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Whirling Winds: Dust Storms & Dust Devils

“Houses were shut tight, and cloth wedged around doors and windows, but the dust came in so thinly that it could not be seen in the air, and it settled like pollen on the chairs and tables, on the dishes. ~John Steinbeck, The Grapes of Wrath, Chapter 1

A study of the weather includes how air moves and anything it might be carrying. Desert dwellers know this may include dust and sand!

Sandstorms and Dust Storms

Sandstorms and dust storms both occur in deserts. The planet Mars has dust storms, too. During sandstorms, the wind lifts up grains of sand and bounces them around. Wind-blown sand acts like a natural sandblaster. It can sculpt rock formations, and can even strip paint from cars.

Sandstorms are a very unpleasant – and painful – fact of life for people who live in the Sahara Desert in North Africa. Howling winds whip up sand which cuts their faces and hands and gets into their eyes, noses, and mouths. Desert nomads protect themselves from the swirling sand grains by pulling their headdresses over their faces when a sandstorm blows up.

The Arabic name for a wild, sand-laden wind is Haboob. Other Saharan winds also have their own special names. Khamsin is an Arabic word meaning “50 days.” This wind sweeps across the desert from March through May, filling the air with sand. The name of the desert wind Harmattan comes from a word in the West African language Twi that means “to tear your breath apart.”

Sand grains are heavy, so they are seldom lifted more than 6 feet. Dust storms, however, can lift choking dust to much greater heights. Dust storms can be dangerous to aircraft because these clouds of dust can reach heights of 5,000 – 10,000 feet or more.

The dust that blows around in dust storms is mostly made of soil. It may also contain pieces of dried leaves, bits of lint, particles of carbon and other solids that come from smoke, volcanic ash, powdered remains of meteorites that have burned up in the atmosphere, plant pollen, mold spores, fungal spores, yeasts, bacteria and germs. Consequently, it’s not a good idea to breathe this dust!

Valley Fever

Valley Fever (coccidioidomycosis) is a lung disease that is endemic in the southwestern United States, California, northwestern Mexico, Central America, and South America. This includes the Central and San Joaquin valleys and desert areas of California, as well as the arid areas of Nevada, Utah, Arizona, West Texas, and New Mexico. It is most common in the Phoenix and Tucson areas of Arizona, and in Kern County, California.

The “cocci” fungus grows naturally in alkaline soil in hot, dry areas. The fungus spores can lie dormant for a long time in harsh environmental conditions including heat and drought. These microscopic spores become airborne when the soil is stirred up by winds, vehicles, construction, agriculture, archaeological excavation, and recreational activities. They can float invisibly in the air for long periods of time, and can be blown by the wind over great distances.

When a person or animal breathes dust that contains these spores, infection occurs in the lung where the spore changes into a larger, multicellular structure called a spherule. The spherule grows and bursts, releasing more spores which develop into more spherules. This process continues until the body develops immunity to the fungus and suppresses the infection, or until an antifungal medication is taken. In general, the more spores inhaled to begin with, the larger and more serious the infection.

Symptoms generally occur within 2-3 weeks of exposure, although about 60 percent of infected persons have no symptoms. The rest develop flu-like symptoms that can last for a month. This may include a low-grade fever, fatigue, dry cough, chest pain, skin rash, sweats, chills, headache, joint aches, and tiredness.

Valley fever is not contagious, and most cases need no treatment. However, it can cause more serious and even life-threatening complications in certain persons, particularly those with weak immune systems. African-Americans and Asians are also at greater risk from valley fever.

Construction workers, farm workers, and others who spend time working in dirt and dust, as well as those who like to go biking or four-wheeling in dusty areas, are most likely to get valley fever. To minimize your risk of getting valley fever, try to avoid exposure to dust and dry soil in areas where valley fever is common, and don’t go out in dust storms or wear a mask if you have to be out in blowing dust.

The rate of valley fever cases in Arizona has been increasing. This is apparently due to all of the new construction on previously undisturbed desert. Your chance of contracting valley fever at some point is higher the longer you live here. Once you have valley fever, you become immune to the fungus and will never get it again. Immunity is indicated by a positive skin test.

Sand Dunes

Sand dunes are hills of sand built by the wind. Dunes are very different from other hills, because dunes move. When the wind blows, a sand dune changes shape. The wind blows sand up one side of the hill and over the top. The sand rolls down on the other side. It shifts up to one foot per day, and much more during a sandstorm. In this way the dune slowly travels across the desert.

Sahara Desert

The Sahara is the world’s biggest desert. It covers approximately three and a half million square miles, which is a third of the African continent, or an area about the size of the United States. The Sahara is one of the hottest, driest places on Earth. Rising and falling winds pull moisture away from the desert, but they rarely bring rain. It receives less than 3 inches of rain a year, and its temperatures can reach 136 F.

The Sahara desert is a parched, forbidding landscape. Crossing it would be almost impossible if not for the oases located there. Even so, the desert is so immense that travelers may go for days before finding the next oasis.

Most of the Sahara is a flat plain covered with stones and gravel. There are a few rocky plateaus and mountain ranges. Sand dunes make up only about 15 percent of the Sahara, but the desert is so huge that even a single dune may be enormous. The sand dune known as the Libyan Erg is as big as France!

The sand dunes are constantly changing. The landscape can look completely different after a sandstorm, and a moving dune has been known to bury a whole village! Satellite photos show that the desert itself regularly shrinks and grows. The border of the Sahara has expanded into other areas and then retreated again. This movement may be related to drought cycles.

The Dust Bowl

One of the worst things that ever happened to farmers in North America was the Dust Bowl of the 1930’s that occurred in parts of Oklahoma, Texas, Kansas, and other Great Plains states. A large number of settlers had been moving into the area. They plowed and mowed the prairie’s natural thick grass without considering the consequences.

A disastrous drought combined with poor farming methods and overgrazed pastures dried up the topsoil into a crumbly powder. Without its cover of tough grass, the soil became vulnerable to wind erosion. Great clouds of this powdery earth were blown into the air.

Blizzards of dust buried crops and houses, suffocated animals, and filled people’s lungs with choking dust. Everyone ate, drank, and breathed dust. The incidence of lung disease rose dramatically. The fine dust got everywhere, even inside cupboards. It clogged machinery and stopped watches.

The drought lasted from about 1932 to 1938. Families were forced to leave their homes. Half a million people abandoned their land and headed for California. A famous novel, The Grapes of Wrath, by the American writer John Steinbeck, tells all about what happened to the farmers who left the dust bowl and went with their families to California.

The U.S. government began a vigorous program of scientific soil conservation to help combat this disastrous problem, and trees were planted as windbreaks. When the drought ended, those who came in and bought the barren land at rock-bottom prices were able to harvest bumper crops.

Dust Devils

Dust devils are common in dry desert locations such as the American Southwest, Africa, Saudi Arabia, and Australia. They are also found on Mars. Some other names for a dust devil are dancing dirvish, desert devil, sand devil, dust whirl, dust swirl, wind devil, and willy-willy.

A dust devil may look like a miniature tornado, but it’s not. Dust devils form when the hot sun beats down on the ground’s surface. They are common in the summer and autumn in dry desert areas. The air immediately above the surface heats up more than the air farther above the ground. At first the differing air temperatures remain stable against each other. But warm air wants to rise, and cool air sinks. So any disturbance caused by a gust of wind, the motion of a car, or even the movement of an animal will cause the warmer air to rush upward. As the hot air rises, it begins to spin as the cooler air rushes in from all directions.

This spinning column of air may move across the landscape and pick up loose dust. This dust makes the vortex visible. Dust devils can also pick up sand, leaves, paper, and sometimes other objects. They have small diameters of only a few feet, but they can rise as high as half a mile.

Most dust devils are nowhere near as powerful as tornadoes. Dust devils usually spin no faster than 25 miles per hour, while an average tornado speed is 100 mph. Nevertheless, some dust devils have been measured up to 70 mph and have damaged small metal buildings. Tornadoes travel in a counterclockwise movement, but dust devils have no preferred sense of rotation.

Individual dust devils usually don’t last for more than a few minutes, but they can form repeatedly for hours. That means when one dies down another one starts up close by, and then when that one dies down, another one starts up, and so on. Dust devils can also travel in groups. A large dust devil is sometimes seen to have small dust devils trailing along behind it.

My family calls the dry valley along Route 60 between the Harquahala and Harcuvar Mountains “Dust Devil Alley” because we see so many dust devils whenever we drive through there. But the most perfectly shaped dust devil we ever saw was at Lake Pleasant. It was a tall, thin, twisting vortex that looked like a tan tornado against a blue sky and must have lasted for 10 minutes.

Did You Know…?

Some ancient desert tribes thought dust devils were spirits, called “ginni.” This is where the genie of the Arabian Nights comes from.

Additional Information

http://vfce.arl.arizona.edu/ValleyFever/valley_fever.htm (Valley Fever and dust storms.)

http://www.valleyfever.com/primer.htm (Valley Fever Primer.)

http://www.calacademy.org/exhibits/africa/exhibit/sahara/index.html (Sahara desert exhibit.)

http://www.gi.alaska.edu/ScienceForum/ASF2/227.html (Dust devil diagrams.)

http://mars.jpl.nasa.gov/gallery/duststorms/ (Dust storms and dust devils on Mars.)


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