During the "Dust Bowl" years of the 1930s, dust storms in the Great Plains smothered crops and left farms in ruin. Dust drifted against barns and farm houses like snow. The dust from these storms wasn't just confined to areas west of the Mississippi. Though, the eastern half of the US didn't have to deal with hefty accumulations of dust, featherdusters were often put to good use. Although the eastern US is perhaps the wettest and thus least dusty region in the country, on occasion, dust is carried here from distant sources. Sometimes the source area is from within North America, but at other times it arrives from afar.

We're not talking about the stuff that gets behind your dresser or on top of table tops. Rather, dust is a generic term used for particles of soil, ash or sand small enough to be lifted from the surface and suspended in the air, for a few feet or for thousands of miles and for a fraction of a second or for a number of months. Of course, the larger and heavier particles are more difficult to lift, however, since smaller particles have greater surface area per weight and their cohesion and surface tension are greater, it take some effort to get them aloft as well.

If dust particles are forced into the upper atmosphere by strong updrafts or by volcanic explosions, they may circle the globe, carried by vigorous jet streams. These high altitude particles, when they scatter light from the rising or setting sun, are often responsible for creating colorful twilights. However, during the day, if the dust is confined to the near-surface area, below a few thousand feet, distant objects may appear pale or colorless.

Dust can be, and often is, composed of mixtures of a number of minerals such as quartz and calcite, and its composition helps provide information as to the location of its source area. The density of the dust, and to a lesser degree the shape of the dust grains, can alter its aerodynamics. Density affects the drag per unit mass area on a dust grain and is a significant factor in determining if and how long the dust will remain airborne. Studies have shown that particles about 40 microns (about 1/1000 inch) are more readily lifted than particles of other sizes. These sized particles are the first to become airborne, and at higher wind speeds, smaller and larger particles can be lifted. However, it's the smaller particles

(less than about 10 microns) that are transported the furthest. In general, steady winds in excess of about 25 miles per hour are required to keep dust particles in suspension.

Because the generation of wind-borne or aeolian dust requires the exposure of bare soil to wind streams, the world's great deserts are the primary sources for dust. Deserts have a greater fetch than do forested areas, for example - there's less in the way to block the wind. Agricultural regions are also important dust producers.

The Sahara desert is the world's largest true desert and is also the largest single source of dust. It may contribute half of the global atmospheric dust load. Persistent desert winds may push dust into the Atlantic, and it's then moved westward by the trade winds to South America, the Caribbean area and even Florida. But the dust that was detected by satellites over the eastern US in April had an even more distant source than Africa, it was carried all the way from eastern Asia.

It's not unusual for dust to be blown away from the arid regions of eastern Asia during the month of April. This has been observed many times by astronauts on the Space Shuttle. The dust comes from the Taklimakan Desert in western China, the Gobi desert in Mongolia, and the thick loess (dust laid down from thousands of years of storms) deposits of central China. When the annual rainfall is below about 10 inches, the vegetation density is sufficiently sparse to allow the wind to lift particles of dust.

When conditions are right (the ground is snow free and newly plowed, the surface and upper air winds are strong and the upper atmosphere circulation is zonal, moving west to east), dust from this region can travel half way around the globe. These conditions are most often met in early and mid spring. Because much of the Mongolia area has been particularly dry for the last year, it has been especially dusty, so vast quantities of dust were available to be removed from the surface once the wind started to blow.

In early April, over eastern Asia, the Total Ozone Mapping Spectrometer (TOMS), detected one of the largest dust clouds since it has been in operation more than 5-years ago. About 10-days later, when the cloud was visible over the central Rockies, it was estimated to be approximately 1,250 miles long and 4 miles thick. It thinned out somewhat when it reached the eastern US, and while hard to detect with our eyes, it was easily observable from satellite sensors looking in the ultraviolet wavelengths.

For more about this see the Earth Science Picture of the Day at: http://epod.usra.edu/archive/epodviewer.php3?oid=55648 and http://epod.usra.edu/archive/epodviewer.php3?oid=56192.