You step out onto the sun-drenched patio to enjoy what's left of yet another stellar summer day. But as you lazily take in the view of the horizon, you can't quite believe what you see. Instead of the clear blue Southwestern sky that usually backdrops the expansive desert landscape, a looming black wall towers miles above the ground, dwarfing the nearby town as if it were a child's LEGO project. You blink. With all due respect to Mother Nature, this thing looks more like the product of computer graphics from a science-fiction movie than anything real. But it is real, and you realize it's moving toward you -- fast. Two minutes later, you're thrown into the dark of night, the wind is howling in your ears, and it feels like someone's repeatedly throwing a shovel full of fine sand in your face. As you fumble to make your way back inside for cover, your eyes, nose and mouth are full of fine gritty stuff. Your stunned brain can only think one thing: Is this what Armageddon feels like?
Often described with apocalyptic terms by the media, what you've just experienced is a haboob, a type of dust storm. Dust storms come in various shapes and sizes, but regardless of their variations, what they do is similar -- they carry dust and sediment from one place to another, regardless of who or what is in their path.
Dust storms are natural events. However, scientists believe human activity is causing them to occur more frequently in certain regions, and their impact can be devastating.
Ever wonder how all that dust gets airborne and why haboobs are the scene-stealers they are? Turns out, it takes more than a windy day.
Where Do Dust Storms Occur?
The erosion of soil in one area and deposition of it in another is a process that has played a role in our global ecosystem since time began. Although dust storms make headlines for the havoc they cause, their occurrence is not always a tale of destruction and mayhem. For instance, 20 million tons of dust are transported from the Bodele depression in the Sahara to the Amazon basin each year, supplying the rain forest with essential minerals and nutrients to keep it fertile and thriving [source: Koren].
From a small, cyclone-like burst of dust that only lasts for a few minutes to storms that blow dust for 50 days straight at near-hurricane speeds, dust storms result from a combination of climate, weather and wind. Central Asia, North America, Central Africa and Australia are home to the most dust storms, but they can kick up anywhere where the conditions are ripe.
The first element needed for any dust storm -- a source of dust -- depends largely on climate. Ideal dust sources occur in areas where the composition of the soil is very dry and loosely held on the surface. This most commonly occurs in arid and semi-arid regions, usually after a prolonged drought. Moisture keeps soil compact and helps maintain vegetation, which protects it from being swept up into passing winds. Not surprisingly, dust storms frequently occur in the desert. However, marginal dry lands are increasingly a source of major dust storms. These areas have fragile, delicately balanced ecosystems. Their degradation, called desertification, makes the soil less resilient to wind during prolonged drought. Desertification sometimes happens naturally; the Bodele Depression, for instance, is a natural dry lake bed that was once a large freshwater lake in northern Chad. Now, an average of 0.7 tons of dust a day is blown up from the dried up lake each winter [source: Koren].
Increasingly, however, desertification results from human activity. Excessive animal grazing, timber cutting and farming methods strip and exhaust dry-land topsoil. When a drought occurs in these areas -- and eventually it will -- all you need is the right weather and you've got yourself a dust storm.
So how do dust storms form? First, we'll examine how wind gets dust into the air and later find out how weather patterns determine what happens to the dust once it's airborne.
How Does Dust Become Airborne?
Without wind, dust will generally remain on the ground. While wind is unquestionably the force that causes dust to rise, additional physical and electrical forces accelerate the process.
When wind passes over a dust source, the loosely held sand and dust particles move. When the soil is dry, it doesn't take much to get them moving; the threshold wind velocity only needs to be at about 9 miles per hour (14.5 kilometers per hour) to disrupt the surface [source: United Nations]. Of course, wind speeds this low don't necessarily create a storm, but it gives you an idea of how little force is needed to sir things up.
The way wind moves particles depends on their size and weight. The smallest particles (less than 0.002 millimeters in diameter) are easily suspended in air and the largest particles (greater than 0.5 millimeters) roll along the surface of the ground, a movement that's called creeping. It's the movement of particles between these two sizes that have the biggest impact on dust emission, however. These particles are lifted temporarily and bounce back onto the surface upwind. When they hit the surface, they bounce back up. They also cause a chain reaction to the particles around them.
Think of the particles on the surface as ping-pong balls. When one ball, propelled by a wind force, bounces onto the others it causes the other balls to bounce. Every time a ping-pong ball hits the surface, additional balls become airborne, regardless of whether wind is there or not. Of course, once airborne, the balls are susceptible to wind forces. This collective action is called saltation. Saltating particles will be lifted higher into the air depending upon their size. Dust particles, also called silt, are between 0.002 and 0.05 millimeters in diameter.
Although very small particles in suspension create their own problems, dust particles are what make most storms so hazardous. Dust can be lifted more than 700 meters (2,296 feet) into the air [source: United Nations].
The physical forces described above increase the amount of airborne dust at a given wind speed, but the effects of saltation don't stop there. Saltating particles also generate electrical energy, which increases the number of saltating particles even more. As particles hit each other and the surface, they acquire a negative charge. The surface, however, acquires a positive charge -- essentially generating a static electric field. Anyone who has rubbed a balloon against his or her head knows how hair will, without wind, fly about in suspension. A similar principle is at work here. The electrification of particles reduces the amount of wind force needed to initiate further saltation. In fact, it can directly lift particles from the surface.
Now that the dust is airborne, how high will it be suspended, where will it go and how long will it remain in the air? In the next section, we'll learn how weather systems determine what a storm will look like.
Weather and Dust Storm Formation
Meteorological conditions play a big role in determining the size, duration, and intensity of dust storms. Weather patterns can be broadly categorized into two categories: large-scale -- or synoptic -- weather systems, and smaller patterns known as mesoscale systems.
Synoptic systems are frontal systems that cover great distances; they range anywhere from 620 to 1,500 miles (998 to 2,414 kilometers) across [source: National Oceanic and Atmospheric Administration]. Dust storms are created when these fronts sweep onto hot, arid surfaces. When cold fronts undercut warm air, the pressure gradient increases, and winds shift and become quite gusty -- as high as 50 to 100 miles per hour (80 to 160 kilometers per hour) [source: University Corporation and Atmospheric Administration].
Additionally, the shifting winds cause turbulence and the high surface temperature creates convective currents at the leading edge of the storm. These forces essentially act to lift particles higher and keep them aloft for longer. Synoptically induced storms can carry dust for very long distances. For instance, each year millions of tons of desert dust ride the trade winds from the North African coast to the Caribbean basin [source: U.S. Geological Survey]. The summer shamals in the Persian gulf and African sirocco winds are synoptic systems that routinely kick up dust storms that can last as long as five days [source: University Corporation and Atmospheric Administration].
Mesoscale systems are smaller weather systems that range from 50 to 100 miles (80 to 160 kilometers) across [source: National Oceanic and Atmospheric Administration]. Although smaller than synoptic systems, mesoscale systems don't always generate smaller storms. Wind currents generated on this scale fall into three groups: down-slope winds, gap winds and convection. Down-slope winds are very strong, gusty winds that blow down the slope of a mountain and reach their peak strength at the foot. Gap winds are low-level winds that pass through relatively small channels or gaps in mountain barriers -- these are the type that blow across the Bodele Depression and end up crossing the Atlantic Ocean. Convection is the vertical movement of air. Although convective winds are generated at all cold fronts, convection is most commonly associated with thunderstorms -- these systems produce haboobs.
Haboobs are what most people think of when they hear the words dust storm. Let's take a closer look at these impressive walls of dust in the next section.
Haboobs are the temper-tantrum-prone siblings in the dust storm family. They're relatively small compared to the massive storms that are caused by synoptic-scale systems, but they tend to erupt out of nowhere and with dramatic fanfare. They're common in the Sudan and North America, namely the Southwest plains. As we just found out, they're associated with thunderstorm activity. But what does a thunderstorm, which we usually associate with rain, have to do with a dust storm? Well, the connective winds that typically associate with thunderstorm activity play a big role in generating a dust storm. Many times, rain will follow a haboob, but it's often unnoticed as the actual rainfall is usually insignificant.
When thunderstorms dissipate, gust fronts, or outflow boundaries, are formed as downdrafts hit the ground and spread out from the storm, where it begins to rain. These gust fronts, which can exceed 50 miles per hour (80.5 kilometers per hour), act as "mini-cold fronts" racing out from the dissipating storm. When the gust fronts sweep across exposed dry land, dust is kicked up into the air similar to the larger scale fronts described above, only with more intensity. True to fashion of your typical three-year old, the resulting tantrum is quite a show; the airborne dust forms into a towering, condensed thick wall, moving with the storm at about one half the speed of the gust front, typically 30 miles per hour (48 kilometers per hour) [source: University Corporation for Atmospheric Research]. However, similar to temper tantrums, these storms don't last very long. Haboobs tend to be over within three hours and visibility improves shortly after the winds subside.
The effects of haboobs are similar to most major dust storms, but they're a bit more chaotic due to the rapid formation and intensity of the convective winds that are responsible for the storm. Strong winds blowing dust around decrease visibility and deposit sediment everywhere, which seriously jams electrical equipment, machinery and even helicopters. Power lines are often blown down and air travel comes to a halt. The whipping dust, which feels like fine sand, can cause respiratory problems, especially for people with asthma.
The most immediate dangers during a haboob are aircraft collisions and highway accidents. Visibility can be reduced to near zero in a matter of two minutes during a haboob -- if you happen to be motoring down the highway, this can be life-threatening. Pile-up collisions are a common occurrence during black blizzards.
While it's likely you'll survive a dust storm, there can be long-term consequences once the dust settles. In the next section, we'll examine how dust storms can affect our health and communities.
The Impact of Dust Storms
During major dust storms, the deposition of dust over populated areas can be wide reaching, often affecting multiple cities and towns. Dust storms can take down trees, bury equipment and cause damage to houses. In the final years of the Dust Bowl, farm animals were found dead in the fields and people started suffering from "dust pneumonia" [source: WGBH].
While the loss of human life during dust storms is relatively small when compared to other natural disasters, long-term health concerns have cropped up recently. This is primarily due to the increased number of storms originating from areas of desertification. The dust in these storms has been shown to contain pollutants and toxins, such as salt, sulfur, heavy metals, pesticides and carbon monoxide to name a few [sources: United Nations, Stewart]. The pollution-laden dust can be carried over hundreds of miles, affecting millions of people who might not necessarily suffer from the acute events of the storm.
The immediate economic impact of dust storms is significant, but it doesn't rival major natural disasters that destroy entire cities. For instance, the damage due to dust storms in China averages at about $6.5 billion per year [source: Ford]. A single major earthquake can do damage to the tune of five times that figure. However, experts argue that the real economic impact of dust storms, particularly those that originate in areas of desertification, is difficult to pin down because of the long-term consequences they have on the livelihood of people who live in the area[source: United Nations]. When dust storms kick up in agricultural dry lands that are degraded, they remove the topsoil, which causes further desertification. As a result, farmers are forced to watch the topsoil, and their livelihood, literally blow away. This cycle, if gone unchecked, threatens to displace whole communities in some regions.
Some dust storm activity can be prevented, but dust storms will always be an integral part of the natural ecosystem. Learn what we can do to prevent and live with dust storms in the next section.
Living with Dust Storms
Co-existing with dust storms can be summarized in three words: prevention, preparation and prediction. In areas where human activity has created dust sources, restoration and preservation of dry land ecosystems has been shown to reduce the number of storms that ravage farmlands. This can be done by changing farming practices, such as reducing tillage frequency to lower disruption of the soil; planting cover crops, such as grass, to prevent erosion; and planting rows of shrubs and/or trees to reduce the impact of wind forces as they move in.
For those who live in areas where seasonal dust storms are unavoidable, preparation is the best way to prevent loss of property and preserve health. Covering computers and machinery with plastic or tarp keeps dust from destroying the electrical components and clogging mechanical parts. Military personnel based in deserts even wrap up helicopters during dust storms. Having goggles and a mask on hand means once the dust starts flying, you can keep it out of your eyes, mouth and nose.
Predicting dust storms obviously helps people to be better prepared, and forecasting large -scale storms is pretty reliable. The methods meteorologists use to do this largely depend on carefully monitoring winds and atmospheric stability in areas known to be prone to dust storms. Scientists have even developed computer models that take weather forecasts and combine them with data from storm research to predict dust storms with reasonable success -- meteorologists can sometimes even predict the size and duration of the storms.
Haboobs are more difficult to forecast. Meteorologists are forced to rely on what's called "real time" forecasting of thunderstorm activity where the environment may be ripe for a dust storm. As you can guess, it's difficult for them to say whether or not a storm will kick up, but at least you can be on the lookout. Also, using data collected by satellites, scientists have developed ways to determine soil moisture and availability in arid regions. This helps assess the likelihood that a haboob may occur when thunderstorms sweep through certain areas.
If you're interested in learning more about dust storms and related phenomena, blow over to the next page for more information.
Frogs! Fish! Birds! They've all been said to appear from clouds. Learn about 10 times the sky rained something other than water at HowStuffWorks.
More Great Links
- Ford, Matt. "Desertification: How to Stop the Shifting Sands." CNN. April 28, 2008. (Jan. 19, 2012) http://edition.cnn.com/2008/WORLD/asiapcf/04/25/es.desertification/index.html
- Kok, Jasper F., and Renno, Nilton O. "Electrostatics in Wind-Blown Sand." Physical Review Letters. Vol. 100, No. 1. 2008.
- Koren, Ilan, et al. "The Bodele depression: a single spot in the Sahara that provides most of the mineral dust to the Amazon forest." Environmental Research Letters. Vol. 1, pp. 1-5. October - December 2006.
- National Oceanic and Atmospheric Administration. National Weather Service Glossary. (Jan. 19, 2012). http://www.weather.gov/glossary
- National Oceanic and Atmospheric Administration. JetStream- Online School for Weather. (Jan. 19, 2012) http://www.srh.noaa.gov/jetstream/synoptic/synoptic_intro.htm
- Scheidt, Stephen. et al. "Determining soil moisture and sediment availability at White Sands Dune Field, NM from apparent thermal inertia data." Journal of Geophysical Research. Vol. 115. 2010.
- Shears, Richard. "Visible from space: The wall of dust marching across the Australian outback to turn the skies over Sydney blood red." The Daily Mail. Sept. 24, 2009. (Jan. 19, 2012) http://www.dailymail.co.uk/news/article-1215443/Australia-dust-storm-sweeps-eastern-coast.html
- Stewart, Robert. "Environmental Science in the 21st Century- An Online Textbook." Ocean World. (Jan. 19, 2012) http://oceanworld.tamu.edu/resources/environment-book/dyingseas.html
- United Nations. "Global Alarm: Dust and Sandstorms from the World's Drylands." August 2001. (Jan. 19, 2012) http://www.unccd.int/publicinfo/duststorms/part0-eng.pdf
- University Corporation for Atmospheric Research- the COMET Program. "Forecasting Dust Storms v2." (Jan. 19, 2012) http://www.meted.ucar.edu/mesoprim/dust/
- U.S. Geological Survey. "African Dust Carries Microbes Across the Ocean: Are They Affecting Human and Ecosystem Health?" USGS Open File Report. January, 2003 (Jan. 19, 2012) http://www.sld.cu/galerias/pdf/red/tiempo/africanduscarries.pdf
- WGBH. "Surviving the Dust Bowl." American Experience. (Jan. 19, 2012) http://www.pbs.org/wgbh/americanexperience/features/transcript/dustbowl-transcript/