Dust Up

  • Published
  • By Melody Higdon
  • Technical Writer, Air Force Combat Climatology Center
Air Force Combat Climatology Center, Asheville, N.C. --  Forecasting dust storms is critical for our forces deployed to dry, arid regions, and involves more than just wind speeds.

Dust storms can occur anywhere there is loose soil, little to no vegetation and strong winds. This combination is mostly found in the world's deserts. In the Sahara, for example, sand dunes dominate and strong winds occur often. Without vegetation to hold it in place, sand and dust lift easily in the wind.

Another region with severe sand or dust storms is Central and Southwest Asia. There, soil is sand or sandy loess (in this case, fine, wind-blown clay). The desert plateaus of Afghanistan are major dust sources for the region. The surface soil, endlessly blown around by unrelenting winds, has been ground into a fine dust. This dust lifts easily and remains suspended in the air for a long time. Sand also lifts in the strong winds, but its effect is largely limited to the first 10-20 feet (3-6 meters) above the ground and the worst effects are limited to the first 2-3 feet (1 meter or less).

Dust storms occur in strong storms with well-defined cold fronts, particularly dry storms. These storms have strong winds that create the problems. Dust and sand lift ahead of 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. Behind cold fronts, even more dust and sand lifts as winds tend to be stronger behind the front than ahead of it. There must be more going on than straight-line winds. First, the ground must be dry. Besides surface winds strong enough to move particles, there must also be upward vertical motion. Upward vertical speed is roughly one fifth of the surface speed. The vertical speed determines how much particulate matter lifts. The larger the particles, the stronger the required wind. Below is a breakdown of wind speeds required to lift particles:

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: 30-35 mph (26.1-30.4 knots)

Well developed desert pavement: 40 mph (36.8 knots)

Rules of thumb for forecasting dust storms:

1. 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 with extreme storms have been documented as high as 35,000-40,000 feet.

2. Dust storms persist for days with stalled or cut-off lows to the southwest or northwest. Southwest storms are the more frequent of the two. Parent storms generally move northeastward but a few move east-southeastward before recurving northeastward.

3. Blowing dust occurs in a zone of maximum winds in the lower atmosphere with converging jet streams at 200-250 mb. Often, the polar and subtropical jets merge as a storm moves in from the west. The converged jets are typically in the southeastern or southern quadrants of the storm. The convergent area stacks vertically into the cooler air (generally northeastward) and the strongest winds and the most dust will occur there. Once a dust storm starts, it can maintain the same intensity even when wind speeds slow to below initiation levels. This is because the bond between the dust particles and the surface is broken and saltation allows dust to lift.

4. 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.

5. Because the air is so dry in deserts, there is a wide diurnal temperature difference. The rapid heat loss at night lowers the inversion and settles the dust. Dust storms generally subside soon after sunset. Western Afghanistan is a major source of dust in Southwest and Central Asia, and these regions can be monitored via satillite images.

6. Visibility forecasting is difficult; restrictions depend on 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, visibility quickly returns to 2-5 miles (3,200-8,000 meters). Visibility will remain at 4-6 miles (5,000-9,000 meters) in dust haze for days after a dust storm. Intense dust storms reduce visibility to near zero in and near source regions with visibility improving away from the source. Dust settles when winds drop below the speed necessary to carry the particles, but some level of dust haze persists nearly constantly in the summer.

7. Small particles restrict visibility more than large particles. In general, however, the worst visibility occurs within 20 feet (6 meters) of the surface. Above that, particles settle out into layers (by particle size) of progressively better visibility conditions. The largest particles are closest to the ground and settle out first. Slant range visibility is typically worse than straight-line visibility, and distortions, halos and coronas may also occur.