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Atmospheric Science

How Do Clouds Condensate?: New Insights into Ice Nucleation

References: Holden, Mark A.; Campbell, James M.; Meldrum, Fiona C.; Murray, Benjamin J.; Christenson, Hugo K. (2021). Active sites for ice nucleation differ depending on nucleation mode. Proc. Nat. Acad. Sci. U.S.A. 118, e2022859118.

DOI: https://doi.org/10.1073/pnas.2022859118

Reading Time: 5 minutes

It’s about 5pm in Virginia Beach, Virginia and the clouds roll in dumping rain. Inside your house you ask yourself, “why does this always happen?” These clouds have condensed and rolled in from inland on their way out over the ocean. “Why did the clouds even form in the first place?” you mutter to yourself now that your beach day is ruined. The nucleation of water molecules on solid particles in the air (which looks like water sticking to things like dust) gives rise to the clouds we see every day, but the picture is not as simple as we think. These solid particles in the air need rare, specific “active sites” where water can collect. How the water molecules condense together changes depending on the conditions around them. Given these uncertainties, how can we accurately determine when and how clouds will form? And what about weather models you see on TV? How do they know when clouds will form?

A schematic of immersion freezing and two types of deposition nucleation (from gaseous and liquid water). Image Credit: Holden et. al.

A team of scientists from the University of Leeds in the United Kingdom decided to solve some of these uncertainties. The group analyzed the nucleation of water molecules into ice on the surface of two relevant particles in the atmosphere: quartz and feldspar. These particles are found at microscopic sizes in the air and have previously been shown to effectively condense water into ice. Water can condense on these substances through two known methods: immersion freezing and deposition nucleation. Immersion freezing is the method where a large droplet of liquid water cools to a very low temperature and freezes at an active site homogenously, and deposition nucleation has either water vapor or a small droplet of liquid water freeze into ice at an active site which then allows more water to condense on top of it. Today, the group investigated the likelihood of ice nucleating through these methods and their impacts on the atmosphere.

What did they find?

The team discovered that there is a significant difference for where immersion freezing and deposition nucleation occurred, so small in fact that only 8% of active sites on both quartz and feldspar had water freeze in both methods! For comparison, around 75% of active sites repeated the same individual method. Interestingly, there was a limiting number of active sites for ice to nucleate on a surface, meaning that the shape of particles in the air determine how water freezes. The group also found that, on average, only 56% of active sites on quartz and 45% of active sites on feldspar had ice form when a freezing experiment is repeated on the same surface. This indicates that the action of crystallization could weather away the substance’s active sites when it freezes. Furthermore, they discovered that colder, less humid conditions had a larger number of active sites with a lower selectivity for where water could freeze. All of these results paint an intriguing portrait of how water freezes in the air.

How did they do it?

The team performed all of these experiments themselves using microscopes to look at water freezing on pieces of quartz and feldspar. The group dropped large droplets of water on the substance, cooled them down while maintaining the humidity of the environment around the experiment, and observed where and how the water froze. They repeated these measurements in multiple cycles to confirm their findings as well as investigate if ice nucleated through different methods at the same active site. To investigate the ability of the same active sites to nucleate ice, the team determined the “repeatability” of ice forming at the same locations (as mentioned above).

Why does it matter?

A collection of images of ice crystals from immersion freezing (left) and deposition nucleation (right) events. Image Credit: Holden et. al.

This investigation tells us a lot about the nucleation of water and clouds in our atmosphere, as they play a crucial part in our world’s climates. Furthermore, we know very little about which types of solid particles in the air nucleate ice well, so these results can help us better understand cloud formation as well as improve our approaches to modelling our weather. With this knowledge, we can discern how our atmosphere responds to our changing climate in the future.



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