This week, sky watchers in the upper United States, Canada and Europe spotted a flurry of shimmering, ghostly lights in the night sky. The blue-silver streaks shine brightly only after the sun has disappeared beyond the horizon, mesmerizing onlookers with their beautiful yet somewhat eerie appearance. These are not your everyday clouds. Researchers say these nocturnal clouds, or nocturnal clouds, are the rarest, driest and highest clouds on Earth. According to satellite data, the recent surge in activity is unlike any seen in at least 15 years. There could be more activity this weekend. “People in the northern US and Canada should definitely be on the lookout for nighttime clouds over the weekend,” Cora Randall, a professor at the University of Colorado Boulder, said in an email. “We are near the peak of nocturnal cloud season and even if there are no extreme events, they can occur over the northern continental US.” Clouds usually occur near the poles, but occasionally occur at lower latitudes. Rare and vivid sightings have been reported from Oregon, Washington, Alberta, the United Kingdom and Denmark in recent days. The best chance to see the clouds is to find a clear view near the horizon and look north. “There’s nothing else like these,” the National Weather Service’s Seattle office wrote on social media. Before sunrise on Friday, they tweeted photos of the “brightest display of night clouds” they claimed had been seen in decades in the region. Nocturnal clouds, also known as the polar mesosphere, occur during the summer in each hemisphere at a height of about 50 miles in the layer of the atmosphere called the mesosphere. They form when water vapor gathers around specks of meteor dust floating in the mesosphere and freeze, forming 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 sinks below the horizon, illuminating the clouds below. “This season has been pretty unusual the last few days,” said Randall, who is also the principal investigator of NASA’s Aeronomy of Ice in the Mesosphere (AIM) mission designed to study nocturnal clouds. “The season started out as a rather average season, but in the last week the cloud frequencies have increased dramatically.” He said the frequency of nocturnal clouds in recent days was higher than ever in at least 15 years of observations by the AIM mission. However, the reason is a bit of a mystery. Nocturnal clouds rely on two main components in the mesosphere: abundant water vapor and low temperatures to aid the formation of ice crystals. Randall and colleague Lynn Harvey said data from the Microwave Limb Sounder on NASA’s Aura satellite showed that temperatures near the mesopause have risen 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 observations. “An increase in temperature would be unfavorable for clouds, but an increase in water vapor would be favorable,” Randall said. Randall said one explanation for the increase in water vapor could be linked to missile launches. Previously, researchers found water vapor released by these missions can lead to the formation of nocturnal clouds. She said this explanation is “preliminary” and her colleagues are currently investigating the lead. Atmospheric scientist Matthew DeLand said the Jan. 15 eruption of the Hunga Tonga undersea volcano also pumped large amounts of water vapor into the stratosphere. The volcano even spewed material 36 miles high into the mesosphere, setting a world record for the highest volcanic plume in a satellite record. However, he said it may take some time to see the effect on cloud behavior. How the Tongan volcano caused a shock wave around the world “This season may be too early to have any effects,” said Deland, a scientist at Science, Systems, and Applications, Inc. at NASA. “The question is how long it takes for the water vapor to move up into the atmosphere in the region where the clouds form.” Deland said the intense cloud activity at lower latitudes like Seattle is unusual, and it’s not certain whether it will continue for the rest of the season. He says it depends on circulation patterns and whether there are built-in waves that allow cold temperatures or extra water vapor to be drawn 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,” Deland said. “Clouds shine in the dark sky.”
