Some unusual clouds were spotted over parts of the western United States
this summer. Those who have seen them say they’re strangely beautiful;
they’re visible only at night, and appear silvery-white and luminous.
They’re called noctilucent (knock-til-loo-cent) clouds. But as the Great
Lakes Radio Consortium’s Wendy Nelson reports, the clouds may be a sign that
all’s not well in the upper atmosphere: