Glowing silver-blue clouds that sometimes light up summer night skies in polar regions, after sunset and before sunrise, are called noctilucent clouds. Scientists studying these clouds, using data from NASA’s AIM (Aeronomy of Ice in the Mesosphere) satellite have found that year-to-year changes in noctilucent clouds are closely linked to weather and climate across the globe. One major discovery they’ve made is that weather conditions in one hemisphere can have a profound effect on the other hemisphere.
Also known as night shining clouds, noctilucent clouds form in the highest reaches of the atmosphere called the mesosphere, as much as 50 miles (80 km) above the Earth’s surface. They’re usually seen during summer in polar regions. After sunset or before sunrise, when the sun is below the ground horizon but visible from the high altitude of noctilucent clouds, sunlight illuminates these clouds, causing them to glow in the dark night sky. Noctilucent clouds are thought to be made of ice crystals that form on fine dust particles from meteors; they can only form when temperatures are incredibly low and when there’s water available to form ice crystals.
NASA’s AIM satellite was launched on April 25, 2007. Weighting just 430 pounds, AIM was placed into a polar orbit, 373 miles in altitude, using a Pegasus-XL launch vehicle out of Vandenberg Air Force Base in California. Its mission is to observe noctilucent clouds using several onboard instruments to collect information such as temperature, atmospheric gases, ice crystal size, changes in the clouds, as well as the amount of meteoric space dust that enters the atmosphere. Scientists will use the data to study how noctilucent are formed and why they change over time.
A video about AIM, that shows the Pegasus-XL rocket launch about 2 min. 30 sec. into the video.
James Russell, an atmospheric scientist at Hampton University in Hampton, VA, and Principal Investigator for AIM, said in a press release,
The question people usually ask is why do clouds which require such cold temperatures form in the summer? It’s because of the dynamics of the atmosphere. You actually get the coldest temperatures of the year near the poles in summer at that height in the mesosphere.
Here’s how it works: during summer, air close to the ground gets heated and rises. Since atmospheric pressure decreases with altitude, the rising air expands. When the air expands, it also cools down. This, along with other processes in the upper atmosphere, drives the air even higher causing it to cool even more. As a result, temperatures in the mesosphere can plunge to as low as -210°F (-134°C).
In the northern hemisphere, the mesosphere predictably reaches these temperatures by mid-May, give or take a week. But that’s not the case in the southern hemisphere where the appearance of noctilucent clouds are not as predictable; for instance, in 2010, the clouds arrived one month later than in 2009.
Atmospheric scientist Bodil Karlsson, at Stockholm University in Sweden and a member of the AIM team, said in the same press release,
Since the clouds are so sensitive to the atmospheric temperatures, they can act as a proxy for information about the wind circulation that causes these temperatures. They can tell us that the circulation exists first of all, and tell us something about the strength of the circulation.
She explained that the appearance of summer noctilucent clouds in the southern hemisphere is linked to an atmospheric phenomenon called the southern stratospheric vortex. It’s a pattern of winter wind circulation above the south pole. In 2010, that vortex persisted into the southern summer season, keeping cold air at lower altitudes which prevented the formation of noctilucent clouds at higher latitudes till later into the summer.
There’s also evidence of a connection between atmospheric conditions in the northern and southern hemispheres. The upwelling of air needed to create noctilucent clouds is part of a larger wind circulation loop traveling between the two poles. Wind activity about 13,000 miles (20,920 km) away in the northern hemisphere appears to influence the southern hemisphere. With AIM, scientists have observed a 3 to 10 day lag between low altitude weather in the north — where mountains create complex wind systems — and its effect on the southern mesosphere during the southern hemisphere summers. However, during northern hemisphere summers, the lower atmosphere in the southern polar region has little variablility. As a result, it produces calmer steadier conditions in the northern hemisphere mesosphere which allows for consistent timing in noctilucent cloud formation. Said Russell:
The real importance of all of that is not only that events down where we live can affect the clouds 50 miles (80 km) above, but that the total atmosphere from one pole to the next is rather tightly connected.
It will take additional analysis to understand the details of this complex northern-southern hemisphere atmospheric interaction. AIM data will also help scientists learn more about how noctilucent cloud seasons vary due to changes in the atmosphere, caused by the sun’s cycles, as well as by natural and human-induced changes. As more information is gathered about these causes and effects, noctilucent clouds could be used to monitor atmospheric processes that are otherwise difficult to observe directly.
Bottom line: NASA’s AIM (Aeronomy of Ice in the Mesosphere) satellite has been observing noctilucent clouds since 2007, gathering data that will help scientists understand the characteristics of these extreme high altitude clouds that are closely linked to weather and climate across the globe. Data has shown, at unprecedented detail, how low altitude weather conditions in the northern hemisphere affect the southern polar mesosphere during the southern summer. And vice versa. A deeper understanding of noctilucent clouds could someday help scientists monitor the effects of climate change in the atmosphere.