The Dunes lie in a high valley surrounded by 13,000-foot peaks of the Sangre de Cristo range.
Air around the peaks is notoriously unstable; weather has a habit of appearing and disappearing mysteriously.

When this group started up the face of the dunes it was likely hot, sunny and cloudless.
But by three o'clock isolated thunderstorms formed above the San Luis Valley,
and by four it was a general melee across a forty-mile front.

Those people were up there to do some sand sledding.