Understanding the Glow of Noctilucent Clouds

Night-Shining Clouds Glitter in the Post-Sunset Twilight

noctilucent clouds
Noctilucent clouds over Solna in Stockholm, Sweden. To see them, it helps to live far north and look after sunset when deep twilight has set in during summer evenings. Kevin Cho, CC-BY-SA, 3.0. Wikimedia Commons

Each summer, people who live at high latitudes north and south of the equator are treated to a fantastically beautiful phenomenon called "noctilucent clouds." These aren't clouds in the normal way we understand them. The clouds we're more familiar with are generally made of water droplets that have formed around particles of dust. Noctilucent clouds are generally made of ice crystals that formed around tiny dust particles in fairly cold temperatures.

Unlike most clouds that float fairly close to the ground, they exist at heights up to 85 kilometers above the surface of our planet, high in the atmosphere that sustains life on Earth. They may look like thin cirrus that we can see throughout the day or night but are generally only visible when the Sun is no more than 16 degrees below the horizon.

Clouds of the Night

The term "noctilucent" means "night shining" and it describes these clouds perfectly. They can't be seen during the day due to the brightness of the Sun. However, once the Sun sets, it illuminates these high-flying clouds from below. This explains why they can be seen in deep twilight. They typically have a bluish-white color and look very wispy.

The History of Noctilucent Cloud Research

Noctilucent clouds were first reported in 1885 and are sometimes linked with the eruption of the famous volcano, Krakatoa in 1883. However, it's not clear that the eruption caused them — there's no scientific evidence to prove it one way or another.

Their appearance may simply be coincidental. The idea that volcanic eruptions cause these clouds was heavily researched and eventually disproved in the 1920s. Since then, atmospheric scientists have studied noctilucent clouds using balloons, sounding rockets, and satellites. They seem to occur pretty frequently and are quite beautiful to observe.

How Do Noctilucent Clouds Form?

The ice particles that make up these shimmering clouds are quite small, only about 100 nm across. That many times smaller than the width of a human hair. They form when tiny particles of dust—possibly from bits of micrometeors in the upper atmosphere—are coated with water vapor and frozen high in the atmosphere, in a region called the mesosphere. During local summer, that region of the atmosphere can be quite cold, and the crystals form at about -100° C.

Noctilucent cloud formation seems to vary as the solar cycle does. In particular, as the Sun emits more ultraviolet radiation, it interacts with water molecules in the upper atmosphere and breaks them apart. That leaves less water to form the clouds during times of increased activity. Solar physicists and atmospheric scientists are tracking solar activity and noctilucent cloud formation to better understand the connection between the two phenomena. In particular, they are interested in learning why changes in these peculiar clouds don't show up until about a year after UV levels change.

Interestingly, when NASA's space shuttles were flying, their exhaust plumes (which were nearly all water vapor) froze high in the atmosphere and created very short-lived "mini" noctilucent clouds.

The same thing has happened with other launch vehicles since the shuttle era. However, launches are few and far between. The phenomenon of noctilucent clouds predates launches and aircraft. However, the short-lived noctilucent clouds from launch activities provide more data points about the atmospheric conditions that help them form.

Noctilucent Clouds and Climate Change

There may be a connection between the frequent formation of noctilucent clouds and climate change. NASA and other space agencies have been studying Earth for many decades and observing the effects of global warming. However, the evidence is still being gathered, and the link between the clouds and warming remains a relatively controversial suggestion. Scientists are following up on all the evidence to see if there is a definite link.

One possible theory is that methane (a greenhouse gas implicated in climate change) migrates to the area of the atmosphere where these clouds form. Greenhouse gases are thought to force temperature changes in the mesosphere, causing it to cool down. That cooling would contribute to the formation of ice crystals that make up the noctilucent clouds. An increase in water vapor (also due to human activities that produce greenhouse gases) would be part of the noctilucent cloud connection to climate change. Much work needs to be done to prove these connections.

Regardless of how these clouds form, they remain a favorite of sky watchers, particularly sunset-gazers and amateur observers. Just as some people chase eclipses or remain out late at night to see meteor showers, there are many who live in the high northern and southern latitudes and actively seek out the sight of noctilucent clouds. There's no doubt of their magnificent beauty, but they are also an indicator of activities in our planet's atmosphere.