4.4.6.2.2 Preventative Measures
One possible way to reduce the amount of dry deposition in the covered areas from blowing sand and dust is to cover the source of the particulate matter. This requires answering the question, how much of the blowing material originates from nearby a particular area and how much is transported from hundreds of miles away. The use of “snow” fences as physical barriers to stop this material from accumulating en masse near the covered area might be helpful.
A review of what sand storms are and how they are created is helpful in evaluating this as an option as well as in planning for surface cleaning. First, the term “sand storm” is a misnomer. The particles of soil that are blown around by desert winds are actually fine soil particles 2-4 microns in diameter, not large grains of sand (150). A more accurate term is “dust storm.” These fine particles are formed as a result of a combination of factors including desertification, droughts, poor land management and the fracturing of the delicate crust-like surface of the desert by vehicular traffic (151).
Dust or sand storms are caused by the outflow from low-pressure cells passing through a desert area from west to east. They can last for hours or days and cover small areas or entire countries, e.g., Kuwait was completely enveloped in such a storm in February 2003 (152, 153). Lack of rainfall and warm temperatures during winter months set up conditions for spring sand storms by drying out the soil, producing fine particles easily swept up by winds. When winds reach 60 mph, a major sand storm is the result.
There are three categories of wind blown dust. The floating dust leftover from previous storms stays airborne without any wind. Floating sand from strong winds and sand and dust from stronger winds make up the rest (154).
In China, the dust storms, called “shachenbao” most often occur in the spring and winter and in the country’s northeast, north and northwest during 10-75 days of winds blowing 2-4 m/sec, about 8 mph (154, 155). In Iraq, in late spring and summer, hot dry winds from the north called “shamals” can produce gusts of 85 mph, raising clouds of sand and dust to several thousand feet (156, 157). Iraq has 20-25 such storms annually (152).
Sand storms can occur throughout the year in the Middle East, but the prime months are May-September when hot, dry air from the Arabian Peninsula blows in as the “As Somoum” wind (153).
The dust from storms originating in Africa and Asia can travel halfway around the world, as far as North and South America (158).
Efforts to reduce sand storms have focused on restoring vegetation in the areas where the loose soil is found (158-160).
In addition to sand storms, there are localized windstorms called dust devils. Large dust devils, which resemble mini tornados, can move as much as 9 tons of dust during their life cycle. In studying the creation of dust devils and sand storms, researchers have learned that when large particles impact on the surface, they cause smaller ones to become airborne, exacerbating the effect (161). This can be seen in the formation of dust devils.
Dust devils form as a result of temperature gradients where patches of relatively cold and warm earth adjoin (162). The dust devils can then begin to raise dust into the atmosphere. This dust begins to create additional temperature gradients which eventually move enough air to raise sand particles that when impacting the ground, raise dust particles. So even though the wind at the surface may be calm, these temperature gradients can feed a developing dust or sand storm.
