<p><strong>Each summer high-north residents are treated to noctilucent, or night-shining, <a href="http://science.nationalgeographic.com/science/earth/earths-atmosphere/clouds-article.html">clouds</a>, such as these pictured above Riga, <a href="http://travel.nationalgeographic.com/travel/countries/latvia-guide/">Latvia</a>, earlier this month—among the first observed this year.</strong></p><p>Too thin and wispy to be seen during the day, noctilucent clouds are high enough that the <a href="http://science.nationalgeographic.com/science/space/solar-system/sun-article/">sun</a>'s steeply raked post-sunset rays hit the clouds even after the ground has gone dark.</p><p>Forming from ice crystals, the rarely seen clouds waft through the mesosphere, slightly more than 50 miles (80 kilometers) above <a href="http://science.nationalgeographic.com/science/space/solar-system/earth/">Earth</a>'s surface—making them the world's highest known clouds.</p><p>(Related:<a href="http://news.nationalgeographic.com/news/2007/06/070629-clouds-picture.html"> "Mysterious 'Night-Shining Clouds' Sighted."</a>)</p><p><em></em></p><p>—Richard A. Lovett</p>

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

Photograph by Sergejs Babikovs, My Shot

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.

Read This Next

The science behind seasonal depression
These 3,000-year-old relics were torched and buried—but why?
How the Holocaust happened in plain sight

Go Further

Subscriber Exclusive Content

Why are people so dang obsessed with Mars?

How viruses shape our world

The era of greyhound racing in the U.S. is coming to an end

See how people have imagined life on Mars through history

See how NASA’s new Mars rover will explore the red planet

Why are people so dang obsessed with Mars?

How viruses shape our world

The era of greyhound racing in the U.S. is coming to an end

See how people have imagined life on Mars through history

See how NASA’s new Mars rover will explore the red planet

Why are people so dang obsessed with Mars?

How viruses shape our world

The era of greyhound racing in the U.S. is coming to an end

See how people have imagined life on Mars through history

See how NASA’s new Mars rover will explore the red planet