This week, skygazers across the upper United States, Canada and Europe have spotted a spate of shimmering, ghostly will-o’-the-wisps in the night sky. Only when the sun has disappeared below the horizon do the blue-silver stripes shine brightly and enchant the viewer with their beautiful, but somewhat eerie appearance.
These are not your everyday clouds.
Researchers say these noctilucent, or noctilucent, clouds are the rarest, driest, and tallest clouds on Earth. According to satellite data, the surge in recent activity has been different from the past 15 years. Other activities may take place this weekend.
“People in the northern US and Canada should definitely keep an eye out for noctilucent clouds over the long weekend,” Cora Randall, a professor at the University of Colorado Boulder, said in an email. “We are nearing the peak of the noctilucent cloud season, and barring exceptional events, they may occur over the northern mainland United States.”
The clouds are most common near the poles, but occasionally appear at lower latitudes as well. Rare and lively sightings have been reported from Oregon, Washington, Alberta, the United Kingdom and Denmark in the past few days. The best chance of seeing the clouds is to find a clear view near the horizon and face north.
“There really is nothing quite like it,” the Seattle office of the National Weather Service wrote on social media. Before sunrise on Friday, they tweeted photos of the “brightest display of noctilucent clouds” said to have been seen in the region in decades.
Noctilucent clouds, also known as polar mesospheric clouds, appear during summertime in each hemisphere at an altitude of about 50 miles in the layer of the atmosphere called the mesosphere. They form when water vapor gathers around patches of meteorite dust floating in the mesosphere and freezes, forming ice crystals.
These thin, wavy ice clouds glow light blue and white and typically appear at 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 from below.
“This season has been quite extraordinary over the past few days,” said Randall, who is also the principal investigator on NASA’s Aeronomy of Ice in the Mesosphere (AIM) mission, which is designed to study the noctilucent clouds. “The season started out as a fairly average season, but around the last week or so cloud counts have increased dramatically.”
She said the frequency of noctilucent clouds in recent days has been higher than what the AIM mission has observed in at least 15 years. But the reason is a bit mysterious.
Noctilucent clouds rely on two main components in the mesosphere: lots of water vapor and cold temperatures to help ice crystal formation. Randall and his colleague Lynn Harvey said data from the Microwave Limb Sounder on NASA’s Aura satellite showed that temperatures near the mesopause have been rising in recent days and are about average for this time of year. But water vapor concentrations are at a record high for this time of year in 15 years of observation.
“The increase in temperature would be unfavorable for clouds, but the increase in water vapor would be favorable,” Randall said.
Randall said one explanation for the increase in water vapor could be linked to rocket launches. Previously, researchers found that water vapor released from these missions can lead to the formation of noctilucent clouds. She said this statement was “preliminary” and her colleagues are currently investigating the lead.
Atmospheric scientist Matthew Deland said the January 15 eruption of the underwater volcano Hunga Tonga also pumped large amounts of water vapor into the stratosphere. The volcano even spewed out material 36 miles high and reached the mesosphere, setting a world record for the tallest volcanic plume on satellite records. However, he said it could take time to see the impact on cloud behavior.
“This season might be too early to have any impact,” said Deland, a scientist with Science, Systems, and Applications, Inc. at NASA. The “question is how long it takes for the water vapor in the atmosphere to move up into the region where the clouds are forming.”
Deland said the brisk cloud activity is unusual at lower latitudes like Seattle and isn’t sure it will continue for the rest of the season. He says it depends on circulation patterns and whether there are embedded waves that allow cold temperatures or additional water vapor to travel to lower latitudes. On rare occasions, the clouds have appeared at latitudes as low as London, central California and Oklahoma in recent years.
“They are truly remarkable to behold,” Deland said. “The clouds really just glow against the dark sky.”