In 1885, two years after the cataclysmic Krakatoa eruption, many onlookers reported luminous, mesmerizing clouds adrift in twilight skies. As far as historians know, these accounts were the first documented sightings of noctilucent — or "night shining" — clouds.
Thin, tufted and faint, the night shiners aren't very dramatic shape-wise. But other qualities really make them stand out. The color scheme of noctilucent clouds usually ranges from electric blue to silver, although they can also look reddish-orange. And as the term "night shining" implies, the rippling wisps glow in the dark.
By and large, noctilucent clouds historically have been observed near the poles, between latitudes 50 and 70 degrees on either side of the equator. Yet, over the past two decades or so, they've expanded their range and made appearances at lower-latitude places — such as France and Kansas — which never previously got to witness them.
Noctilucent clouds are only visible shortly before sunrise or just after sunset. Although we still have much to learn about them, astronomers do know that they're restricted to a specific portion of the space above our heads. If you were to board a rocket ship and fly it straight upwards, you'd eventually hit the mesosphere. The third layer in the atmosphere, this expanse begins 31 miles (50 kilometers) above the planet's surface. Its uppermost boundary is a region called the mesopause, which is located 28 miles (35 kilometers) still higher off the ground.
Noctilucent clouds form at these altitudes when drifting particles — including dust left behind by meteors — become coated with ice crystals at low temperatures. When the sun is 6 to 16 degrees below the horizon (as it is 30 to 60 minutes before it rises or after it sets), solar rays hit them at an angle that makes the light scatter and the clouds shine. This renders said clouds visible to the naked eye.
However, according to the World Meteorological Organization, none of that can happen unless the temperatures in the mesosphere dip below approximately —184 degrees F (—120 degrees C). Only then will there be enough ice crystals up there to cover orbiting debris, kicking off the whole illumination process.
Due to the way heated, ground-level air rises, expands, and cools, the mesosphere above either pole is at its coldest during the local summer. Hence noctilucent clouds are mainly seen from November to February south of the equator and between early May and late August in the northern hemisphere.
Humanity's methane emissions over the past century or so have led to increased water vapors up in the mesosphere. As a result, night-shining clouds have been getting brighter. That may sound like good news for stargazers, but environmentalists see it as a harrowing sign of the times. To quote a July 3, 2018 paper on the subject from Yale Environment 360, noctilucent clouds are currently "a long-term indicator for climate change."