Explainer: Why some clouds glow in the dark | Science News for Students

Explainer: Why some clouds glow in the dark

In a warming world, noctilucent clouds might become more common far from polar skies
Aug 2, 2019 — 11:56 am EST
a photo of clouds glowing blue after twilight

Noctilucent clouds glow in blue hues after twilight. They are rare in non-polar skies. But with climate change, that might change, scientists report.

Sjo/iStock/Getty Images Plus

A glowing cloud lit up Northern California’s skies on December 19, 2018. Thousands of people in San Francisco, Calif., gawked at the eerie neon-blue spiral that loomed for an hour after sunset. Even the National Weather Service was baffled by what could have caused it. 

Then dashcam video emerged. It showed the instigator was from out of this world. A meteor left a trail of dust that created the noctilucent (Nok-tih-LU-sint) cloud. The cloud’s name comes from Latin words for “night-lit.”

A car’s dashcam picked up a meteor (glowing white streak) streaming through the night sky near Daly City, Calif., on December 19, 2018. Daly City is around 13 kilometers (8 miles) south of San Francisco, Calif.

Smoke from the burning space rock “seeded” Earth’s upper atmosphere with dust. Water vapor can condense around those dust bits to form clouds. Meteors burn up high in the atmosphere. So these noctilucent clouds also form high up.

Given the curvature of the Earth, objects high in the sky can still catch some sunlight well after the sun sets closer to the ground. Noctilucent clouds’ extreme height is what keeps them shining in the dark. And they appear blue because all of the other wavelengths of light have scattered.

Noctilucent clouds typically emerge at high latitudes, meaning near or over the poles. They almost never appear above the lower 48 U.S. states — not unless the atmosphere there gets some help, as it did that December night. 

Reports of the glowing cloud had begun pouring in around 5:40 p.m. Onlookers flooded the local National Weather Service office with pictures. Many also began guessing at the cloud’s cause. A rocket launch, for instance, might explain it.

United Launch Alliance did have a launch scheduled for that night. This company specializes in building and launching spacecraft. That night, a rocket armed with a top-secret spy satellite was due to take off from Vanderberg Air Force Base, south of San Francisco. But 9 minutes before blastoff, the launch was scrubbed. So its rocket did not produce the eerie cloud.

The next day, the American Meteor Society (AMS) described 180 eyewitness accounts of what did: a meteor. A so-called fireball, it appeared brighter than Venus as it burned up in Earth’s atmosphere. AMS estimated that the space rock broke apart over open water some 56 kilometers (35 miles) west of the Golden Gate Bridge. 

Although space rocks commonly enter Earth’s atmosphere, they have seldom spawned clouds. The reason: Those rocks tend to break apart too high. The mesosphere, where the breakups typically happen, is some 81 kilometers (50 miles) above the ground. It hosts very little water.

But that could change. More water is entering the upper atmosphere as Earth’s climate warms. 

A pivotal role for space rocks

For a noctilucent cloud to form, the mesosphere must be super cold — below –40° Celsius (–40° Fahrenheit). These temps develop above Earth’s poles in the summer. Near the Arctic, that means peak noctilucent season is June through August. Peak season near Antarctica is December through February. 

At those low temperatures, the air is dry. And at such high altitudes, the air is also relatively dust-free. Without some dust particle to glom onto, any moisture here tends not to freeze; it is “supercooled.”

a sattelite image of a ring of neon-blue clouds over the South Pole
NASA’s AIM Spacecraft detects neon-blue noctilucent clouds forming a doughnut-like ring high above the South Pole. Such clouds may appear for up to a week during summer in the Arctic and Antarctic. 
LASP/Univ. of Colorado/NASA    

But that can change with the arrival of meteor smoke. With something to freeze onto, the supercooled droplets rapidly turn to ice. Once one ice crystal forms, more join it in what becomes a chain reaction. If the process is large enough, a noctilucent cloud develops.

About 3 percent of each ice crystal in a noctilucent cloud comes from meteors, says atmospheric scientist Mark Hervig. He works at the aerospace company GATS, Inc., in Newport News, Va. Hervig led a team that found the strong link between meteor smoke and noctilucent clouds.

The researchers analyzed data collected by NASA’s AIM mission. AIM stands for Aeronomy of Ice in the Mesosphere. The team’s findings suggest meteor smoke is the main trigger for the formation of these luminous clouds. Tiny smoke particles serve as the core around which ice crystals form. 

The inner solar system is littered with meteors of all shapes and sizes, but mostly small stuff. Earth’s atmosphere collects tons of these itty bitty meteors. Once inside Earth’s atmosphere, they’ll burn up. This leaves behind a haze of tiny particles suspended at an altitude of 70 to 100 kilometers (43 to 62 miles).

“It's no coincidence that noctilucent clouds form 83 kilometers high, squarely inside the meteor smoke zone,” Hervig says.

The coming climate for noctilucent clouds

Today, noctilucent clouds seldom develop outside the Arctic and Antarctic. But that might not be true for long. Indeed, these clouds have already begun to creep down into the regions between the poles and the tropics. One reason seems to be the increasing presence of methane at high altitudes.

High up in the mesosphere, methane takes part in a complex chemical reaction that forms new molecules of water. “Water vapor can increase if methane increases,” says atmospheric scientist James Russell. Every methane molecule can produce two water molecules in the mesosphere, Russell explains. He works with NASA's AIM mission at Hampton University in Virginia. There, he is part of a group that studies noctilucent clouds.

The atmospheric sciences community has likened noctilucent clouds outside of polar skies as a potential symptom of climate change. 

Methane, a potent greenhouse gas, can be released into the sky by thawing permafrost, burping cows, biomass burning and more. Increasing methane levels may boost the amount of water in the mesosphere. In turn, that could improve chances for noctilucent clouds.

Rising levels of another greenhouse gas, carbon dioxide, may also play a role. As CO2 boosts air temperatures near the ground, it can cause temps in the mesosphere to drop, Russell explains. That cooling effect could help supercool more water — a key ingredient for noctilucent clouds.

In step with rising greenhouse gases, the breadth and frequency of glowing clouds have increased over the last few decades, climate research hints.

Gary Thomas is an atmospheric scientist at the University of Colorado, Boulder. From 1964 to 1986, noctilucent clouds covered more and more of the sky high above the poles, his team found. These clouds also ventured toward Earth’s equator, beyond their normal territory. And increased methane played a key role in the clouds’ spread. The team reported its findings in 2001 in Advances in Space Research

Glowing clouds are not only spreading farther across the sky. Since 1998, they have also been appearing more often and getting brighter. A team of German researchers reported those findings in a 2015 study.

Russell says the expansion of noctilucent clouds could be an indicator of climate change. More study is needed to confirm this, he says. But it certainly makes scientists wonder, he says: “Is climate change occurring on the edge of space?”

Power Words

(more about Power Words)

Arctic     A region that falls within the Arctic Circle. The edge of that circle is defined as the northernmost point at which the sun is visible on the northern winter solstice and the southernmost point at which the midnight sun can be seen on the northern summer solstice. The high Arctic is that most northerly third of this region. It’s a region dominated by snow cover much of the year.

asteroid     A rocky object in orbit around the sun. Most asteroids orbit in a region that falls between the orbits of Mars and Jupiter. Astronomers refer to this region as the asteroid belt.

atmosphere     The envelope of gases surrounding Earth or another planet.

chemical     A substance formed from two or more atoms that unite (bond) in a fixed proportion and structure. For example, water is a chemical made when two hydrogen atoms bond to one oxygen atom. Its chemical formula is H2O. Chemical also can be an adjective to describe properties of materials that are the result of various reactions between different compounds.

chemical reaction     A process that involves the rearrangement of the molecules or structure of a substance, as opposed to a change in physical form (as from a solid to a gas).

climate     The weather conditions that typically exist in one area, in general, or over a long period.

climate change     Long-term, significant change in the climate of Earth. It can happen naturally or in response to human activities, including the burning of fossil fuels and clearing of forests.

cloud     A plume of molecules or particles, such as water droplets, that move under the action of an outside force, such as wind, radiation or water currents. (in atmospheric science) A mass of airborne water droplets and ice crystals that travel as a plume, usually high in Earth’s atmosphere. Its movement is driven by winds. 

colleague     Someone who works with another; a co-worker or team member.

condense     To become thicker and more dense. This could occur, for instance, when moisture evaporates out of a liquid. Condense can also mean to change from a gas or a vapor into a liquid. This could occur, for instance, when water molecules in the air join together to become droplets of water.

crystal     (adj. crystalline) A solid consisting of a symmetrical, ordered, three-dimensional arrangement of atoms or molecules. It’s the organized structure taken by most minerals. Apatite, for example, forms six-sided crystals. The mineral crystals that make up rock are usually too small to be seen with the unaided eye.

extraterrestrial     Anything of or from regions beyond Earth.

fireball     A lump of rock or metal from space that hits the atmosphere of Earth. Fireballs are meteors that are exceptionally bright and large.

force     Some outside influence that can change the motion of a body, hold bodies close to one another, or produce motion or stress in a stationary body.

greenhouse gas     A gas that contributes to the greenhouse effect by absorbing heat. Carbon dioxide is one example of a greenhouse gas.

haze     Fine liquid or solid particles scattered through the atmosphere that make it hard to see. Haze can be caused by harmful substances such as air pollutants or by water vapor.

latitude     The distance from the equator measured in degrees (up to 90). Low latitudes are closer to the equator; high latitudes are closer to the poles.

literally     A term that the phrase that it modifies is precisely true. For instance, to say: "It's so cold that I'm literally dying," means that this person actually expects to soon be dead, the result of getting too cold.

mesosphere     The highest part of Earth’s atmosphere where all of the gases are all still well-mixed, not merely layered on the basis of each gas’s mass. This layer, found immediately above the stratosphere, is 35 kilometers (22 miles) thick. It’s the uppermost layer of the atmosphere with enough gas to cause friction for incoming space rocks. That’s why this region is where most meteor’s burn up. It varies somewhat in height, but tends to span about 50 to 80 kilometers (30 to 50 miles) above Earth’s surface.

meteor     (adj. meteoritic) A lump of rock or metal from space that hits the atmosphere of Earth. In space it is known as a meteoroid. When you see it in the sky it is a meteor. And when it hits the ground it is called a meteorite.

meteoroid     A small rock- or metal-based piece of natural space debris that orbit the sun. Far smaller than asteroids, these range in size from small dust grains to big pebbles. When they get close to Earth, some may be pulled into the planet's atmosphere. NASA estimates that every day some 3,000 metric tons of this space debris enters Earth's atmosphere. Friction caused by their fast past through the air will cause most to burn up. The light streaks they cause as this happens is known as a meteor.

methane     A hydrocarbon with the chemical formula CH4 (meaning there are four hydrogen atoms bound to one carbon atom). It’s a natural constituent of what’s known as natural gas. It’s also emitted by decomposing plant material in wetlands and is belched out by cows and other ruminant livestock. From a climate perspective, methane is 20 times more potent than carbon dioxide is in trapping heat in Earth’s atmosphere, making it a very important greenhouse gas.

microscopic     An adjective for things too small to be seen by the unaided eye. It takes a microscope to view objects this small, such as bacteria or other one-celled organisms.

model     A simulation of a real-world event (usually using a computer) that has been developed to predict one or more likely outcomes. Or an individual that is meant to display how something would work in or look on others.

molecule     An electrically neutral group of atoms that represents the smallest possible amount of a chemical compound. Molecules can be made of single types of atoms or of different types. For example, the oxygen in the air is made of two oxygen atoms (O2), but water is made of two hydrogen atoms and one oxygen atom (H2O).

NASA     Short for the National Aeronautics and Space Administration. Created in 1958, this U.S. agency has become a leader in space research and in stimulating public interest in space exploration. It was through NASA that the United States sent people into orbit and ultimately to the moon. It also has sent research craft to study planets and other celestial objects in our solar system.

National Weather Service     An agency of the National Oceanic and Atmospheric Administration. Created in 1870, its current role is to collect weather, precipitation and climate data. It also issues forecasts and warnings 24 hours a day for the entire United States, focusing on signs of possible conditions that could threaten lives and structures. 

occultation     A celestial eclipse-like event in which an object that appears large from Earth, such as the moon, obscures a smaller-seeming object, such as a distant star.

particle     A minute amount of something.

planet     A celestial object that orbits a star, is big enough for gravity to have squashed it into a roundish ball and has cleared other objects out of the way in its orbital neighborhood. 

potent     An adjective for something (like a germ, poison, drug or acid) that is very strong or powerful.

rocket     Something propelled into the air or through space, sometimes as a weapon of war. A rocket usually is lofted by the release of exhaust gases as some fuel burns. (v.) Something that flings into space at high speed as if fueled by combustion.

satellite     A moon orbiting a planet or a vehicle or other manufactured object that orbits some celestial body in space.

smoke     Plumes of microscopic particles that float in the air. They can be comprised of anything very small. But the best known types are pollutants created by the incomplete burning of oil, wood and other carbon-based materials.

society     An integrated group of people or animals that generally cooperate and support one another for the greater good of them all.

solar system     The eight major planets and their moons in orbit around our sun, together with smaller bodies in the form of dwarf planets, asteroids, meteoroids and comets.

supercooled     An adjective for a liquid or a gas that has been slowly cooled to below its freezing point without it becoming a solid.

symptom     A physical or mental indicator generally regarded to be characteristic of a disease. Sometimes a single symptom — especially a general one, such as fever or pain — can be a sign of any of many different types of injury or disease.

Venus     The second planet out from the sun, it has a rocky core, just as Earth does. Venus lost most of its water long ago. The sun’s ultraviolet radiation broke apart those water molecules, allowing their hydrogen atoms to escape into space. Volcanoes on the planet’s surface spewed high levels of carbon dioxide, which built up in the planet’s atmosphere. Today the air pressure at the planet’s surface is 100 times greater than on Earth, and the atmosphere now keeps the surface of Venus a brutal 460° Celsius (860° Fahrenheit).

water vapor     Water in its gaseous state, capable of being suspended in the air.

wavelength     The distance between one peak and the next in a series of waves, or the distance between one trough and the next. It’s also one of the “yardsticks” used to measure radiation. Visible light — which, like all electromagnetic radiation, travels in waves — includes wavelengths between about 380 nanometers (violet) and about 740 nanometers (red). Radiation with wavelengths shorter than visible light includes gamma rays, X-rays and ultraviolet light. Longer-wavelength radiation includes infrared light, microwaves and radio waves.

weather     Conditions in the atmosphere at a localized place and a particular time. It is usually described in terms of particular features, such as air pressure, humidity, moisture, any precipitation (rain, snow or ice), temperature and wind speed. Weather constitutes the actual conditions that occur at any time and place. It’s different from climate, which is a description of the conditions that tend to occur in some general region during a particular month or season.


Tweet: American Meteor Society https://t.co/YNXizJuiHR December 20, 2018

Tweet: National Weather Service Bay Area https://twitter.com/NWSBayArea/status/1075568066879057920 December 19, 2018

Article: Loren Grush Stay up late to watch ULA launch a secret satellite with its most powerful rocket TheVerge.com December 8, 2018

Tweet: SpaceX http://t.co/abzSF6YeON August 11, 2018

Meeting: L. Megner. Meteoric dust, ice nucleation and noctilucent cloud growth. Abstract C5.2-5-18 (p. 1009), 42nd COSPAR Scientific Assembly July 14-22, 2018, Pasadena, Calif.

Blog: G. Doyen. Noctilucent Clouds: A rare phenomenon happening this summer! A posting from the French telescope manufacturer, Vaonis. June, 23, 2018. 

Journal: J. Fiedler et al. Long-term variations of noctilucent clouds at ALOMAR. Journal of Atmospheric and Solar-Terrestrial Physics. Vol. 162, August 2016. doi: 10.1016/j.jastp.2016.08.006.

Article: T. Phillips. Meteor smoke makes strange clouds. NASA Science. August 7, 2012.

Journal: G. Thomas and J. Olivero. Noctilucent clouds as possible indicators of global change in the mesosphere. Advances in Space Research. Volume 28, Issue 7, 2001, p. 937. https://doi.org/10.1016/S0273-1177(01)80021-1.