I got my first lesson in the physics of sand dunes in ... we found a 50-foot-tall barchan to give us some shelter for the night. We awoke before dawn. The wind on the dune crest had died down ...

Related: See all the best images of Earth from space A group of five barchan, or "horned," sand dunes can be seen in a zoomed-in version of the 2013 photo (see below). Over time, these arrowhead ...