Dust storms need a source for lots of loose soil or sand. Deserts and arid plains subject to dry spells work splendidly. They also need the right weather conditions in order to kick up a lot of dusta certain set of conditions on sea and land. It may seem odd that what happens far away on the ocean can trigger storms that will happen days later well inland, but it can and does.
To see why, let’s consider the effect of what meteorologists call a steep temperature gradient-rapid cooling or heating over a short period of time in any given location. The gradient steepens in areas affected by movements of the jet stream, a ribbon of air thousands of miles long, flowing rapidly at an altitude of between six to nine miles. The jet stream has the energy to be able to shove hot and cold fronts around like a kitten playing with its toy. It tends to form around a front, and in turn feeds that front. The jet stream is a catalyzing system for extreme weather because low fronts tend to form underneath them as the jet stream plows through. Generally, in the Northern Hemisphere, cool air stays to the north of the jet stream while warm air rides along its southern edge.
The Earth is heated unequally; the equatorial regions receiving far more sunlight than the polar regions. Great differences in temperature tend toward the average in the great chaotic turbulence that is our atmosphere. They cannot remain stable, just as two gases released into a room cannot remain separated. Therefore winds build up and push warm and cold fronts, easing the temperature gradient for a time. As a result, low pressure areas form and winds drive warmer air northward and send cold air south. Hence the formation of the jet stream. For those of you Down Under, the reverse applies for descriptions of the Southern Hemisphere.
Any overall changes in the pattern of jet stream activity over a comparatively long period of time can induce at least short-term climate changes, drying out what usually is a wetter region or the reverse. The jet stream is sensitive to changes in normal patterns of sea surface temperatures in the world’s oceans. The pattern of warmer surface water in the western and central Pacific, for example, with colder water in the eastern portion, tends to generate thunderstorms in the west, which enhance the power of the jet stream, generating powerful fronts driving toward the United States.
Things get further complicated with temperature differences between the land and the sea, leading to strong winds off of coastlines. This is how weather patterns, oscillating patterns of winds, distribute heat and cold around the world, ease Earth’s constantly building and ebbing regional temperature differences.
Now, we are in a position to discuss dust storms. Consider the nature of Africa, home of the largest dust storms, with some as large as the United States. These monster storms form out of the enormous temperature gradient always emerging between the cooler Sahel region and the very hot Sahara Desert to the north. The resulting trade winds rise and blow west. Obviously, the Sahara feeds its sand into the mix from time to time. Such storms are extremely dry and are able to loft sand particles up as high as three miles, creating a slight sun screen over the region.
Africa’s dust storms seem to suppress hurricane generating processes in the Atlantic Ocean. When sand blows westward, it blocks a fraction of solar energy that would normally reach the ocean’s surface. The Atlantic is cooled slightly. Hurricanes breed and grow in the normally energetic tropical waters off of West Africa. If these waters cool for any reason, fewer hurricanes reach the Gulf of Mexico or the East Coast of the United States.
Consider the nature of Arizona, with its arid and semi-arid regions. Sudden dust storms arise on windy days quite often there. One day as I relaxed out on a covered patio of a Sun City residence while taking a vacation break from a frigid Minnesota winter, a wind slowly rose in speed from a soft breeze until it began blasting me and the newspaper sections I had left scattered about on the patio table. Unlike the short gusts of wind I was accustomed to in Minnesota, this one kept blowing harder and harder for at least two minutes. My goose bumps rose as I chased down the paper. It was eerie. I was worried. Finally, the wind very slowly eased, eased down some more, and then became a gentle breeze again. I expected another such wind to return in a few minutes and waited for it, but it never did. This, I realized later, was a small example of the kind of powerful, sustained wind that can bring dust storms down upon unwary golfers and commuters with no prior notice.
A real dust storm is much, much worse than the zephyr I experienced. It arrives as the horrified observer spots an advancing dust wall, miles long, several thousands of feet high and coming fast. It begins as a mass of extremely dry, hot air baking the ground. A large mass of cold, unstable air approaches, forcing the dry, hot air downward where it spreads rapidly and picks up any loose soil or dust lying in its path. The resulting turbulence is called a “haboob” by Arabs, a violent wind that drives across the desert at 50 to 60 miles per hour. Had I been in Arizona in the summer, that zephyr may well have turned into a haboob, since Arizona is the only place in the United States where that kind of dust storm can form.
Historians believe that the once Fertile Crescent of the Middle East that had fostered the growth of numerous civilizations several thousands of years ago was ravaged by overgrazing and deforestation. These human activities permitted the dust storms to grow in intensity by prohibiting the formation of much needed rain clouds. Along with naturally caused changes in temperature gradients in the atmosphere and the oceans, and lower precipitation, the dust storms fed on themselves, sending their clouds of fine bits of feldspar, quartz, and mica high into the troposphere where strong winds spread them around the world. The Sahara Desert kept growing as a result. It’s still growing today.
Consider the nature of the Great Plaines of the United States during the 1930s in the days of the Dust Bowl. Winds blew tons of topsoil off of each farm, carrying it away in storm clouds for hundreds of miles. With gallows humor, a Kansas farmer would say his farm was blowing all the way to Missouri.
The dust storms had been building in intensity for years until that grim day, known as Black Sunday-April 14, 1935-when a 60 mile-per-hour storm hit. People caught outdoors groped their way home blinded. It was worse than the worst winter blizzard because the fine sand got into eyes and lungs. There was no daylight on Black Sunday. No light could penetrate the dust.
The rain stayed away, month after month, year after year, kept out as I said above by the dust storms themselves. The “black blizzards” reflected sunlight back into space. No evaporation could take place, hence no clouds could form. When conditions are right, dust storms feed on themselves. They absorb heat and remain dry. What had been set off, perhaps, by lack of erosion control, had become self-generating, and would not ease until prevailing weather patterns shifted in the 1940s.
Historians had assumed for years that the dust storms started and grew worse as a result of the growth of farming in land that had formerly been grassland. They thought what had happened in the Middle East was happening all over again in America. As a result of straight line plowing before planting and after harvesting, heavy topsoil no longer could stay in place in strong prairie winds. The prairie grass that used to do the job was gone. Farmers had yet to learn good soil management practices. They simply had assumed that practices that worked well enough back East would work fine out West.
However, puzzled researchers in the field of historical meteorology recently noted that the drought in the Great Plaines in the 1950s stayed in the South and didn’t spread north, unlike the one during the Dust Bowl years. These dry cycles occur roughly every 20 years on the Great Plaines. They also noted that the Fifties drought wasn’t nearly as severe and didn’t last for years with little or no rain. None of the other previous and subsequent droughts noted in records in that region suffered from the same widespread devastation, either. They were much more similar to the drought of the 1950s. What was so special about the 1930s? Could all that horror really been nature’s response merely to poor farming practices? If so, one would think that the same kind of devastation would’ve hit the Great Plaines earlier, say around the turn of the century. But it didn’t.
Scientists believe, with increasing evidence gathered from meteorological records and computer modeling, that the Plaines region is in a long-term oscillating pattern of climate, ranging from somewhat wet to dry to very dry and back again. And that this oscillating pattern tracks the natural oscillating patterns of warm and cool ocean patterns called El Nino and La Nina (the former-the boy-named for the Christ child because it’s observed off the Pacific coast of South America by researchers during the Christmas season. The latter-the girl-so named because it’s the opposite of El Nino). Usually, sea surface temperatures affect the Southern Great Plaines during La Nina (not the Northern Great Plains) according to those models. The North gets a number of storms from the West and the Northwest.
Researchers also studied tree ring growth patterns in sample cross sections preserved by previous researchers, many cored out of truly ancient trees, some rescued from logs dredged up in peat bogs or lakes, some rescued during the razing of old buildings, others chipped from petrified forests. They were able to match overlapping sequences of ring patterns from a wide variety of samples, creating a historical series of rings dating back centuries.
No such severe drought patterns like the Dust Bowl were detected elsewhere in the modern period. Other than during the 1930s, the ordinary oscillations maintained their ordinary pattern of growth rings and less growth, then more growth all the way back through the history of the United States, the age of European colonization, and the age of exploration. It wasn’t until researchers checked tree rings dating back a thousand years that they found evidence of truly devastating droughts, the kind that matched the 1930s. Those barely visible rings showed signs of virtually no growth. A thousand years ago. Medieval times in Europe. The Little Climate Optimum in Europe. Was this a coincidence? Researchers don’t think so.
La Nina severely restricts the number of thunderstorms moving off the Pacific into the Southwest. Cold water, or at least water colder than normal, exists in the Eastern Pacific under the influence of La Nina, reducing the temperature gradient, and therefore reducing the chance that the jet stream will form off the Pacific coast in the Southwest. Instead, the jet stream moves north over the Alaskan and Canadian coast. From there, it dives southward toward the Midwest.
Less precipitation forms over Southern California and Arizona, and as a result, less precipitation moves into the Southern Great Plaines. While the Northwest may be wet, the Southwest will be dry. Since most of the moisture arriving in the Southern Great Plains comes from the Southwest, La Nina can and has led directly to droughts in several American states on a regular basis. If La Nina stays around longer than normal, the drought worsens.
This is precisely what researchers assert happened in the Great Plains during the Dust Bowl years. The weather pattern was so severe, the dust storms so thick, La Nina lasted so long, that the affected region crept up north to the Dakotas and spread to the Rockies and the Northwest. It was the drought of the millennium. And farming, as it turns out, had little to do with it. These are the kinds of killer droughts that have destroyed civilizations in the past, with untold numbers of human fatalities left in the wreckage. Fortunately for us, American civilization is far more resilient than that.
Researchers were forced to consider how small changes, subtle changes triggered by La Nina in the Pacific, held on long enough to ramify through thousands of miles of atmospheric turbulence and cause catastrophe in the middle of the North American continent. It probably stretched the technology to the breaking point, but a computer program was written, loaded with the data gathered, and run to model the way similar patterns might hold in the real world, both in the 1930s and medieval times. The results matched precisely the effects observed by American farmers and meteorologists in the Great Plaines.
But what of Europe? Years of La Nina-like conditions in the Pacific coincided with a subtropical-like North Atlantic according to the model. Data was loaded, including observed world-wide effects that occur during the lack of a strong ocean temperature gradients. Result: Devastation for any American tribal peoples trying to survive on buffalo meat in the Great Plaines simultaneous with bountiful expansion for European farmers ringing in the High Middle Ages. The land with no farmers devastated. The land with numerous farmers flourishing. Both astonishingly different effects growing out of the same set of natural causes ramifying world-wide. Dust Bowls for America. Little Climate Optimum for Europe. A deep, strange irony, if true.
Think of dust storms as signs of how the little things in life can grow into big things and go horribly wrong or astonishingly right without any input from us at all.
Sources:
Check out these web sites for further information on the many points I brought up in my essay:
http://www.wrh.noaa.gov/fgz/science/pdo.php?wfo=fgz
http://www.worldclimatereport.com/index.php/2008/10/31/atlantic-ssts-and-saharan-dust-and-hurricanes/
http://www.livinghistoryfarm.org/farminginthe30s/water_02.html
http://www.nationmaster.com/encyclopedia/Medieval-climate-optimum
http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/cyc/upa/jet.rxml
