Particles are transported by winds through suspension, saltation, and
creep.
Small particles may be held in the atmosphere in _suspension_. Upward
currents of air support the weight of suspended particles and hold them
indefinitely in the surrounding air. Typical winds near the Earth’s
surface suspend particles less than 0.2 millimeters in diameter and
scatter them aloft as dust or haze.
_Saltation_ is downwind movement of particles in a series of jumps or
skips. Saltation normally lifts sand-size particles no more than one
centimeter above the ground, and proceeds at one-half to one-third the
speed of the wind. A saltating grain may hit other grains that jump up
to continue the saltation. The grain may also hit larger grains that are
too heavy to hop, but that slowly _creep_ forward as they are pushed by
saltating grains. Surface creep accounts for as much as 25 percent of
grain movement in a desert.
Eolian turbidity currents are better known as _dust storms_. Air over
deserts is cooled significantly when rain passes through it. This cooler
and denser air sinks toward the desert surface. When it reaches the
ground, the air is deflected forward and sweeps up surface debris in its
turbulence as a dust storm.
[Illustration: Saltation moves small particles in the direction of the
wind in a series of short hops or skips.]
Crops, people, villages, and possibly even climates are affected by dust
storms. Some dust storms are intercontinental, a few may circle the
globe, and occasionally they may engulf entire planets. When the Mariner
9 spacecraft arrived at Mars in 1971, the entire planet was enshrouded
in global dust.
[Illustration: Dust storm along the Mohave River near Daggett,
California, October 24, 1919 (photograph by D. G. Thompson).]
Most of the dust carried by dust storms is in the form of silt-size
particles. Deposits of this windblown silt are known as _loess_. The
thickest known deposit of loess, 335 meters, is on the Loess Plateau in
China. In Europe and in the Americas, accumulations of loess are
generally from 20 to 30 meters thick.
Small whirlwinds, called _dust devils_, are common in arid lands and are
thought to be related to very intense local heating of the air that
results in instabilities of the air mass. Dust devils may be as much as
one kilometer high.