Rippled Sand Dunes, Death Valley National Park, California
All it takes is a bit of breeze (16 kilometers/hour or 10 miles/hour) to whisk fine sand into motion. The grains may be suspended in the air, bounce along, or nudged along by impacts from bouncing grains, depending upon the grain size and wind strength. Almost all blowing sand remains within a meter of the surface as it migrates. Once sand begins to pile up, ripples and dunes can form. Wind continues to move sand up to the top of the pile until the pile is so steep that it collapses under its own weight. The collapsing sand comes to rest when it reaches just the right steepness to keep the dune stable. This angle, usually about 30-34°, is called the angle of repose. Every pile of loose particles has a unique angle of repose, depending upon the properties of the material it's made of. The repeating cycle of sand inching up the windward side to the dune crest, then slipping down the dune's slip face allows the dune to inch forward, migrating in the direction the wind blows. All of this climbing then slipping leaves its mark on the internal structure of the dune.
Size: 3872px × 2592px
Location: Mesquite Flat, Death Valley National Park, California
Photo credit: © Robert Bush / Alamy / Afripics
License: Licensed
Model Released: No
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