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
To understand how ‘night-shining’ clouds form, scientists made one themselves
A rocket, a bathtub’s worth of water and a high-altitude explosion reveal how water vapor cools the air to form shiny ice-crystal clouds.
www.sciencenews.org
By Maria Temming
MARCH 5, 2021 AT 10:25 AM
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.
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.
