Space traffic boosts noctilucent clouds

Jan 27, 2020
Noctilucent clouds over Anchorage AK on July 26, 2022. Credit: Todd Salat.

Never before have so many rockets been launched. In 2021, the space agencies of Earth broke the all-time record for global rocket launches with 133, and in 2022 it looks like the record will be broken again with more than 150. China and SpaceX are big contributors to this increase.

High above Earth, something else is increasing: Noctilucent Clouds (NLCs). It's no coincidence. A new paper just published in the AGU journal for Earth and Space Science confirms that "space traffic has a strong influence on the interannual variability of these bright mesospheric clouds."

Noctilucent clouds are a natural phenomenon. During summer months, wisps of water vapor rise up to the mesosphere, 83 km high, and crystalize around specks of disintegrated meteoroids. Sky watchers at northern latitudes can easily see the clouds, which are filled with fine ripples and shine at night with an electric blue color.

Rocket launches are boosting NLCs. A team of researchers led by Michael Stevens of the Naval Research Lab in Washington, DC, looked at data from NASA's AIM spacecraft, which was launched in 2007 to study noctilucent clouds. They found a strong correlation between the number of rockets launched each July and the abundance of clouds in the mesosphere.

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July rocket launches (black) vs. the abundance of noctilucent clouds (red)

The link is simple: Rockets produce plumes of water vapor. Winds carry these plumes toward the polar mesosphere where they become raw material for NLCs. Rockets launched in the "morning" between 11 pm and 10 am local time are most effective. During those times, diurnal wind patterns push the plumes toward the noctilucent zone.

Researchers have long known that rockets can produce NLCs. Seminal studies led by Stevens in the early 2000s linked specific space shuttle launches to outbursts of the clouds over both of Earth's poles (refs: #1, #2). The shuttle program ended in 2011, but the "rocket effect" has continued and probably increased (researchers are still investigating the trends at mid-latitudes during the satellite era).

This is good news for sky watchers who love seeing "frosted meteor smoke" light up the night. Rockets make NLCs both brighter and more widespread. Researchers, on the other hand, may have mixed feelings. NLCs can be a sensitive indicator of changes to Earth's climate system--e.g., revealing long-range teleconnections and the abundance of greenhouse gases in the upper atmosphere. Rocket launches could swamp these delicate signals.

Either way, the launch schedule continues. Be alert for noctilucent clouds!


The first satellite designed to study noctilucent clouds, NASA’s Aeronomy of Ice in the Mesosphere (AIM) mission released the first view of these clouds over the entire Northern Hemisphere in 2007, at a resolution of approximately 5 kilometers (3 miles). This image, acquired on June 11, 2007, shows some of the first data AIM collected about these clouds. In this image, centered on the North Pole, white indicates clouds with the greatest density of ice particles, and dark blue indicates clouds with the lowest. Because ice particles reflect sunlight, a greater concentration of such particles creates a higher albedo—the ratio of reflected light to total incoming light. Areas of no data appear in black, and landmass outlines appear in blue-green.

Noctilucent clouds, also known as polar mesospheric clouds, form in a part of the atmosphere roughly 50 to 86 kilometers (30 to 54 miles) above the surface of our planet. In the months following AIM’s early observations, researchers working with the satellite shared some of their findings. They discovered that the clouds appear daily, are widespread, and vary on an hourly to daily basis. They also found that the clouds’ brightness varies over horizontal scales of about 3 kilometers (2 miles). To their surprise, the researchers also noticed that the ice in the mesosphere—the layer of the atmosphere where the clouds form—occurs in a single, continuous layer stretching from about 82 to 89 kilometers (51 to 55 miles) above the Earth’s surface.

By late 2007, AIM had documented the life cycle of noctilucent clouds in the Northern Hemisphere, noting that they first appeared around May 25 and lasted through late August. Although AIM had provided researchers with valuable information on noctilucent clouds by the end of the year, many other questions remained, and the researchers planned to keep watching AIM’s progress over the life of the mission.


The summer polar mesopause is the coldest region of the Earth's atmosphere, reaching temperatures as low as -140°C. It is sufficiently cold for noctilucent ('night shining') clouds to form in summer, at altitudes around 83 km. Noctilucent clouds can only be seen when the sun is shining on them (at ~83 km) and not on the lower atmosphere, i.e. when the sun is between 6 and 16 degrees below the horizon. They are a summer, polar phenomena but because of the restrictive viewing conditions they are most commonly observed at latitudes between 55 and 65 degrees. Noctilucent clouds were first reported in 1885 when they were independently observed in Germany and Russia. This was two years after the volcanic explosion of Krakatoa in the Straits of Java.

One hypothesis is that the initial observation of noctilucent clouds was related to an increase in the number of observers of the twilight skies attracted by the spectacular displays resulting from the globally distributed volcanic debris of Krakatoa.

Alternatively, water vapour injected into the upper atmosphere by the volcano ultimately reached the cold, dry upper mesophere. Subsequent observations have proved that noctilucent clouds are not solely related to volcanic activity, and their volcanic association is now scientifically contentious. It has been alternatively claimed that the appearance of noctilucent clouds is the earliest evidence of anthropogenic climate change. They have been observed thousands of times in the northern hemisphere, but less than 100 observations have been reported from the southern hemisphere. It has not been resolved if this is due to inter-hemispheric differences (temperature &/or water vapour) in the atmosphere at these altitudes, or the lack of observers and poorer observing conditions in southern latitudes.


Dark colors have an albedo close to zero, meaning little or no energy is reflected. Pale colors have an albedo close to 100%, meaning nearly all the energy is reflected.

Forests, for example, have an albedo of about 15%, which means that 15% of the sunlight that hit a forest is reflected out to space. Fresh snow, on the other hand, can have an albedo of 90%, which means that 90% of the sunlight that hits a snow-capped peak is reflected out to space.

The amount reflected back out to space is called the planetary albedo. It’s calculated by averaging the albedo of all Earth surfaces – including the land, ocean, and ice. Above the Earth surface, clouds reflect a large amount of sunlight out to space too. Earth's planetary albedo is about 31% meaning that about a third of the solar energy that gets to Earth is immediately reflected out to space.

Why do we care what happens to sunlight that gets to Earth? Understanding how much energy from the Sun is reflected back out to space and how much is absorbed becoming heat is important for understanding climate.

If Earth's climate is colder and there is more snow and ice on the planet, albedo increases, more sunlight is reflected out to space, and the climate gets even cooler. But, when warming causes snow and ice to melt, darker colored surfaces are exposed, albedo decreases, less solar energy is reflected out to space, and the planet warms even more. This is known as the ice-albedo feedback.

Large bright surfaces are needed to maintain a temperature so that planet earth, and the life that it sustains, can thrive. Therefore, white and snow-covered areas such as the Arctic and the Antarctic are essential – they have a very high albedo effect. In fact, these areas are the earth’s most important cooling mechanisms. Naturally, the effect is at its highest in wintertime, as fresh snow reflects up to 90 % of the radiation from the sun. Other surfaces with a high albedo are clouds and deserts.


On an interesting note:

Noctilucent clouds (NLCs) may also aid in propagating radio waves, according to a 21 May 2019 article on Polar Mesospheric Summer Echoes:
The underlying physics of these echoes is still uncertain. A leading theory holds that the ice particles in noctilucent clouds are electrically charged, and this makes them good reflectors of radio waves. However, NLCs are not always visible when the radar echoes are observed and vice versa.
According to Rob Stammes of the Polarlightcenter in Lofoten, Norway:
“I detected these [56.25MHz] VHF signals coming from transmitters in Eastern Europe. Before they reached my receiver in Norway, they bounced off something in the mesosphere. The patterns were recognizable and very strong.”

Noctilucent cloud observations over the last 30 years show an increasing trend in the number of nights on which the clouds are observed each summer season. Competing anthropogenic explanations for this increasing occurrence of noctilucent clouds focus on either their excessive greenhouse cooling of the middle atmosphere combined the greater amount of water vapor linked to the increasing number of rockets being launched through the atmosphere whose by-product is a tower of water vapor injected into the atmosphere from the launch location up to the 30-54 mile altitude of the mesosphere. Once this water vapor freezes into the ice crystals which make up the noctilucent clouds, where temperatures may be as low as -140F, resulting in the addition of the noctilucent clouds' own albedo to an overall increase to the total planetary albedo.