These might look a little like the northern lights, but they are, in fact, an utterly different atmospheric phenomenon known as nacreous clouds. Sometimes referred to as mother-of-pearl clouds, they form at relatively high altitudes, as far as 15 miles above sea level. That means that even after dusk, these clouds remain high enough to be illuminated by the sun.

The clouds are not uncommon in the polar regions, but they are rarely seen in the middle latitudes, as they are here, photographed in Colorado in 2006.