The Washington PostDemocracy Dies in Darkness

Rare clouds that glow in the dark are the most vibrant in 15 years

Sky watchers in Oregon, Washington, England and Canada have spotted “noctilucent clouds,” the rarest on Earth

Oliver Schwenn captured this image of noctilucent clouds on Thursday in Aarhus, Midtjylland, Denmark. (Oliver Schwenn/Spaceweathergallery)

This week, sky watchers in the upper United States, Canada and Europe have spotted a flurry of shimmering, ghostlike wisps in the night sky. The blue-silver streaks shine brightly only after the sun has disappeared beyond the horizon, entrancing viewers with their beautiful yet somewhat eerie appearance.

These are not your everyday clouds.

Researchers say these noctilucent, or night-shining, clouds are the rarest, driest and highest clouds on Earth. The uptick of recent activity has been unlike any seen in at least the past 15 years, according to satellite data. More activity could be on the way this weekend.

“Folks in the northern U.S. and Canada should absolutely be on the lookout for noctilucent clouds over the long weekend,” Cora Randall, a professor at the University of Colorado Boulder, said in an email. “We are near the peak of the noctilucent cloud season, and even in the absence of extraordinary events, they can appear over the northern continental U.S.”

The clouds most commonly occur near the poles but occasionally appear at lower latitudes as well. Rare and vibrant sightings have been reported from Oregon, Washington, Canada, Britain and Denmark in the past few days. The best chance to see the clouds is to find a clear view close to the horizon and look north.

“There’s really nothing else quite like them,” the National Weather Service office in Seattle wrote on social media. Before sunrise on Friday, they tweeted photos of the “most vivid displays of noctilucent clouds” claimed to be seen in decades in the area.

Noctilucent clouds, also known as polar mesospheric clouds, appear during the summertime in each hemisphere at about 50 miles high in the layer of the atmosphere called the mesosphere. They form when water vapor congregates around specks of meteor dust floating in the mesosphere and freezes, forming ice crystals.

These thin, wavy icy clouds glow bright blue and white and typically appear around dusk or dawn. Unlike other clouds, they form so high in the atmosphere that they can continue to reflect sunlight after the sun dips below the horizon, illuminating the clouds from below.

“This season has been quite extraordinary in recent days,” said Randall, who is also the principal investigator for the Cloud Imaging and Particle Size instrument on NASA’s Aeronomy of Ice in the Mesosphere (AIM) mission that was designed to study the night-shining clouds. “The season began as a rather average season, but in about the last week the cloud frequencies have increased dramatically.”

She said the frequency of noctilucent clouds in the past few days has been higher than ever observed in at least 15 years of observations by the AIM mission. Yet the reason is a bit of mystery.

Noctilucent clouds rely on two main ingredients in the mesosphere: plenty of water vapor and cold temperatures to aid ice crystal formation. Randall and colleague Lynn Harvey said data from the Microwave Limb Sounder on NASA’s Aura satellite showed temperatures near the mesopause increased in the past few days and are about average for this time of year. But water vapor concentrations are at a record high for this time of the year in 15 years of observations.

“The increase in temperature would be unfavorable for clouds, but the increase in water vapor would be favorable,” said Randall.

Randall said one explanation for the increase in water vapor could be tied to rocket launches. Previously, researchers found water vapor released from these missions can lead to the formation of noctilucent clouds.

At least 16 rockets were launched in June, “any of which could have been responsible for some clouds forming,” said Michael Stevens at the Naval Research Laboratory. He said two launches from the United States on June 18 and 19 are probably the best candidates for contributing to the most recent uptick in cloud sightings, as it can take up to 10 days for the water vapor plume from the rocket to reach the latitudes where the clouds form. This explanation is preliminary, however, and would require a quantitative analysis to confirm.

Atmospheric scientist Matthew Deland said the eruption from the underwater volcano Hunga Tonga on Jan. 15 also pumped large quantities of water vapor into the stratosphere. The volcano even spewed material 36 miles high and reached the mesosphere, setting a world record of the highest volcano plume on satellite record. However, he said it might take some time to see the effect on the behavior of the clouds.

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“This season might be too soon to get some impacts,” said Deland, a scientist with Science, Systems, and Applications, Inc. at NASA. “[The] question becomes how long does it take to transport the water vapor upward in the atmosphere to the region where the clouds form.”

Deland said the vibrant cloud activity at lower latitudes like Seattle is uncommon and is not sure if it will persist for the rest of the season. He said it depends on circulation patterns and if there are embedded waves that allow cold temperatures or extra water vapor to get drawn down to lower latitudes. On rare occasions in recent years, the clouds have appeared in latitudes as low as London, central California and Oklahoma.

“They’re really remarkable to see,” said Deland. “The clouds really just shine against the dark sky.”