Afternoon clouds boil above Great Sand Dunes National Park in southern Colorado.
The dunes are formed by wind which funnels toward three mountain passes here - Mosca, Medano, and Music Passes. The sand accumulates in a natural pocket. The winds blow from the valley floor toward the mountains, but during storms the winds blow back toward the valley. These opposing wind directions cause the dunes to grow vertically
Canon EOS 50D, f/13 @ 200 mm, 1/400, ISO 200, No Flash
© Gary Grossman Photography