Near the Earth’s North and South Poles, wispy, iridescent clouds often shimmer high in the summer sky at dusk and dawn. These nocturnal, or noctilescent, clouds are sometimes also spotted farther from the poles, at a rate that varies considerably from year to year. Morning rocket launches are partly responsible for the appearance of low-latitude clouds.
“Space traffic plays an important role in the formation and variation of these clouds,” says Michael Stevens of the Naval Research Laboratory, lead author of a paper reporting the findings in the journal Earth and Space Sciences. This is an important finding as scientists try to understand whether the increase in noctilucent clouds is related to climate change, human activities, or possibly both.
Noctilucent clouds appeared in the sky over Edmonton, Alberta, Canada on July 2, 2011. Credits: NASA/Dave Hughes
First documented in the late 1800s, noctilescent clouds are the highest clouds in our atmosphere. While rain clouds typically do not rise more than 10 miles (16 kilometers) above the Earth’s surface, noctilescent clouds float about 50 miles (80 kilometers) high in a layer of the atmosphere called the mesosphere. (For this reason, they are also known as mesospheric clouds.) They shine at night because they are so high that sunlight can reach them even after the Sun has set for observers in the sky. floor. These high-flying clouds form when water ice crystals condense on particles of meteorite smoke – tiny debris from meteors that burned up in our atmosphere.
Noctilescent clouds most often appear at high latitudes near the Earth’s poles (where they are also known as polar mesospheric clouds), but they sometimes emerge farther from the poles, below 60 degrees of latitude. Between 56 and 60 degrees north latitude (over regions such as southern Alaska, central Canada, northern Europe, southern Scandinavia and south-central Russia), for example, the frequency of these clouds can vary by a factor of 10 from one year to the next.
Previous studies have shown that water vapor released into the atmosphere by space shuttle launches can cause an increase in noctilucent clouds near the poles. “The prevalence of noctilescent clouds at mid-latitudes, however, has been shrouded in mystery and the underlying cause has been disputed,” Stevens said. The last space shuttle was launched in 2011, but other rockets have carried satellites and people into space since then, adding water vapor to the atmosphere. “This study shows that space traffic, even after space shuttle launches have ceased, controls the year-to-year variability of mid-latitude noctilucent clouds,” Stevens concluded.
Stevens and his team studied observations of noctilucent clouds taken by the Cloud Imaging and Particle Size (CIPS) instrument on NASA’s AIM satellite, which launched in 2007 to determine why clouds glow at night. form and change over time.
The team compared AIM observations to the timing of rocket launches south of 60 degrees north latitude. The analysis revealed a strong correlation between the number of launches that took place between 11 p.m. and 10 a.m. local time and the frequency of mid-latitude noctilescent clouds observed between 56 and 60 degrees north latitude. In other words, the more early morning launches, the more mid-latitude noctilucent clouds appeared.
The researchers also analyzed the winds just above the noctilucent clouds and found that winds moving north were strongest during these early morning launches. This suggests that winds can easily carry exhaust from morning rocket launches at lower latitudes, such as from Florida or southern California, toward the poles. There, the rocket exhaust turns into ice crystals and descends to form clouds.
These graphs compare the number of rockets launched each year (green dots) to the frequency of noctilucent clouds (orange dots) observed between 56 and 60 degrees north latitude each July from 2007 to 2021 by the Cloud Imaging and Particle Size instrument (CIPS) on the Aeronomy of Ice in the Mesosphere (AIM) satellite. (CIPS did not collect cloud data in 2017 due to operational issues.) The top chart includes worldwide rocket launches throughout the day and shows little correlation with the frequency of noctilescent clouds. The bottom graph, however, shows a stronger correlation when only morning launches (between 11 p.m. and 10 a.m. local time) south of 60 degrees north latitude are considered. Credits: NASA/Michael Stevens (Naval Research Laboratory) et al.
Additionally, observations revealed no general upward or downward trend in the frequency of noctilucent clouds at mid-latitudes over the duration of the study, nor any correlation between their frequency and the solar cycle of 11 years, indicating that changes in solar radiation are not causing clouds to vary from year to year.
“Changes in the number of noctilucent clouds at mid-latitudes correlate with morning rocket launches, consistent with the transport of exhaust gases by atmospheric tides,” Stevens concluded.
“This research, linking changes in the frequency of mesospheric clouds to rocket launches, helps us better understand observed long-term changes in the occurrence of these clouds,” said the NASA Heliophysics Program Scientist, John McCormack, at agency headquarters in Washington, who contributed to the study.
As the atmosphere near the Earth’s surface warms, the mesosphere cools and more water vapor ends up in the upper atmosphere. Both of these effects could facilitate the condensation of water crystals and the formation of noctilescent clouds. AIM’s observations, along with efforts to model cloud formation processes under changing atmospheric conditions, are helping scientists understand how naturally induced changes in noctilucent clouds are and how are influenced by human activities.
By Vanessa Thomas NASA Goddard Space Flight Center, Greenbelt, Maryland.