Summer is the season to spot rare and luminescent Noctilucent Clouds. From May to early August, these ethereal clouds show their best displays thirty minutes after sunset or before sunrise.

Noctilucent translates from the Latin words “nocto,” meaning night, and “lucent,” meaning shining.

We are mid-season for the luminous tendrils of blue-silvery clouds that heighten in brightness as the brightest stars become visible. These are the highest of Earth’s clouds, forming 47 to 53 miles above the surface. They are also the least dense of clouds, wispier than Cirrus clouds, too thin to be seen in daytime. Only when the sun has dropped six degrees below the horizon is it dark enough for the tendrils to shine.

As with the mystery and majesty of the Northern Lights, atmospheric dynamics and nearness to the polar regions offer the best opportunities for sightings. Noctilucent clouds form in regions where the air is extremely dry and cold. At that great height, the air is coldest around the poles during summer. Warm air rises, expanding at high altitudes and cooling repeatedly in a process that can drop temperatures to -210 degrees Fahrenheit. Ice crystals latch onto dust particles and reflect angled sunlight in electric blue tints.

Once spotted every other year in the early 1900s, upticks in noctilucent cloud viewing have been linked to higher atmospheric methane levels and frequent rocket launches.