Rippling Over Riga
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