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This week, sky watchers across the upper United States, Canada and Europe spotted a flurry of glowing ghostly plumes in the night sky. The blue-silver streaks shine brightly only after the sun has hidden behind the horizon, captivating onlookers with their beautiful yet somewhat eerie appearance.
These are not your everyday clouds.
Researchers say these nocturnal or nocturnal clouds are the rarest, driest and highest clouds on Earth. The recent increase in activity is unlike any seen in the past 15 years, according to satellite data. There could be more activity this weekend.
“People in the northern US and Canada should absolutely watch for clear 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 nocturnal cloud season, and even in the absence of extreme events, they may occur over the northern continental US.”
Clouds most often occur near the poles, but sometimes occur at lower latitudes. Rare and vivid sightings have been reported from Oregon, Washington, Canada, Great Britain and Denmark over the past few days. The best chance to see the clouds is to find a clear view near the horizon and look north.
“There really is nothing else like them,” the National Weather Service office in Seattle wrote on social media. Before sunrise on Friday, they tweeted pictures of the “brightest display of translucent clouds” said to have been seen in decades in the area.
Nostilucent clouds, also known as polar mesospheric clouds, occur during the summer in each hemisphere about 50 miles high in the layer of the atmosphere called the mesosphere. They form when water vapor collects around patches of meteoric dust floating in the mesosphere and freezes to form ice crystals.
These thin, wavy ice clouds glow bright blue and white and usually 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 has dipped below the horizon, illuminating the clouds below.
“This season has been pretty unusual over the last few days,” said Randall, who is also the principal investigator for NASA’s Aeronomy of Ice in the Mesosphere (AIM) mission, which is designed to study nocturnal clouds. “The season started off as a pretty average season, but in the last week or so the cloud frequency has increased dramatically.”
She said the frequency of luminescent clouds in the past few days was higher than ever seen in at least 15 years of observations by the AIM mission. Yet the reason is a bit mysterious.
Noctilucent clouds rely on two main ingredients in the mesosphere: abundant water vapor and cold temperatures to support the formation of ice crystals. Randall and colleague Lynn Harvey said data from the Microwave Limb Sounder on NASA’s Aura satellite show that temperatures near the mesopause have increased over the past few days and are about average for this time of year. But water vapor concentrations are a record high for this time of year in 15 years of observations.
“Increasing temperature would be unfavorable for clouds, but increasing water vapor would be favorable,” Randall said.
Randall said one explanation for the increase in water vapor could be related to rocket launches. Researchers have previously detected water vapor emitted by these missions can lead to the formation of nocturnal clouds.
At least 16 missiles were fired in June, “any of which could have been responsible for the cloud formation,” said Michael Stevens of the Naval Research Laboratory. He said two launches from the United States on June 18-19 were likely the best candidates for contributing to the most recent increase in cloud sightings because it could take up to 10 days for the rocket’s water vapor plume to reach latitudes , where the clouds form. However, this explanation is preliminary and requires quantitative analysis to confirm.
Atmospheric scientist Matthew DeLand said the Jan. 15 eruption of the Hunga Tonga submarine volcano also pumped large amounts of water vapor into the stratosphere. The volcano even spewed material 36 miles high and reached the mesosphere, setting a world record for the tallest volcanic column in the satellite record. However, he said it may take some time to see the effect on cloud behavior.
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“This season may be too early to have any impact,” said DeLand, a scientist at Science, Systems, and Applications, Inc. at NASA. “The question is how long it takes for the water vapor to be transported up in the atmosphere to the region where the clouds form.”
DeLand said brisk cloud activity at lower latitudes like Seattle is unusual, and he’s not sure if it will continue through the rest of the season. He said it depends on circulation patterns and whether there are built-in waves that allow cold temperatures or extra water vapor to be 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 watch,” DeLand said. “The clouds really just sparkle against the dark sky.”
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