27 FEBRUARY 2021
If you're in the right place at the right time, and you turn your eyes to the sky, you just might catch sight of something eerily beautiful: Wisps of cloud, high in the sky, glowing softly even though the Sun has already slipped below the horizon.
These are noctilucent, or night-shining, clouds, appearing at twilight primarily in the summer months, at high latitudes (but never observed from within the polar circle). They consist of collections of ice crystals high up in the atmosphere, at mesospheric altitudes between 76 and 85 kilometres (47 to 53 miles) above Earth's surface.
These wisps are too faint to be seen during the day, but after the Sun has sunk below the horizon, its last rays reach high into the atmosphere, even though the star itself is no longer visible from the planet's surface. There, these rays light up the clouds, causing them to glow against the darkening sky. For this reason, these glowing clouds are also known as polar mesospheric clouds.