SCRIPT FOR THE COMET MODULE: ATMOSPHERIC DUST
Chapter 1: The Social and Economic Impacts of Dust Storms
Page 1: 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.
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.
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.
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This plot from NOAA’s HYSPLIT model makes the dust transport even clearer.
If you’ve never experienced an intense dust storm like this, take a few minutes to watch some of the videos listed
below. (If a link is not working, try another one.)
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Beijing, China, 20 to 22 March 2010, 1 minute, English,
http://www.youtube.com/watch?v=a1sgOvYUHO8
Riyadh, Saudi Arabia, 10 March 2009, 3 minutes, English,
http://www.youtube.com/watch?v=BD7IedPZvt8&NR=1
Al Asad, Iraq, 27 April 2005, 2.5 minutes, English, http://www.youtube.com/watch?v=cv4BhZV5mAA
Hassakeh, Syria, 16 May 2007, 1 minute,
http://www.youtube.com/watch?v=T8sxhTutawY&feature=related
Page 2: 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:
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Their size; the average soil particle size within a dust cloud decreases 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, which 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. It’s unclear
how many people develop health problems from dust storms each year, but some estimate as many as a million.
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Page 3: Impact on Air Traffic
As you’d expect, Sydney’s dust storm had a major impact on its air traffic. On 23 September, visibility at the airport
was reduced to 400 metres and flights were delayed for hours, with some diverted to cities hundreds of kilometers
away.
Dust can also have global impacts on air traffic management. For example, a major dust storm that started over
northern Africa on 2 March 2004 reached the Portuguese island of Madeira several days later. This MSG visible
image shows how the dust cloud extended from the coast of Guinea and Guinea-Bissau over the Cape Verde
islands and Madeira and 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.
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Page 4: Visibility
We’ve seen how 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.
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 airport from above even when the reported horizontal surface visibility is about 5 km or
more.
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Page 5: Agriculture & 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 kilometres 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.
The United Nations Convection to Combat Desertification (UNCCD) has found that dust or sand encroachment in
China has been expanding around 3500 square kilometres per year since the late 1990s.
Page 6: Fertilization of the Ocean 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.
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As you can see, these events coincided with prolonged drought in the Sahel region of North Africa. The bar graph
shows strong negative anomalies for annual precipitation, which favors the occurrence of dust events.
For more information on 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 to why Caribbean
coral reef ecosystems are deteriorating and human health may be 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.
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Page 7: Cloud Microphysics 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.
Mud rain occurs when dust is removed from the air by rain, 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.
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Before dust settles with rain, a complex interaction occurs between the dust, clouds, and precipitation regime.
There’s no clear consensus as to what that interaction is—if the dust increases or decreases precipitation.
Observational studies suggest that dust inhibits precipitation or at least makes it lighter than it would otherwise be.
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. This prevents or reduces precipitation.
But studies using cloud models suggest that dust behaves like a 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 does, in fact,
decrease precipitation, a positive feedback mechanism may be at work in which deserted areas contribute to less
precipitation and less precipitation enhances deserted areas. Naturally, this has implications for climate and
climate change.
Scientists are investigating many other dust-related topics, such as the following.
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The impact of African dust storms on hurricane activity in the North Atlantic Ocean
The possibility that upper-tropospheric flow features weaken Atlantic tropical cyclones near Saharan Air Layer
regions
The impact of dust storms on the Asian summer monsoon regime
SECTION 2: Physical Processes
Subsection 1: Dust
First page: Moving Sediment
Dust moves through several processes, described below. The latter two are integral to the formation of dust
storms since they loft dust into the air.
Click to view animation.
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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.
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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.
Next page: 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.
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 (much smaller
than the width of a human hair).
Question
Of the following types of particles, which fit this description? (Choose all that apply.)
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Clay particles, which have diameters of less than 2 micrometers
Silt particles, which range from 2 to 50 micrometers
Sand-size particles, which are greater than 75 micrometers
Feedback: 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. Tenmicrometer 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.
Next page: Sources of Dust: Desert
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.
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Click to view animation.
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.
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.
Next page: Other Dust Sources
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This montage shows other sources of dust. Are you surprised to see ocean sediments and glacial deposits in the
list? Click each image to view more information on that topic or simply scroll down to view all.
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.
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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.
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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.
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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.
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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.
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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. Highresolution (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.
Next page: 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).
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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).
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.
Subsection 2: Wind
Next page: 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.
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The first sand and dust particles to move are those from 0.08 to 1 mm in diameter. This occurs with wind speeds
of 10 to 25 knots.
As a rule of thumb, winds at the surface need to be 15 knots 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.
Next page: 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.
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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 10,000 feet
(3000 m).
Click to view animation.
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 15 knots, the wind at 1,000 feet (305 meters)
must be about 30 knots to keep the dust particles aloft.
Next page: 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.
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.
Next page: Friction Velocity
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As 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.
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.)
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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.
Next page: 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 to view animation.
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.
Next page: 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.
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Be sure to examine winds at 925 mb (approximately 2,500 feet or about 750 meters 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.
Subsection 3: Dust Removal
Page 1: 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.)
On the following pages, we will discuss three processes that remove dust: dispersion, advection, and entrainment
in precipitation. Gravity also plays a role, although we will not discuss it.
Next page: Life Cycle of a Dust Storm
This animation depicts the life cycle of a typical summer dust storm in Iraq, called a shamal.
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Click to view animation.
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.
Next page: Dispersion
In the shamal example, the dust dissipated through two processes: dispersion and advection. We’ll start by
looking at dispersion.
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.
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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.
Next page: 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: Mechanical turbulence, Turbulence caused by shear, and
turbulence caused by buoyancy.
Mechanical turbulence is caused by air flowing over rough features, such as hills or buildings.
Click to view animation.
Turbulence from shear can result from differences in wind speed and/or direction.
Click to view animation.
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.
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Click to view animation.
Turbulence acts to disperse dust plumes and keep the dust particles in suspension. Without turbulence, dust
generally settles at a rate of 1,000 feet (305 meters) per hour. However, this is highly dependent on environmental
conditions. Any turbulence will slow the settlement rate.
Next page: 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.
This graphic shows dust plumes dispersing under both stable and unstable conditions.
Question 1
When the local environment is unstable, how do dust plumes disperse? (Choose the best answer.)
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Horizontally
Vertically
In both directions
Feedback: The correct answer is c). Dust disperses in both directions although the effect is significantly more
pronounced for the vertical component.
Question 2
When the atmosphere is stable, dust disperses: (Choose the best answer.)
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Primarily in the horizontal direction
Primarily in the vertical direction
In both directions
Feedback: The correct answer is a). A stable atmosphere tends to suppress the vertical dispersion of dust, but
horizontal transport is still possible.
Question 3
Under neutral stability conditions, dust plumes spread: (Choose the best answer.)
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More horizontally
More vertically
Roughly equally in both directions
Feedback: The correct answer is c). When the atmosphere has neutral stability, dust plumes disperse roughly
equally in both directions because neither one is favored.
Next page: 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 15,000 feet (4600 meters), 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 to view animation.
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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.
Next page: 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 to view animation.
Particles between 10 and 50 micrometers fall at about 1,000 feet (305 meters) per hour. Using that rate, if dust is
lifted to 5,000 feet (1500 meters) and the wind ceases, the dust will settle in about 5 hours.
Over how large an area? If winds are 10 knots and there’s little to no vertical motion, the dust will typically settle
up to 50 nautical miles downstream 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.
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