood plain of the Tigris and Euphrates Rivers in southern Iraq serves as the source region for many dust storms, particularly during shamal events. (A shamal is a northwesterly wind that blows over Iraq and the Persian Gulf states. It is often strong during the day and decreases at night.)

While river channels carry fairly sandy sediment, they deposit mud in the flood plain when they rise and flood. When the area dries out and desertifies, the rapid evaporation results in the formation of a salty white crust.

Ocean sediments: Ancient ocean sediments in the Baja California peninsula are the source region for the prominent dust plumes in this SeaWiFS image. The desiccated sediments were once a muddy sea floor that was lifted up above sea level.

Glacial deposits: Dust storms occur outside of the world's deserts. This satellite image shows a dust storm blowing out from Iceland's southern coast. The bright white areas are glaciers. The melt water that emerges from beneath them carries a tremendous load of pulverized rock, or glacial flour. This material gets deposited on large mud flats referred to as outwash plains. The harsh climate and constantly shifting channels prevent vegetation from becoming established. During dry periods, the dust is picked up and carried offshore by high winds associated with storms in the North Atlantic. Similar glacial deposits can be found at high latitudes or high elevations around the world. For example, ancient deposits of wind-blown glacial flour, referred to as loess, fuel the prodigious dust storms of the Gobi desert in northwest China.

Dry lake beds: Dry lake beds are called playas in the U.S. and sabkhas in the Middle East. They arise as water erodes rocks and forms fine-grained soils. The erosion can occur over long periods of time or can happen quickly as a result of recent precipitation events.

When lakes dry up, the fine-grained deposits inhibit plant growth, which further contributes to dust availability. The salty deposits tend to be much lighter in color than the surrounding ground on satellite imagery, making them easy to detect.

The MODIS true color image above shows long plumes of dust coming off of dry lake beds and dry wetlands in the Sistan Basin. Straddling southern Afghanistan and eastern Iran, it's one of the world's driest basins. High-resolution (250-m) images like this show that entire flood plains, dried lakes, and agricultural areas do not erode. Rather, numerous small point sources with diameters of one to tens of kilometers erode to produce numerous individual dust plumes. It is these individual plumes that merge downstream to form mesoscale dust clouds and dust fronts.

Physical Processes » Dust » Point Sources of Dust

As we've seen, most dust comes from a number of discrete areas that are small enough to be regarded as point sources, much like smokestacks. Many of the point areas are much lighter than the surrounding ground on satellite imagery, indicative of salt or gypsum-type compounds vs. the reddish-brown coloration of desiccated river flood plains (alluvial dust).

The black plus marks on this map are dust source areas in Iraq. Many of the red pluses are areas that were active before 2005 but are no longer so. The larger black pluses are additional dust source areas that were located by the Naval Research Laboratory (NRL).

MODIS true color image of SW Asia overlaid with dust point sources from COAMP and other sources

These photographs show how the wetlands of southern Iraq have been restored, eliminating some source areas for blowing dust. However, the most prevalent ones between the Tigris and Euphrates rivers remain.

Photograph showing how the wetlands of southern Iraq changed from before 1991 to after 2006

Physical Processes » Wind

Physical Processes » Wind » Minimum Wind

After an appropriate source, the next key ingredient for dust storm generation is wind from the surface through the depth of the boundary layer that's strong enough to move and loft dust particles.

Table of threshold dust-lofting wind speeds for different desert environments

The first sand and dust particles to move are those from 0.08 to 1 mm in diameter. This occurs with wind speeds of 5 to 13 m/s (12 to 29 mph).

As a rule of thumb, winds at the surface need to be 8 m/s (17 mph) or greater to mobilize dust. The table shows the wind speeds required to lift particles in different source environments.

Once a dust storm starts, it can maintain the same intensity even when wind speeds slow to below initiation levels. That's because the bond between the dust particles and the surface is broken and saltation allows dust to lift.

Physical Processes » Wind » Turbulence

Lofting of dust typically requires substantial turbulence in the boundary layer. This image shows dust being mobilized during a downslope windstorm on the lee slope of the Sierra Nevada mountains in California.

Photo of eastern Sierra Nevada and Owens Valley, CA showing laminar flow across part of the valley, entraining dust, then lofting dust 3km AGL when the flow becomes turbulent

Laminar flow in the right half of the photograph carries the dust close to the valley floor. Further left, the flow slows down and quickly becomes extremely turbulent. During the transition, the dust is lofted approximately 3000 m (10,000 ft). Click Play to view the animation.

Animated schematic of turbulence due to shear

Typically, wind shear creates the turbulence and horizontal roll vortices that loft dust up and away from the surface. As a rule of thumb, if the wind at the surface is blowing 8 m/s (18 mph), the wind at 305 meters (1,000 feet) must be about 15 m/s (34 mph) to keep the dust particles aloft.

Physical Processes » Wind » Stability

Because vertical motions are required to loft dust particles, it stands to reason that dust storms are favored by an unstable boundary layer. In contrast, stable boundary layers suppress vertical motion and inhibit dust lofting. Click Play to view the animation.

Schematic showing the impact of surface heating on low-level atmospheric stability

With the lack of vegetation in dust-prone regions, the ground can experience extreme daytime heating, which creates an unstable boundary layer. As the amount of heating increases, the unstable layer deepens.

Physical Processes » Wind » Friction Velocity

NOGAPS Analysis of friction velocity, wind, and ground wetness for North Africa, 12Z 7 Jan 2003As we've seen, it's not enough to have strong wind; the wind must be sufficiently turbulent to loft dust and must occur in a reasonably unstable environment. Wouldn't it be nice to have a single parameter that expresses wind speed, turbulence, and stability? We do. It's called the friction velocity.

In more technical terms, dust mobilization is proportional to the flux of momentum, or stress, into the ground. A friction velocity of 60 centimeters per second is typically required to raise dust.

Friction velocity is computed by many NWP models. This NOGAPS analysis for northwest Africa on 7 January 2003 at 12Z shows surface winds, ground wetness, topography, and friction velocity values greater than 60 cm/s.

Note the high friction velocities plotted in red and magenta across the Sahara, particularly near the west coast.

SeaWiFS true-color image of northern Africa 7 Jan 2002 showing a dust plume blowing westward out to sea off the west coast These parallel the area of blowing dust in this SeaWiFS true color image. Since both are from January, the dust in both cases is probably being lifted by the remnants of frontal boundaries manifested as shear lines across equatorial Africa. (Note: The image is from a year prior to the friction velocity chart but is still relevant.)

The whiter plumes are clearly visible in the center of the satellite image, as is an area of higher friction velocities to their north. The plumes are oriented northeast-southwest and are enhanced by the funneling of winds between two areas of higher terrain to the north and the south of the area.

The remnants of the cold front appear as cloud cover over the Red Sea, with cold air cumulus over the northern part.

Notice how the plume of dust blowing out to sea lines up nicely with the region of high friction velocity on land.

Physical Processes » Wind » Diurnal Effects

Dry desert air has a wide diurnal temperature difference. Strong radiative cooling leads to rapid heat loss after sunset. This quickly cools the lowest atmosphere, resulting in a surface-based inversion that can have a strong impact on blowing dust. Click Play to view the animation.

Animation showing how radiative cooling leads to a surface-based inversion

While a 10-knot wind can raise dust during the day, it may have little impact at night. This effect accounts for much of the diurnal variation in summer shamal dust storms, which we will discuss later.

The formation of a surface-based inversion has little effect on dust that's already suspended higher in the atmosphere. Furthermore, if winds are sufficiently strong, they will inhibit the formation of an inversion or even remove one that has already formed.

If you've heard that dust storms always go away at night, that's not necessarily true; occasionally they persevere. Dust RGB products enable us to detect dust storms at night, something that was not possible with earlier surface and satellite observations.

If you're not familiar with RGBs, the acronym stands for Red, Green, Blue processing. The products are made from several spectral channels or channel differences and highlight specific features, such as dust. For more information, see the COMET module Multispectral Satellite Applications: RGB Products Explained at http://www.meted.ucar.edu/npoess/multispectral_topics/rgb/index.htm.

Physical Processes » Wind » Forecasting Tips

Forecasting tips:

  • When you are evaluating the potential for dust lofting, be aware of when the boundary layer has a dry adiabatic lapse rate, for the strongest winds aloft can be brought down to the surface, creating gusty conditions.
  • Be sure to examine winds at 925 hPa (approximately 750 meters or 2,500 feet) above the surface when at sea level) where stronger winds allow more dust to be suspended aloft and persist for longer periods due to turbulent mixing.

Physical Processes » Dust Removal

Physical Processes » Dust Removal » Four Processes

This section addresses the fate of suspended dust once it's been lofted high into the atmosphere. Eventually that dust will settle, although it may travel half way around the globe before doing so. As a forecaster, you need to be concerned about the processes that lead to lower dust concentrations, improved visibility, and reduced hazards. (But you should continue to look for conditions that can lower visibility again.)

MSG Dust RGB showing dust-laded Saharan air over the Atlantic Ocean at 21Z 01 Aug 2009

On the following pages, we will discuss three processes that remove dust:

  • Dispersion
  • Advection
  • Entrainment in precipitation

Gravity also plays a role, although it will not be covered.

Physical Processes » Dust Removal » Life Cycle of a Dust Storm

This animation depicts the life cycle of a typical summer dust storm in Iraq, called a shamal.

Animation of the life cycle of a summer shamal dust storm

The initial dust plume extends in a narrow swath immediately downwind from a relatively small source region. As the wind continues to blow, the plume expands laterally and also continues to move downwind.

Sometime later, the wind starts to diminish, eventually falling below the threshold required to continue raising dust. Although no new dust is being raised, the existing dust remains in suspension. The plume detaches from the source region and continues to move downstream and spread. Eventually the dust concentration diminishes through lateral dispersion and settling.

Physical Processes » Dust Removal » Dispersion

In the shamal example, the dust dissipated through two processes: dispersion and advection. We'll start by looking at dispersion.

Schematic of of dust plume geometry

The fanning of a dust plume as it moves downstream from its source region is caused by dispersion, which is a diluting process. Basically, the more air you mix with a dust plume, the more it dilutes, spreads out, and disperses. This is similar to what you see if you pour dye into a river and watch how the color fades as the water moves downstream. Dispersion processes always act to dilute; plumes never re-concentrate.

This figure shows a highly idealized view of a plume dispersing as it moves downstream from a point source. Note that the concentration is not uniform throughout the plume. The highest concentration remains in the center and falls off away from it.

Physical Processes » Dust Removal » Dispersion and Turbulence

Dispersion is primarily governed by turbulence, which mixes ambient air with the plume. Any increase in turbulence increases the rate at which the plume disperses.

Three kinds of turbulence act to disperse a plume:

  1. Mechanical turbulence
  2. Turbulence caused by shear
  3. Turbulence caused by buoyancy

Mechanical turbulence is caused by air flowing over rough features, such as hills or buildings. Click Play to view the animation (and the others on this page).

Schematic animation of turbulence induced by terrain, buildings, and trees

Turbulence from shear can result from differences in wind speed and/or direction.

Animated schematic of turbulence due to shear

Buoyancy turbulence can be caused by something as dramatic as an explosion or as simple as parcels of air rising during the diurnal heating of the surface. Particularly in the latter case, buoyancy is governed by the stability of the atmosphere.

Schematic animation of turbulence induced by buoyancy

Turbulence acts to disperse dust plumes and keep the dust particles in suspension. Without turbulence, dust generally settles at a rate of 305 meters (1,000 feet) per hour. However, this is highly dependent on environmental conditions. Any turbulence will slow the settlement rate.

Physical Processes » Dust Removal » Dispersion and Stability

We've seen how unstable conditions favor the lofting of dust and formation of dust storms. Stability also has a strong influence on how dust disperses.

Schematic of 2 model plumes with different dispersions due to different stabilities

This graphic shows dust plumes dispersing under both stable and unstable conditions.

When the local environment is unstable, how do dust plumes disperse? (Choose the best answer.)

The correct answer is C.

Dust disperses in both directions although the effect is significantly more pronounced for the vertical component.

When the atmosphere is stable, dust disperses: (Choose the best answer.)

The correct answer is A.

A stable atmosphere tends to suppress the vertical dispersion of dust, but horizontal transport is still possible.

Under neutral stability conditions, dust plumes spread: (Choose the best answer.)

The correct answer is C.

When the atmosphere has neutral stability, dust plumes disperse roughly equally in both directions because neither one is favored.

Physical Processes » Dust Removal » Advection

Our initial shamal schematic showed the dust plume detaching from the source area when the winds dropped below the threshold to loft dust. Visibility would be expected to improve substantially in the source area soon after this happened. The dust that was lofted simply moved away from its source. Where does the dust go? Recall that dust storms are typically several thousand feet high and frequently extend up to 4600 meters or 15,000 feet, and that wind shear contributes to the turbulence needed to loft dust. Therefore, winds aloft may very well carry dust in a direction that's different from the wind direction on the ground. Click Play to view the animation.

Shematic of a dust plume rising into a zone of wind shear

When predicting where a dust plume will travel, you should check the vertical wind profile. As this animation shows, dust that leaves the ground going one direction can rise to a level where it travels in an entirely different direction. Fortunately, dust forecast models can do the hard work for you, accounting for the complex evolution of dust plumes in a three-dimensional framework.

Physical Processes » Dust Removal » Settling of Dust

Particle size plays an important role in both lifting and settling thresholds. Longer suspension times for smaller particles result in long periods of dust haze in arid areas. Click Play to view the animation.

Schematic of a dust plume settling due to gravity

Particles between 10 and 50 micrometers fall at about 305 meters (1,000 feet) per hour. Using that rate, if dust is lifted to 1500 meters (5,000 feet) and the wind ceases, the dust will settle in about 5 hours.

Over how large an area? If winds are 5 m/s (12 mph) and there's little to no vertical motion, the dust will typically settle up to 50 nautical miles downstream (26 m/s or 58 mph) from the source. Settling is by particle size, with the largest particles falling out first and the smallest ones falling out last. Therefore, the larger, heavier particles will settle near the source area, with the smaller ones settling farther away.

Most dust particles are hygroscopic, or water-attracting. In fact, they usually form the nucleus of precipitation. Because of this affinity to moisture, precipitation very effectively removes dust from the troposphere.

Schematic of a dust plume settling due to precipitation

Dust Source Regions

Dust Source Regions » Identifying Dust-Prone Regions

Dust storms can only form if there’s an appropriate source region. In this section, we’ll look at where these regions are located and the key factors that contribute to their formation.

Eight land covers are thought to produce dust:

  • Low sparse grassland in Mongolia
  • Bare desert equatorward of 60 degrees latitude
  • Sand desert
  • Semi-desert shrubs equatorward of 60 degrees latitude
  • Semi-desert sage
  • Polar and alpine desert
  • Salt playas/sabkhas
  • Sparse dunes and ridges

When these land cover types are combined with wetness values, we get a bulk measure of erodibility. The map shows how the world's deserts dominate the resulting pattern.

Global plot of dust-producing land cover

Identifying dust-prone regions based on land cover characteristics can be refined by incorporating satellite data. For example, the Total Ozone Mapping Spectrometer Aerosol Index (TOMS AI) provides a near-real-time measurement of absorbing aerosols in the atmosphere.

This plot bases the dust productivity of the earth surface on the observed frequency of high aerosol values and results in a much more refined view of global dust source regions than the land cover type database. Clearly, the majority of the world’s dust storms arise in relatively few areas: the Sahara, Middle East, Southwest Asia (notably Iran, Iraq, and Pakistan,) China, Mongolia, southwestern North America, west coast of South America, and Australian interior.

Global plot of dust-producing land cover determined using the TOMS Aerosol Index

Desertification is on the increase. For example, the United Nations Convection to Combat Desertification (UNCCD) has found that dust or sand encroachment in China has been expanding around 3500 square kilometers per year since the late 1990s.

Dust Source Regions » World's Major Dust Source Regions

Let’s take a closer look at the world’s major dust source regions. Click each label for a brief description. The description will display below the map.

Terra/MODIS True Color View of the Earth with the world's largest deserts labeled north america thar turkestan iranian australia atacama patagonia namib kalahari sahara arabian east_asia antarctica

NORTH AMERICAN DESERT: The North American Desert includes all of the deserts on the continent. It stretches down the western side of the North American continent from southern Oregon and Idaho to northern Mexico. The individual deserts vary based on differences in latitude, elevation, climate, topography, vegetation, and soil.  The four primary deserts are the cold Great Basin Desert and the warm Sonoran, Mojave, and Chihuahuan deserts.

THAR DESERT: This desert, also known as the Great Indian Desert, is a large, arid region that forms a natural boundary along the border of India and Pakistan.

TURKESTAN DESERT: This desert is in Turkmenistan, south of the Aral Sea.

IRANIAN DESERT: Iran’s arid interior plateau contains the Kavir and Lut deserts, which are some of the hottest and most arid areas in the world. The Kavir Desert is a great salt desert whose crust results from little to no precipitation and intense surface evaporation.

PATAGONIAN DESERT: This is the largest desert in South America and the seventh largest in the world. It covers 673,000 square kilometers and is located primarily in Argentina, with small portions in Chile.

AUSTRALIA DESERTS: Thirty eight percent of Australia is desert, with most of it in the central and northwestern parts. The largest deserts include the Great Victoria Desert, Great Sandy Desert, Tanami Desert, and Simpson Desert.

KALAHARI DESERT: The Kalahari is a large arid to semi-arid sandy area that covers most of Botswana and parts of Namibia, Angola, Zambia, and South Africa. August is the height of the dry season, when scorching sun and little to no precipitation parch the vegetation and winds excite dust storms.

NAMIB DESERT: This desert covers Namibia and southwest Angola. It is caused by the descent of dry air from the Hadley Cell, which is cooled by the cold Benguela current along the coast. The area gets less than 10 mm of rain a year and is almost completely barren.

ARABIAN DESERT: This huge desert occupies most of the Arabian Peninsula, stretching from Yemen to the Persian Gulf and Oman to Jordan and Iraq.

ANTARCTICA: Antarctica is the world’s largest desert due to very low precipitation rates (200 mm/yr along the coast, far less inland).

SAHARA: The Sahara is the world’s largest hot desert and covers most of Northern Africa. The Bodélé depression in northern Chad is actually the world's dustiest spot. Dust clouds from the Sahara can travel long distances, drifting to the Caribbean in summer, the Amazon in winter, and Europe during many months of the year.

EAST ASIA: Many great deserts, such as the Taklamakan and Gobi, and much semi-arid land lie in East Asia, where dust outbreaks are very common and especially severe in spring. Asian dust storms have been documented for thousands of years, helping us determine past and current climate change.

ATACAMA DESERT: The Atacama Desert lies along the Pacific Coast west of the Andes Mountains and covers 105,000 km of land in Northern Chile. The desert is the one of the driest in the world due to the rain shadow on the leeward side of the Chilean Coast Range and a coastal inversion layer created by the cold offshore Humboldt Current.

Dust Source Regions » Characteristics of Dust-Prone Areas

Several factors make an area prone to dust storms, including soil type, topography, and climate.

Soil type
Even in bare desert, sandy areas generally do not generate dust storms. Areas rich in silt and clay are responsible for the majority of dust storms. These fine-grained soils are found in areas with dry lake beds and river flood plain deposits, such as the Middle East.

monthly temperature and precipitation for Baghdad, Iraq

Topography
Low-lying regions are particularly prone to dust storm generation because prevailing winds are unimpeded by higher terrain. We see this in the low-lying regions of the eastern Arabian Peninsula, southern Syria, and western Iraq.

Climate
Finally, an area's potential for dust storm generation is related to its climate (its precipitation patterns, prevailing wind direction and speed, and normal location of low- and high-pressure centres). These graphics show how the world’s arid desert and semi-arid climate zones correlate with the major deserts.

Graphic of the earth with the dry climate zones marked
Terra/MODIS True Color View of the Earth with the world's largest deserts labeled

Dust Source Regions » Inter-Annual Variations in Dust Source Regions

Periods of extended drought dry out lakes, wetlands, and otherwise productive agricultural land, often resulting in new and expanded dust sources. The opposite occurs with wet winters, when numerous storms, heavy rains, and/or above-average snowfall can flood lakes, rivers, and streams and shut off active dust sources.

For example, Southwest Asia experienced an extended drought from 1998 to 2005. Then in 2005, heavy rain and melting snow led to numerous floods in southern Afghanistan. This MODIS true colour image shows the Sistan Basin (one of the world’s driest) on 21 February 2005 before the heavy rains and snow melt.

MODIS true color image of the Sistan Basin, 21 Feb 2005

This false colour image from 7 March 2005 shows how much the basin changed in just a few weeks. The dark blue indicates clear, deep water, the light blue mud-laden water.

MODIS false color image of the Sistan Basin, 7 March 2005

After a long, hot summer, Lake Saberi was still filled with muddy, brown water in October. This decreased the production of dust plumes and dust storms.

MODIS true color image from 12 Oct 2005 showing Lake Saberi filled with muddy water after a long, hot summer

These NRL dust enhancement product (DEP) images of Pakistan and Afghanistan on 2 May 2003 and 12 October 2005 also demonstrate the difference between a drought-ravaged basin and one that has experienced a wet period.

NRL DEP images of Pakistan and Afghanistan showing the difference between a drought-ravaged basin (2 May 2003) and one that's experienced a wet winter or spring (12 October 2005)

Dust Source Regions » Satellite Products Used to Identify Dust Source Regions

Various technologies are used to identify and classify dust source regions, from ground-based observations to satellites and numerical models. Satellites alone can provide frequent and reliable local, regional, and global observations, making them essential for detecting and monitoring dust sources areas as well as the life cycle of dust storms. Satellite-derived products also play an important role in initializing and validating regional models, global transport models, and general circulation models that include dust along with other major aerosol types. For example, they help validate and constrain model simulations of the spatial and temporal distribution of dust. They also help validate the seasonal cycle and inter-annual variability of dust, especially in areas where ground-based observations have poor spatial and temporal sampling.

Now we'll explore several types of satellite products used to identify dust source regions. Click each tab. When you are finished, proceed to the next page.

Aerosol Index

The Aerosol Index (AI) indicates the presence of elevated absorbing aerosols in the Earth's atmosphere, such as desert dust and biomass burning aerosols. In addition to identifying dust-prone areas, it is also used to monitor the spatial extent and inter-annual variability of dust. This image shows the Aerosol Index for Chile's Puyehue-Cordón Caulle Volcanic Complex after it erupted on 5 Jun 2011.

SSSSS

Aerosol Optical Depth

Aerosol optical depth or AOD indicates the degree to which aerosols prevent the transmission of light. The AOD product measures the integrated columnar aerosol load. AOD products are better at detecting dust over dark ocean surfaces than bright desert surfaces. Therefore, they are typically used to track dust over water. AOD products are available from several NASA instruments, including MISR on Terra, MODIS on Terra and Aqua, and OMI on Aura.

Modis Terra Aerosol Optical Depth at 550nm Averaged From Jan 2005 to Jan 2010

Dust Enhancement

The U.S. Naval Research Lab Dust Enhancement Product (DEP) identifies dust emission areas in Southwest Asia. Its 1-km resolution allows for the identification of individual plume heads that often measure 10 km or less across.

Compare the MODIS image with the NRL DEP image over southern Afghanistan, north-western Pakistan, and eastern Iran on 20 August 2003. Small dust plumes are readily apparent in the box in the dust enhancement product while those in the MODIS image are barely visible.

Comparison between an area of dust on a MODIS true color image vs. an NRL dust enhanced product from 20 Aug 2003

The close-up view of this localized dust plume indicates that it originated from many point sources to the north and merged into a single small plume as it dispersed to the south.

Close up of a NRL dust enhancement product showing dust plumes in Iran on 20 Aug 2003

Infrared Diff. Dust Index

The Infrared Difference Dust Index (IDDI) is a satellite dust product that detects dust in arid regions such as the Sahel and Sahara. The product is used in various ways:

  • For climatologic applications
  • For operational use by, for example, China's National Satellite Meteorological Center
  • In dust forecast models, such as the Chinese Unified Atmospheric Chemistry Environment for Dust (CUACE)

The IDDI is based on the different absorption and emission properties of dust aerosols and the surface and is made from midday Meteosat infrared imagery.

Climatology

Climatology » Dust Storm Seasonality and Frequency

Climatologies tell us what happened in the past, which helps forecasters anticipate future events and improve their forecasting. Climatology provides several types of data that help with forecasting the location, seasonality, frequency, and severity of dust storms.

We've seen how data from the TOMS Aerosol Index helps us map dust source regions. That same data can help us determine seasonal variations in dust storms.

This animation shows the seasonal variability of dust storms in the dust belt that stretches from western Africa up through the Taklamakan Desert in central Asia. Note the strong seasonal dependence of dust storm frequency. For example, dust storms in this desert show a pronounced peak in May while the maximum values for West African dust storms shift northward from winter to summer.

Maps showing seasonal variability of dust sources in the dust belt that stretches from western Africa up through Mongolia.

Climatology » Dust Storm Frequency and Severity

Climatologies compiled by the U.S. Air Force Weather Agency Metsat Applications Branch show the monthly frequency of dust storms. Note how the number of storms in the Gobi Desert spikes in March and April and tapers off from May through July.

Bar chart showing the frequency of dust storms by month for the Gobi Desert

Categorizing dust storms by visibility provides even more detail. Now we see that the majority of severe dust storms in the Gobi Desert occurs in March and April, more than the rest of the year combined.

Bar chart showing the monthly frequency of dust storms by visibility for the Gobi Desert

Climatology » Building a Dust Source Climatology

If you are forecasting in a region and don't have access to information on the frequency of dust storms, you may be able to infer a climatology by examining other climatologic data such as the frequency of dust events vs. annual precipitation rates (PR).

monthly temperature and precipitation for Baghdad, Iraq

Obviously, drier, hotter conditions favour more dust storms. Here we see a minimum for precipitation events and a maximum for temperature in central Iraq through the summer months, the dustiest time of the year.

You can also build a dust storm climatology from archived satellite imagery to establish the most prevalent source areas. For example, this sequence of images reveals that the same light-coloured areas in western Afghanistan repeatedly serve as the source for dust storms. Once you know the colour characteristics of source areas in a given region, you should look for other potential areas with a similar appearance. Click Play to view the animation.

Five true color SeaWiFS/MODIS satellite images of dust storms in Afghanistan

You have reached the end of this chapter. Click the Table of Contents button at the top of the page and select Chapter II to proceed.