Dust storms occur anywhere there is loose soil, little to no vegetation and strong winds. This combination is mostly found in desert areas around the world. In the Sahara Desert, for example, sand dunes dominate the terrain and strong winds occur often. Without vegetation to hold it in place, sand and dust lift easily and are carried down wind. The American Southwest has deserts that get dust storms on occasion, and sand dunes in the White Sands area are infamous for burying roads and gradually scouring telephone poles into nothing.
Another region vulnerable to sand or dust storms is Central and Southwest Asia. There, soil is primarily sand or sandy loess (in this case, fine, wind-blown clay). The sand and loess are saline, silicate-based, and often, alkaline. The desert plateaus of Afghanistan are major sources of dust for the whole region. The surface layers of sand and loess, endlessly blown around by the unrelenting winds in this region, have been ground down into an extremely fine dust at the surface layer.
This dust lifts easily and remains suspended in the air for a long time. Sand also lifts in the strong winds, of course, but its effect is largely limited to the first 10-20 feet (3-6 meters) above ground and the worst effects are limited to the first 2-3 feet (1 meter or less). The Hindu Kush (mountain) range is barren of soil except where wind-borne dust has settled over thousands of years.
Why Dust Storms are a Problem
Dust and sandstorms play havoc with human activity. They not only restrict visibility to varying degrees, they are highly destructive. Dust, especially the fine, powdery dust typical in Southwest and Central Asia, gets into the tiniest openings to contaminate water and food even through containers considered impermeable.
Sand and dust scours surfaces and wears away protective coverings. Glass and other transparent surfaces quickly become frosted. Wire wrap wears away and electrical circuits ground out. Textile materials wear quickly and ropes/guy wires must be replaced often.
Dust gets into electronics and machinery, and renders some weapons useless. The more sophisticated a system (weapon or otherwise) is, the more dust effects it. Additionally, dust compacts easily, solidifies into a hardened mass with the slightest moisture added, and easily combines with lubricants to become a superior grinding powder. At times, extreme measures must be taken to protect stores and equipment.
Dust storms and sandstorms set up electrostatic discharges that, while not fatal of themselves, can produce a spark or electrical shock that can have negative consequences in fueling operations, computer or electrical systems, etc. People are reluctant to touch things when they know a heavy shock might result.
How Dust Storms Occur
Dust storms occur with strong low-pressure (storm) systems with well-defined cold fronts, particularly dry systems. These storms have strong winds associated and the winds create the problems. Dust and sand start to lift ahead of the cold fronts as winds increase in tightening gradient. Oddly, the finest particles are not the first to lift; small sand particles of the “favored” size are moved on the surface first in a process called saltation.
In saltation, wind-driven particles bounce along the ground, thus break the surface tension (which holds the particles together), and lift more easily. Behind the cold fronts, dust and sand are lifted even more as winds tend to be stronger behind the front than ahead of it. The stronger the cold front, the stronger the winds and the greater the sand/dust load they carry aloft.
Before sand or dust can be lifted, there must be more going on than simple straight-line winds. First, of course, the ground must be dry enough. Besides surface winds strong enough to move particles of sand or dust, there must also be sufficient upward vertical motion as well. Upward vertical motion speed can be calculated at roughly one fifth of the surface wind speed.
It is the vertical speed that determines the amount of particulate matter lifted. The larger the particles, the stronger the required wind. Additionally, the larger the particle, the shorter distance it lifts vertically or is carried horizontally.
Below is a general breakdown of wind speeds required to lift particles of varying sizes.
|Particle Size||Wind Speed|
|Fine to medium sand in dune covered areas||10-15 MPH (8.7-13 knots)|
|Sandy Areas with poorly developed desert pavement||20 MPH (17.4 knots)|
|Fine material, desert flats||20-25 MPH (17.4-21.7 knots)|
|Alluvial fans and crusted sabkhas (playas)||30-35 MPH (26.1-30.4 knots)|
|Well developed desert pavement||40 MPH (36.8 knots)|
|Note: A sabkha (also spelled sabka) is an area of crusted-over sand and dirt, generally found in wadis. Once the crust is disturbed, dust is easily lifted from these sabkhas.|
General Rules of Thumb for Forecasting Dust Storms
- The average height of a dust storm is 3,000-6,000 feet and stronger storms have dust to 8,000-10,000 feet. Haze and dust associated with extreme storms, however, have been documented as high as 35,000-40,000 feet.
- Dust storms persist for days with stalled or cut-off lows to the southwest or northwest with southwest storms the more frequent of the two. The parent storm systems generally move northeastward across and then out of the area but a few move east-southeastward before recurving northeastward once again.
- Blowing dust occurs in a zone of maximum winds in the lower atmosphere associated with converging jet streams at 200-250 mb. Often, the polar and subtropical jets merge as a low-pressure system moves into the region from the west. The converged jets are typically in the southeastern or southern quadrants of the low. The convergent area stacks vertically into the cooler air (generally northeastward) and the strongest winds and the most dust will occur in this area. Once a dust storm starts, it will continue at the same dust intensity even with lower wind speeds than were needed to get it started. This is because the bond between the dust particles and the surface is broken and saltation allows dust to lift.
- Summer dust storms have greater vertical motion over a larger area due to high temperatures and resultant convective currents. Additionally, frontal inversions that cap dust storms tend to be higher in summer as well.
- Because the air is so dry in desert areas, there is a wide diurnal temperature difference between day and night. The rapid heat loss at night lowers the inversion and settles the dust. Dust storms generally subside at the source soon after sunset. Dust sources are easily identifiable on satellite photos. The western half of Afghanistan is considered a major source in Southwest and Central Asia.
- Visibility forecasting is difficult; restrictions depend on too many variables. On the edges of blowing dust and within 150 NM downstream, visibility is 1/2-3 miles (800-4,800 meters). Beyond that, and as dust settles, visibility returns to 2-5 miles (3,200-8,000 meters) fairly quickly. Visibility will often remain at 4-6 miles (5,000-9,000 meters) in dust haze for days after a dust storm. In summer, dust haze persists nearly constantly. Intense dust storms reduce visibility to near zero in and near source regions and visibility is progressively better with distance from the source. Dust will settle when the winds die down to below the speed necessary to carry the particles (generally considered to be 15 knots or less).
- Particle size is not necessarily a guide for the amount of visibility restriction that occurs. Small particles of the right composition restrict visibility more than large particles constructed of something else. In general, however, the worst visibility occurs within 20 feet (6 meters) of the surface and above that, particles begin to settle out into layers (by particle size) of progressively better visibility conditions. (The largest particles are closest to the ground and settle out the earliest.) Slant range visibility is typically worse than straight-line visibility. Distortions, halos and coronas can also occur.
- Dust devils, although small, can be teacup tempests. These tiny tornado-like whirlwinds lift sand dust and small objects into the air. They tend to move generally upslope and have average wind speeds of 15-25 knots but have been clocked as strong as 50 knots. Most common in March-October, they can occur at any time of year under the right conditions and peak in July and August. Overall weak synoptic-scale winds, warm temperatures (80F/27C or hotter) and clear to fair skies are required for dust devils to develop. These tiny whirlwinds are easily evaded or can be safely waited out with the proper precautions taken.
Written by Melody L. Higdon, 14 WS/DOPA