Noctilucent clouds.Submitted photo

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Every day we check the local weather forecast to see if we will need sunglasses or an umbrella.  It is also possible to check the space weather forecast (at spaceweather.com, earthsky.org and other astronomy websites), to see if we are expecting meteor showers or whether an approaching geomagnetic storm will prompt a northern lights display.

“Space weather” takes place so high above Earth’s surface it is considered the edge of space. Probably the best-known space weather event is the colorful aurora borealis, or Northern Lights. Other space weather events include shooting stars and fireballs (exceptionally bright shooting stars), glowing rings of ice crystals called sun halos or moondogs, and colorful blue jets, red sprites and green elves – unique light bursts that occur above thunderstorms.  And now that summer is here, there might also be luminous blue noctilucent clouds (NLCs).  

Wispy noctilucent (“night shining”) clouds shimmer wondrously across high latitude skies during summer in the northern hemisphere, and during our winter in the southern hemisphere.  (See a brief time-lapse movie of NLCs at tinyurl.com/clouds-movie) They become visible 30-60 minutes after the sun dips below the horizon, generally from far northern states, Canada and northern European countries, but occasionally farther south.  Besides being magical to look at, NLCs are studied by environmental scientists for their ties to global warming.  

Tiny particles called “meteor smoke” are the seeds of noctilucent clouds, also called polar mesospheric clouds because they form close to both poles in the mesosphere – an area 30 to 50 miles above our planet’s surface, far higher than the normal cloud zone. Meteoroids blast through Earth’s atmosphere and leave a radiant trail of fine dust, or “smoke” – what we see as shooting stars, if we’re lucky. Water vapor freezes around that dust and hangs in the mesosphere, where it catches rays from the setting sun and glows blue in darkening northern skies.  

Noctilucent clouds appear blue for the same reason the sky is blue in daytime:  the scattering of blue light waves. When the setting sun’s light hits that frozen meteor dust, shorter blue light waves are scattered more intensely than longer light waves in other parts of the color spectrum, making the clouds appear blue.

Summer is noctilucent cloud season because, ironically, the mesosphere is coldest during summer months – as cold as -200˚ Fahrenheit and even colder than that – so the conditions for NLC formation are optimal. Ordinary water vapor could not reach let alone survive, in such a high and cold environment, but methane (CH4) and rocket exhaust can.  Methane – an abundant byproduct of industry, waste management, wetlands, agriculture and, yes, livestock flatulence – rises to the upper atmosphere where it reacts with atomic oxygen (O1) to produce water vapor. (Atomic oxygen has only one oxygen atom, while the oxygen we breathe has two, thus O2.) Those billowy plumes of exhaust from SpaceX and other rockets also supply water vapor to the polar mesosphere for NLC formation, and when it was active, Space Shuttle contributed as well.  

Climate scientists study noctilucent clouds because in recent years they have become brighter and more often visible at lower latitudes. NLCs have been seen from as far south as Colorado and Kansas, the probable result of increased carbon dioxide (CO2) and methane, two gases associated with global warming.  

“The way the atmosphere works, we put more carbon dioxide into the air and
it causes global warming at the surface,” explains Cora Randall, Ph.D., a professor of atmospheric sciences at the University of Colorado and co-investigator on NASA’s Aeronomy of Ice in the Mesosphere mission, called AIM. (Aeronomy is the study of the upper atmosphere, which includes the upper stratosphere, mesosphere, and thermosphere.) Since its launch in 2007, instruments on the AIM satellite have been examining ice particles, cosmic dust, and the temperature and atmospheric gases involved in forming noctilucent clouds.  

As carbon dioxide warms the earth’s surface, it also chills the upper atmos-phere, says Dr. Randall, so with increased CO2 in the air we expect the mesosphere to become colder and for NLCs to increase. Humans have also been releasing more methane into the atmosphere, she says, which increases the amount of water vapor in the mesosphere for more frequent NLC formation. Dr. Randall adds that data from the series of Solar Backscatter Ultraviolet (SBUV) instruments onboard National Oceanic and Atmospheric Administration satellites show an increase in northern hemisphere noctilucent clouds since 1979.

In the near future, additional noctilucent cloud data will come from a NASA funded citizen-science program called Project PoSSUM (Polar Suborbital Science in the Upper Mesosphere). PoSSUM is training high school and college students to collect data about NLCs from manned suborbital spacecraft – specialized planes that fly into the mesosphere. PoSSUM is also designing camera systems to fly on unmanned high-altitude balloons, in order to take high-resolution images of NLCs for the study of atmospheric dynamics.   

Noctilucent clouds were first documented in 1885, two years after the catastrophic Krakatoa volcano eruption in Indonesia.  Volcanic dust from Krakatoa stayed in the atmosphere for years, temporarily lowering temperatures around the globe and causing colorful sunsets worldwide.  Night time sky watching became an international past time and people started observing the mysterious iridescent clouds.  It is not known whether NLCs were caused by Krakatoa’s volcanic dust or by something else and simply were not widely seen before then.  

While checking the space weather forecast and awaiting the next meteor shower (the Perseids in mid-August), tune in to an a capella musical ode to noctilucent clouds:  http://tinyurl.com/NLC-song.