Night-glowing clouds crept south this summer | Science News for Students

Night-glowing clouds crept south this summer

More atmospheric moisture may be fueling these high-flying clouds that catch the sun’s rays after dark
Aug 2, 2019 — 6:45 am EST
a photo of notilucent clouds over water

Noctilucent clouds that catch sunlight after dark are showing up farther south than usual this year. Scientists are studying the upper atmosphere to figure out why.

Matthias Süssen/Wikimedia Commons (CC BY-SA 4.0)

This summer, scientists have reported seeing a surprising number of night-shining clouds in the Northern Hemisphere. High in the sky, such noctilucent clouds remain aglow even after sundown. Typically, these sunlit wisps develop up high in polar skies. Making it as far south as Oklahoma and New Mexico — as some did this summer — is a rarity.

These clouds gleam blue or white as they catch the sun’s rays, even after sundown. “They’re beautiful,” observes James Russell. He’s an atmospheric scientist at Hampton University in Virginia. “It’s hard to take your eyes off of them,” he says, “because they’re so iridescent.” They float in the mesosphere, some 80 kilometers (50 miles) up, and normally at high latitudes.

The clouds form at −130° Celsius (-200° Fahrenheit), when water vapor condenses onto and then freezes around dust particles. This makes nanometer-sized ice crystals. What stood out in June was how wet the mesosphere was. “It’s record-setting,” says Lynn Harvey, an atmospheric scientist at the University of Colorado Boulder.

One possible explanation is that more moist air than usual rose this summertime. There may also have been a rise in methane within the upper atmosphere. It can enter chemical reactions that form water vapor.

a sattelite image showing a ring of Arctic noctilucent clouds
This satellite image measures sunlight reflected off of noctilucent clouds that covered the Arctic on June 12. White areas denote areas reflecting sunlight the most. Dark blue areas reflected light least.
Joshua Stevens/NASA Earth Observatory 

NASA’s Earth Observatory released a satellite image that shows these noctilucent clouds covering the Arctic on June 12. It showed in easy-to-see white areas where sunlight was reflecting off of the clouds most effectively. 

Russell and Harvey have been part of a team that’s been monitoring these clouds for 13 years. They’re trying to learn more about how these clouds form and whether they might reveal atmospheric changes due to global warming.

The scientists plan to use computers to model the conditions under which clouds form. They’re hoping to explain what seems to be a trend of these clouds to increasingly form outside of polar skies.

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.

atmosphere     The envelope of gases surrounding Earth or another planet.

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).

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. 

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.

develop     To emerge or come into being, either naturally or through human intervention, such as by manufacturing.

global warming     The gradual increase in the overall temperature of Earth’s atmosphere due to the greenhouse effect. This effect is caused by increased levels of carbon dioxide, chlorofluorocarbons and other gases in the air, many of them released by human activity.

iridescent     Adjective that describes something that seems to change color with a shift in the angle at which it is viewed or at which lighting is applied.

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.

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.

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.

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.

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.

particle     A minute amount of something.

reflective     Adjective that refers to the ability of something to reflect light strongly. Such objects can produce a strong bright glare when sunlight bounces off of them. Examples of reflective objects include a mirror, a smooth metal can, a car window, a glass bottle, ice, snow or the watery surface of a lake.

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

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


Report: M. Carlowicz. Clouds Light the Night. NASA Earth Observatory. June 27, 2019.