Page 553 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

Chapter 16 Oceans, Coastal Systems, and Wind Processes 517
(a) Wind-sculpted tree near South Point, Hawai`i. Nearly constant tradewinds keep this tree naturally pruned.
▲Figure 16.24 The work of wind. [Bobbé Christopherson.]
Since the viscosity and density of air are much lower than those of other transporting agents such as water and ice, the ability of wind to move materials is correspond- ingly weaker. Yet, over time, wind accomplishes enor- mous work. Consistent local wind can prune and shape vegetation and sculpt snow surfaces (Figure 16.24).
Eolian Transport of Dust and Sand
Just like water in a stream picking up sediment, wind exerts a drag, or frictional pull, on surface particles until they become airborne. Grain size, or particle size, is important in wind erosion. Intermediate-sized grains move most easily, whereas movement of the largest and the smallest sand particles requires the strongest winds. Stronger wind is needed for the large particles because
(b) Wind-eroded snow, called sastrugi, usually forms irregular grooves or ridges that are parallel to the wind direction.
they are heavier, and for the small particles because they are mutually cohesive and because they usually present a smooth (aerodynamic) surface that minimizes frictional pull. Eolian processes only work on dry surface materi- als, since wet soils and sediments are too cohesive for movement to occur.
The distance that wind is capable of transporting par- ticles in suspension also varies greatly with particle size (for comparison, Figure 15.13 shows the transport of stream sediment in suspension, a similar process). The finer ma- terial suspended in a dust storm is lifted much higher than are the coarser particles of a sandstorm (Figure 16.25). Thus, the finest dust particles travel the farthest dis- tances. As discussed in previous chapters, atmospheric circulation can transport fine material, such as volcanic debris, fire soot and smoke, and dust, worldwide within days (please review Chapter 3, Figure 3.7, and Chapter 6, Figure 6.1). In some arid and semiarid regions, winds
Wind direction
Impact
cm 10
Suspension 8 Saltating grains
Grains bounce high off pebbles
6
4
Surface 2 creep
Impact 0
(a) Eolian suspension, saltation, and surface creep are transportation mechanisms.
▲Figure 16.25 How the wind moves sand. [(b) Robert Christopherson.]
Animation
How Wind Moves Sand
(b) Sand grains saltating along the surface in the Stovepipe Wells dune field, Death Valley, California.
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