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Sand Dunes
Mon Feb 23, 2004
Listen in RealAudio 
You may think sand dunes are purely geological formations. But dunes are as much a child of
weather, as of earth. Hi, I'm Bryan Yeaton for The Weather Notebook.
To form sand dunes, you need three things: sand, steady winds, and obstacles such as
vegetation or rocks to stop the sand. Dunes appear stationary, but they actually migrate
downwind with geological slowness. The process starts when winds drive sand grains on a dune
windward slope by creep -- the rolling of sand grains -- or saltation -- the hopping of sand.
That's right -- hopping.
In saltation, Latin for leaping, wind lifts sand grains less than an inch off the ground,
tossing them several inches downwind. On returning to the sandy surface, a saltating sand
grain strikes another grain, sending both airborne. The process continues in a chain reaction
until the wind diminishes, unable to loft the sand.
Grains that cross the dune ridge roll down the opposite slope. Sand accumulating at the crest
eventually becomes unstable and avalanches downslope. Over time, perhaps decades, a new dune
forms the dune crest crawling downwind.
Dunes form several common shapes, which reflect the local wind-direction variability.
Barchan dunes are crescent-shaped, formed where winds are constant and the sand supply
moderate. When sand is abundant, barchan dunes merge to become transverse dunes, resembling
large sand ripples. Linear dunes, long and straight, arise where sand supplies are limited and
wind directions converge.
Shifting dunes may engulf houses and roads, requiring continuous removal. But, along
coastlines, they can continually replenish that sandy beach.
Thanks to our contributing writer, meteorologist Keith Heidorn We are funded by Subaru and the
National Science Foundation.
Today's Links
Coastal Protection International
http://www.coastalprotection.com/solutions.phtml
The Life & Love of Sand Dunes
http://www.desertusa.com/magjan98/dunes/jan_dune1.html
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