A dust storm or sand storm is a meteorological phenomenon common in arid and semi-arid regions.
Dust storms arise when a gust front or other strong wind blows loose sand and dirt from
a dry surface. Particles are transported by saltation and suspension, a process that moves
soil from one place and deposits it in another. Drylands around North Africa and the Arabian
peninsula are the main terrestrial sources of airborne dust. Also with some contributions
from Iran, Pakistan and India into the Arabian Sea, and China's significant storms deposit
dust in the Pacific. It has been argued that recently, poor management of the Earth's drylands,
such as neglecting the fallow system, are increasing dust storms size and frequency
from desert margins and changing both the local and global climate, and also impacting
local economies. The term sandstorm is used most often in the
context of desert sandstorms, especially in the Sahara Desert, or places where sand is
a more prevalent soil type than dirt or rock, when, in addition to fine particles obscuring
visibility, a considerable amount of larger sand particles are blown closer to the surface.
The term dust storm is more likely to be used when finer particles are blown long distances,
especially when the dust storm affects urban areas.
Causes
As the force of wind passing over loosely held particles increases, particles of sand
first start to vibrate, then to saltate. As they repeatedly strike the ground, they loosen
and break off smaller particles of dust which then begin to travel in suspension. At wind
speeds above that which causes the smallest to suspend, there will be a population of
dust grains moving by a range of mechanisms: suspension, saltation and creep.
A recent study finds that the initial saltation of sand particles induces a static electric
field by friction. Saltating sand acquires a negative charge relative to the ground which
in turn loosens more sand particles which then begin saltating. This process has been
found to double the number of particles predicted by previous theories.
Particles become loosely held mainly due to drought or arid conditions, and varied wind
causes. Gust fronts may be produced by the outflow of rain-cooled air from an intense
thunderstorm. Or, the wind gusts may be produced by a dry cold front, that is, a cold front
that is moving into a dry air mass and is producing no precipitation—the type of dust
storm which was common during the Dust Bowl years in the U.S. Following the passage of
a dry cold front, convective instability resulting from cooler air riding over heated ground
can maintain the dust storm initiated at the front.
In desert areas, dust and sand storms are most commonly caused by either thunderstorm
outflows, or by strong pressure gradients which cause an increase in wind velocity over
a wide area. The vertical extent of the dust or sand that is raised is largely determined
by the stability of the atmosphere above the ground as well as by the weight of the particulates.
In some cases, dust and sand may be confined to a relatively shallow layer by a low-lying
temperature inversion. In other instances, dust may be lifted as high as 20,000 feet
high. Drought and wind contribute to the emergence
of dust storms, as do poor farming and grazing practices by exposing the dust and sand to
the wind. One poor farming practice which contributes
to dust storms is dryland farming. Particularly poor dryland farming techniques are intensive
tillage or not having established crops or cover crops when storms strike at particularly
vulnerable times prior to revegetation. In a semi-arid climate, these practices increase
susceptibility to dust storms. However, soil conservation practices may be implemented
to control wind erosion.
Physical and environmental effects
A sandstorm can transport large volumes of sand unexpectedly. Dust storms can carry large
amounts of dust, with the leading edge being composed of wall of thick dust as much as
1.6 km high. Dust and sand storms which come off the Sahara Desert are locally known as
a simoom or simoon. The haboob is a sandstorm prevalent in the region of Sudan around Khartoum,
with occurrences being most common in the summer.
The Sahara desert is a key source of dust storms, particularly the Bodélé Depression
and an area covering the confluence of Mauritania, Mali, and Algeria.
Saharan dust storms have increased approximately 10-fold during the half-century since the
1950s, causing topsoil loss in Niger, Chad, northern Nigeria, and Burkina Faso. In Mauritania
there were just two dust storms a year in the early 1960s, but there are about 80 a
year today, according to Andrew Goudie, a professor of geography at Oxford University.
Levels of Saharan dust coming off the east coast of Africa in June were five times those
observed in June 2006, and were the highest observed since at least 1999, which may have
cooled Atlantic waters enough to slightly reduce hurricane activity in late 2007.
Dust storms have also been shown to increase the spread of disease across the globe. Virus
spores in the ground are blown into the atmosphere by the storms with the minute particles then
acting like urban smog or acid rain.
Prolonged and unprotected exposure of the respiratory system in a dust storm can also
cause silicosis which, if left untreated, will lead to asphyxiation; silicosis is an
incurable condition that also may lead to lung cancer. There is also the danger of keratoconjunctivitis
sicca which, in severe cases without immediate and proper treatment, can lead to blindness.
Economic impact Dust storms cause soil loss from the dry lands,
and worse, they preferentially remove organic matter and the nutrient-rich lightest particles,
thereby reducing agricultural productivity. Also the abrasive effect of the storm damages
young crop plants. Dust storms also reduced visibility affecting aircraft and road transportation.
In addition dust storms also create problems due to complications of breathing in dust.
Dust can also have beneficial effects where it deposits: Central and South American rain
forests get most of their mineral nutrients from the Sahara; iron-poor ocean regions get
iron; and dust in Hawaii increases plantain growth. In northern China as well as the mid-western
U.S., ancient dust storm deposits known as loess are highly fertile soils, but they are
also a significant source of contemporary dust storms when soil-securing vegetation
is disturbed. Extraterrestrial dust storms
Dust storms are not limited to Earth and have been known to form on other planets such as
Mars. These dust storms are much larger and more powerful than dust storm here on Earth
with wind speeds topping 100 miles per hour. Martian dust storms are formed when solar
heating warms the Martian atmosphere and causes the air to move, lifting dust off the ground.
The chance for storms is increased when there are great temperature variations like those
seen at the equator during the Martian summer.
See also List of dust storms with visibility of 1/4
mile or less, or meters or less
References
External links 12-hour U.S. map of surface dust concentrations
Mouse-over an hour block on the row for 'Surface Dust Concentrations'
Slide Show of a Dust Storm in Lubbock, Texas on December 15, 2003
Dust in the Wind Video of a dust storm in Al Asad Iraq, April
27, 2005 Photos of the April 14 1935 and September
2 1934 dust storms in the Texas Panhandle hosted by the Portal to Texas History.
The Bibliography of Aeolian Research University of Arizona Dust Model Page
Photos of a sandstorm in Riyadh in 2009 from the BBC Newsbeat website
Dust storm in Phoenix Arizona via YouTube