Noctilucent, or “night-shining”, clouds are the rarest type of cloud and only occur after the Sun has dipped below the horizon or before it rises. Each summer, for just a couple of months, they make an appearance in the Northern Hemisphere sky, but this year, they are the most vibrant in 15 years so get outside and look up.
Noctilucent clouds appear in the sky as long, thin wisps that seem to shimmer and glow blue. They form in the upper part of Earth’s atmosphere, tens of kilometers above weather clouds in the mesosphere. Too thin to be seen in daylight, as the Sun drops below the horizon, the light continues to be reflected by the high-altitude clouds, illuminating them against the darkening sky.
They are seen between June and August in the Northern Hemisphere, usually above the poles, but a recent flurry of lovely images shared online shows they are visible across North America, Canada, and Europe right now. In fact, according to the Washington Post, they are the best seen in 15 years, with views ranging from Oregon to Denmark and the UK in the last week.
Considered by scientists to be the rarest, driest, and highest clouds, the clouds form when water vapor gathers on specks of meteorite dust in the mesosphere and freezes, forming ice crystals. They are so high in the atmosphere that they can continue to reflect the Sun's light long after it has set.
First recorded in 1885, just two years after the Krakatoa eruption spewed huge amounts of water vapor into the atmosphere, it was thought large events like this were needed for the clouds to appear. But they have noticeably been on the rise for many years, and many things are contributing to this.
A 2018 study concluded that the increased visibility of noctilucent clouds was down to humanity's impact on the climate. The extraction and burning of fossil fuels has released greenhouse gases into the atmosphere, including methane, which at high altitudes produces water vapor. Since 1871, the report found, there had been a 40 percent increase in water vapor 80 kilometers (50 miles) high at mid-northern latitude, meaning that there is now a good chance of seeing the clouds several times each summer.
“This season has been quite extraordinary in recent days,” Professor Cora Randall, principal investigator for the Cloud Imaging and Particle Size instrument on NASA’s Aeronomy of Ice in the Mesosphere (AIM) mission that studies noctilucent clouds, told the Post. “The season began as a rather average season, but in about the last week the cloud frequencies have increased dramatically.”
As to why they might be so prolific right now, according to Randall, temperatures in the mesopause are about average at the moment but water vapor is at a record high in 15 years of observations with AIM.
So get outside after the Sun has dipped down and look up!