Subscribe to our free daily newsletters
. 24/7 Space News .

Subscribe to our free daily newsletters

How ice in clouds is born
by Staff Writers
Salt Lake City UT (SPX) Nov 14, 2017

A diagram of the free energy barrier depicts the energy required to sustain ice nucleation, with a small diagram of a sufficiently-sized crystallite at the peak of the barrier. A larger-scale model (right) shows a cubic segment of a larger crystallite, with cubically-stacked molecules in red and hexagonally-stacked molecules in blue.

Something almost magical happens when you put a tray full of sloshing, liquid water into a freezer and it comes out later as a rigid, solid crystal of ice. Chemists at the University of Utah have pulled back the curtain a little more on the freezing process, particularly in clouds.

Their research shows that when water droplets freeze in clouds, the structure of the ice crystal isn't necessarily the classic hexagonal snowflake structure. Rather, a more disordered ice structure forms more easily than hexagonal ice under certain cloud conditions, allowing the water droplets in clouds to turn to ice more rapidly than previously predicted. The work reconciles theoretical models of clouds with observations of freezing rates. The study is published in Nature.

Even in warm climates, precipitation usually starts with water droplets in clouds turning to ice. Why? "These droplets of liquid can grow to a certain size," says Valeria Molinero, chemistry professor at the University of Utah, "but to grow to a size that is large enough that it can fall from the sky, these droplets have to grow much larger."

The best way to grow larger is to turn to ice. A small atmospheric particle, called an aerosol, can start the process of freezing in chilled water. Or the process can start spontaneously, with a small region of ordered water molecules appearing within the droplet.

If that "crystallite" is large enough, then the droplet can freeze and continue to grow by pulling in the surrounding water vapor. The process of crystals growing from a small nucleus is called nucleation.

Overcoming the barrier
Small crystal nuclei face a barrier to growth. Because of the interactions between a small solid and its liquid surroundings, a crystallite has to grow to a certain size to be able to continue growing and not simply melt away.

Picture a hill. If you push a rock up a hill but don't make it all the way to the top, the rock rolls back down to where you started. But if you push it far enough, it rolls down the other side. The top of the hill (called the free energy barrier) sets the critical size for continuing to grow the crystallite.

"The focus of our paper is showing what the structure of the crystallite at the top of this barrier is and what is the implication for the rate of nucleation," Molinero says.

Previously, chemists assumed that the structure of ice at the top of the energy barrier was the hexagonal structure seen in snowflakes (although snowflakes are much larger than crystallites). It's a very stable structure. "The assumption that it's hexagonal is the more intuitive one," says Laura Lupi, a postdoctoral scholar and first author on the Nature paper.

Jumbled layer cake
Previous simulations found that under some cloud conditions, however, crystallites with a disordered structure were more favored. These "stacking disordered" structures are a layer-cake mix of molecules that don't settle into either the hexagonal or cubic crystal structure.

In their study, Lupi and Molinero found that at a temperature of 230 K, or -45 degrees Farenheit, the free energy barrier for the stacking disordered crystallite is 14 kJ/mol smaller than that for hexagonal ice. In other words, disordered ice has a "hill" much smaller than hexagonal ice and forms around 2,000 times faster.

This helps cloud modelers understand better their observational data regarding freezing rates in clouds. Previous nucleation models using hexagonal ice couldn't capture all of a cloud's behavior because those models extrapolated nucleation rates across cloud temperatures without understanding the effects of temperature on those rates.

Lupi and Molinero's study begins to correct those models. "Rates of ice nucleation can only be measured in a very narrow range of temperatures," Molinero says, "and it is extremely challenging to extrapolate them to lower temperatures that are important for clouds but inaccessible to the experiments."

By virtue of their size, snowflakes are more stable as hexagonal ice, Lupi and Molinero say. Their findings only apply to very small crystallites. Lupi says that their work can help cloud modelers create more accurate models of the phase of water within clouds.

"If you have so many water droplets at a certain temperature, you want to predict how many will turn into ice droplets," she says. Better cloud models can lead to better understandings of how clouds reflect heat and produce precipitation.

Molinero says that their work improves fundamental understanding of how quickly water forms ice - a process that plays out in clouds and freezers every day. And it is a process, not an instantaneous event, Molinero adds.

"The transformation is not just that you go below zero and that's it," she says. "There's a rate at which the transition happens, controlled by the nucleation barrier. And the barrier is lower than previously anticipated."

Research paper

NASA Estimates the Global Reach of Atmospheric Rivers
Pasadena CA (JPL) Nov 01, 2017
A recent study by NASA and several partners has estimated, for the first time, the global impact of atmospheric rivers on floods and droughts, as well as the number of people affected by these atmospheric phenomena. Atmospheric rivers are relatively long, narrow, short-lived jets of air that transport water vapor across significant portions of Earth's mid-latitude oceans, onto the continen ... read more

Related Links
University of Utah
Earth Observation News - Suppiliers, Technology and Application

Thanks for being here;
We need your help. The SpaceDaily news network continues to grow but revenues have never been harder to maintain.

With the rise of Ad Blockers, and Facebook - our traditional revenue sources via quality network advertising continues to decline. And unlike so many other news sites, we don't have a paywall - with those annoying usernames and passwords.

Our news coverage takes time and effort to publish 365 days a year.

If you find our news sites informative and useful then please consider becoming a regular supporter or for now make a one off contribution.

SpaceDaily Contributor
$5 Billed Once

credit card or paypal
SpaceDaily Monthly Supporter
$5 Billed Monthly

paypal only

Comment using your Disqus, Facebook, Google or Twitter login.

Share this article via these popular social media networks DiggDigg RedditReddit GoogleGoogle

NASA Moves Up Critical Crew Safety Launch Abort Test

NanoRacks launches Full External Cygnus Deployer on OA-8 to ISS

Robotic arm reaches out and grapples Cygnus

Colossal SoftBank fund could shake up tech world

Orbital ATK launches eighth cargo mission to space

The state of commercial spaceports in 2017

Vega launches Earth observation satellite for Morocco

Orbital ATK Successfully Tests First Motor Case for Next Generation Launch Vehicle

Mars 2020 Mission performs first supersonic parachute test

New partnership on Mars drone applications research

Powering up NASA's human reach for the Red Planet

NASA Opens $2 Million Third Phase of 3D-Printed Habitat Competition

China's reusable spacecraft to be launched in 2020

Space will see Communist loyalty: Chinese astronaut

China launches three satellites

Mars probe to carry 13 types of payload on 2020 mission

Astronaut meets volcano

European Space Week starts in Estonia

New Chinese sat comms company awaits approval

Myanmar to launch own satellite system-2 in 2019: vice president

Leonardo tapped by British Royal Air Force for radar testing equipment

A new way to mix oil and water

Building better silk

Plasma from lasers can shed light on cosmic rays, solar eruptions

Astronomers See Moving Shadows Around Planet-Forming Star

NASA plans mission to study why planets lose their atmospheres

Scientists find potential 'missing link' in chemistry that led to life on earth

18-Month Twinkle in a Forming Star Suggests a Very Young Planet

Jupiter's Stunning Southern Hemisphere

Watching Jupiter's multiple pulsating X-ray Aurora

Help Nickname New Horizons' Next Flyby Target

Juno Aces 8th Science Pass of Jupiter, Names New Project Manager

Memory Foam Mattress Review
Newsletters :: SpaceDaily :: SpaceWar :: TerraDaily :: Energy Daily
XML Feeds :: Space News :: Earth News :: War News :: Solar Energy News

The content herein, unless otherwise known to be public domain, are Copyright 1995-2017 - Space Media Network. All websites are published in Australia and are solely subject to Australian law and governed by Fair Use principals for news reporting and research purposes. AFP, UPI and IANS news wire stories are copyright Agence France-Presse, United Press International and Indo-Asia News Service. ESA news reports are copyright European Space Agency. All NASA sourced material is public domain. Additional copyrights may apply in whole or part to other bona fide parties. All articles labeled "by Staff Writers" include reports supplied to Space Media Network by industry news wires, PR agencies, corporate press officers and the like. Such articles are individually curated and edited by Space Media Network staff on the basis of the report's information value to our industry and professional readership. Advertising does not imply endorsement, agreement or approval of any opinions, statements or information provided by Space Media Network on any Web page published or hosted by Space Media Network. Privacy Statement