To understand how ‘night-shining’ clouds form, scientists made one themselves

A high-altitude explosion shows how water vapor can cool air to rapidly form ice crystals

noctilucent cloud as seen from International Space Station

Noctilucent clouds (seen here from the International Space Station) shimmer high in the dark sky after sunset, when sunlight from beyond the horizon illuminates ice crystals in the upper atmosphere.

International Space Station, NASA’s Johnson Space Center

NASA’s Super Soaker mission was basically an extreme DIY project: To better understand how noctilucent, or night-shining clouds form, researchers made one from scratch.

One frigid, predawn morning in January 2018, researchers launched a rocket hauling a bathtub’s worth of water from Poker Flat Research Range in Chatanika, Alaska. When this rocket was 85 kilometers off the ground, its water cargo exploded — spraying the upper mesosphere with a plume of vapor that froze into a cloud of ice crystals. When such high-flying hazes of ice are illuminated by sunlight from beyond the horizon after sunset, they are seen in the dark sky as shimmery noctilucent clouds (SN: 7/16/19).

The cloud in this experiment was too small to see from the ground, but reflections from a ground-based laser aimed at the “Super Soaker” rocket detected a cloud of ice crystals 18 seconds after the explosion. According to computer simulations of cloud formation, that cloud could have formed so quickly only if the temperature inside the vapor plume was about 25 degrees Celsius cooler than surrounding air, the researchers report in the February Journal of Geophysical Research: Space Physics.

The rapid cooldown, from a starting temperature of about –45° Celsius, suggests the water vapor released by the rocket not only provided the H2O to make ice crystals, but also actively cooled the air to trigger cloud formation.

Water vapor can cool the upper atmosphere because H2O is very good at emitting infrared radiation, and gas high in the atmosphere is sparse enough that this heat escapes easily into space, says study coauthor Richard Collins, an atmospheric scientist the University of Alaska Fairbanks.

At these altitudes, then, water vapor itself can readily cool the air enough to create noctilucent clouds.

time-lapse photo of three rocket launches and chemical tracers in night sky
This time-lapse photo shows the bright trails of three rocket launches from Poker Flat Research Range in Alaska. One rocket exploded a container of water to create a cloud (not visible, but detected using a green laser beam pointed at the sky). The other two rockets released bright chemical tracers into the air to track wind movements (creating the blue-white smear in the upper right).Zayn Roohi, Poker Flat Research Range, NASA’s Wallops Flight Facility

The Super Soaker experiment “was a very innovative idea,” says Xinzhao Chu, a physicist at the University of Colorado Boulder not involved in the work. Noctilucent clouds have become brighter and more frequent in the last few decades. But it’s not clear how much of that is due to climate change — which is expected to make the mesosphere colder and wetter — versus more rocket launches pumping extra water vapor into the atmosphere, she says. Better understanding rocket-generated clouds could help isolate climate change’s impact on the upper atmosphere.  

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