Rippling Over Riga
Each summer high-north residents are treated to noctilucent, or night-shining, clouds, such as these pictured above Riga, Latvia, earlier this month—among the first observed this year.
Too thin and wispy to be seen during the day, noctilucent clouds are high enough that the sun's steeply raked post-sunset rays hit the clouds even after the ground has gone dark.
Forming from ice crystals, the rarely seen clouds waft through the mesosphere, slightly more than 50 miles (80 kilometers) above Earth's surface—making them the world's highest known clouds.
(Related: "Mysterious 'Night-Shining Clouds' Sighted.")
—Richard A. Lovett
Pictures: First Night-Shining Clouds of 2012
Rippling clouds glow against dark night skies in one of summer's strangest sights, which may be getting more common as Earth warms.