- Noctilucent clouds are the rarest and highest-altitude type, forming about 50 miles above Earth's surface.
- The night-shining clouds, which glow against the night sky, have become more visible in recent years.
- Experts say rising methane emissions and more rocket launches may play a role.
Each year, from the beginning of June to the start of August, a type of cloud at the edge of space makes an appearance in the evening sky over high latitudes.
Experts say these eerie, glowing clouds are becoming brighter and more visible over a wider area of the planet. The likely culprit: increasing pollution from fossil fuels and rocket launches.
Noctilucent — or night-shining — clouds are the Earth's highest, forming at altitudes of more than 50 miles above Earth's surface, in an upper layer of our planet's atmosphere called the mesosphere. The wispy blue-white tendrils, spreading against the night sky, were first reported in 1885, two years after the Krakatoa volcanic eruption.
"The clouds occur every summer, beginning in mid-May in the north and mid-November in the south," said James Russell, co-director of the Center for Atmospheric Sciences at Hampton University in Virginia. Russell is also the primary investigator of NASA's AIM mission, a satellite that has been observing noctilucent clouds from 370 miles above the Earth's poles since 2007.
Scientists think noctilucent clouds form when water molecules freeze around specks of smoke from meteors burning up in the atmosphere, forming ice crystals that make up the lofty clouds. They rely on extremely cold temperatures and form in the summertime, when temperatures in the mesosphere are colder than -180 degrees Fahrenheit.
In the 19th and 20th centuries, reports of noctilucent clouds were confined to the far northern and southern reaches of the planet, in places like Canada and Alaska. But in recent years, they've appeared closer to the equator, in Colorado and Utah. In 2019, they were seen as far south as California's Joshua Tree.
"The summer of 2019, northern hemisphere clouds were especially bright and were seen more abundantly at mid-latitudes," Russell told Insider, adding, "The current season so far appears to be behaving in a similar way to 2019."
Surging methane emissions, a greenhouse gas that contributes to climate change, may be making them appear more often, according to Russell. One of the major sources of methane emissions is burning fossil fuels, according to the International Energy Agency.
When methane reaches the upper atmosphere, it's broken down by absorbing the sun's rays and yields water vapor, Russell said. This extra water vapor creates ice crystals and forms the noctilucent clouds. Noctilucent clouds can act as a canary in the coal mine for methane gas, according to Russell.
"This says the rarefied upper atmosphere, literally on the edge of space, is a potential harbinger of key information on global change," Russell said.
Researchers say rocket launches, which have become more common with the rise of commercial space travel, may also play a role in the uptick in noctilucent clouds. As rockets leave Earth's atmosphere, they leave behind water vapor as exhaust. "In the last couple of days we saw a huge spike in the clouds," Cora Randall, a scientist and professor at the University of Colorado, told Spaceweather.com last week.
"We're speculating that the spike might be due to extra water vapor transported to higher latitudes from rocket launches," Randall said, although more analysis is needed to confirm whether rocket launches are creating more of the rare clouds.
Russell pointed to a growing body of research which suggests small — but statistically significant — long-term increases in methane and water vapor in the upper reaches of the atmosphere. Taken with the increase in noctilucent clouds, "these results could be interpreted as an early indication of climate change on the edge of space," Russell told Insider. Still, more data collected over a longer time period is needed to draw definitive conclusions, he added.