Clouds Are Cool, But What Makes A Cloud Super-Cool?

We have all spent time gazing up at clouds, looking for shapes, or hoping they will not rain on us. We think most people would agree that clouds are pretty cool! But what makes a cloud super-cool? It all starts with water. Clouds are made up of tiny droplets of water. Within these water droplets are tiny little particles that come from many sources, such as the ocean, the land, or pollution. These particles are often referred to as cloud “seeds”. If the seeds are large enough, they are known as cloud condensation nuclei. For any cloud droplet to form, cloud condensation nuclei must be present. You can think of cloud condensation nuclei as the building blocks of clouds!

The other ingredient needed to form a cloud is water—of course! But not water in a liquid form. Clouds need water to be in the gas phase first, which is known as water vapor. Colder air can hold less water vapor than warmer air can.

When cool air touches a surface, the water vapor in it turns to liquid. In other words, it condenses on that surface. In wintertime, have you ever noticed water condensing on your windows? A similar process happens in clouds. In the atmosphere, as air rises, it cools. The water vapor in the cold air will condense on cloud condensation nuclei surfaces if they are present, forming liquid droplets. This is how the droplets in clouds, called cloud droplets, are formed. These cloud droplets then stick together and grow, until they get so heavy that they fall out of the sky as rain!

Some clouds are happy to remain liquid their whole lives, especially in warm places! But in places like the Southern Ocean, temperatures are well-below freezing. When water cools to below 0°C, it wants to freeze—and in most cases, it will. However, in some unique environments, water can stay as a liquid well-below 0°C! We call this super-cooled liquid water. A cloud that exists in temperatures below 0°C and remains liquid is known as a super-cooled liquid water cloud. Above the Southern Ocean, there are more super-cooled liquid water clouds than anywhere else in the world [1].

Super-cooled liquid water can exist only if the water is extremely pure and does not contain a special type of cloud condensation nuclei known as an ice nucleating particle. If the water droplet contains an ice nucleating particle, or if it mixes with air that has ice nucleating particles in it, the water will freeze.

Only some types of cloud condensation nuclei can also act as ice nucleating particles—it depends on what the particle is made of. Pollution and dust are two examples of particles that can help water freeze into ice crystals. If a cloud droplet does not make contact with an ice nucleating particle, it will stay liquid until the temperature drops below -38°C! These processes are shown in Figure 1.

The Southern Ocean Is A Unique Environment

The Southern Ocean is very far away from land and humans (Figure 2). This means the water in the clouds has had very little contact with pollution or dust. Because of how far away it is, few ice nucleating particles are present in the Southern Ocean. Clouds in the Southern Ocean are mostly formed with cloud condensation nuclei from the ocean, such as sea spray, or from the gases released by tiny plants that live in the ocean, such as phytoplankton [2]. Some particles in sea spray can act as ice nucleating particles. However, we are not sure how big a role those particles play yet.

Not many other parts of the world, including other oceans, are as pure as the Southern Ocean. The lack of ice nucleating particles causes more clouds in the Southern Ocean to be made up of super-cooled liquid water than anywhere else in the world. The Earth’s other polar ocean, the Arctic Ocean, is very close to land and human influences, meaning that more ice nucleating particles exist there. Super-cooled liquid water clouds can form in the Arctic under the right conditions, but satellite data show they occur much less frequently.

The remoteness of the Southern Ocean makes it unique. But its remoteness also makes it one of the most difficult places in the world to study!

Super-Cool And Super-Important For The Planet!

In addition to sounding cool, super-cooled liquid water clouds are also important for Earth’s weather and climate.

Weather and climate are different. The weather is the current state of the atmosphere, which can often change quickly. For example, if it is sunny one day and raining the next—that is the weather. The climate consists of the average conditions over a long time. For example, you may know that summertime is usually warm and dry, or warm and raining at your place, depending on where you live. Both weather and climate need to be understood because they impact the choices we make every day. Should I take an umbrella today? Check the weather! I want to spend my holidays swimming at the beach—where would I go? Somewhere with a warm and dry climate!

Super-cooled liquid water clouds are important for both weather and climate because they behave differently compared to ice clouds. They last longer than ice clouds and they also reflect more sunlight back out into space (Figure 3). Both of these behaviors can help to cool the surface of the ocean when the sun is shining. This means that whether clouds are made of liquid or ice can influence how warm the surface of the ocean is.

This is important for the long-term climate in balancing the amount of energy in the Earth’s atmosphere, as well as for all the marine animals that live near the surface and are influenced by the weather.

How Do We Study Southern Ocean Clouds?

As scientists, we want to understand super-cooled liquid water clouds so that we can understand and predict the weather and climate of the Southern Ocean and the Earth. One way to do this is by using computers to make models. Climate models (and weather forecast models) use our knowledge of physics and chemistry to create a “virtual” version of our atmosphere on a computer. Right now, our climate models have some issues. They think that the Southern Ocean has the same amount of ice nucleating particles as the rest of the world’s oceans [3]. We know this is not true. Because of this, the models predict fewer super-cooled liquid water clouds than what we observe, which means they also predict too much sunlight reaching the ocean [4]. All of this means that our climate models are less accurate about many other parts of the Southern Ocean climate, like the ocean surface temperatures!

Scientists are working hard to fix these problems by using observations to test and improve the models, but the data are hard to get—the Southern Ocean is a remote and harsh place, so very few measurements of the clouds over the Southern Ocean exist compared to other parts of the world. Most of the observations we have for the region were made during only the summer, and the expeditions often only last a month or so [5]. This lack of data over the Southern Ocean is one of the reasons the climate models think that there are many more ice nucleating particles in the region than there really are.

More Measurements A Must

Thankfully, scientists around the world are planning lots of research expeditions into the Southern Ocean over the next few years. These expeditions include ships, planes, drones, satellites, and land-based stations on islands and on the Antarctic continent. Scientists will be studying many areas of the vast Southern Ocean, and they will also try to take measurements in seasons besides summer, to try to fill in the gaps in our knowledge.

Many of these expeditions will be trying to understand these particles and how they change the clouds. Measurements will include how many particles are in the air, how big they are, what they are made of, and if they can act as ice nucleating particles. We also want to know where the particles came from, so we will measure what is in the ocean and look at what the wind might be carrying from far away.

We will also look at the types of clouds that are present and how much sunlight is passing through them. Super-cooled liquid water clouds look like any other clouds, so it is difficult for us to tell them apart just by looking at them. This is another reason why new measurements are so important. For example, we can use a technique called Light Detection and Ranging (LiDAR), which shines a laser into the clouds and measures what is reflected back, giving us information that helps to identify what the clouds are made up of. LiDAR instruments can be carried by ships, to look up at the clouds, or can be located on satellites and planes, to look down at them.

Measurements like these will then be fed into our computer models, to improve both weather forecasts and climate predictions!

Supercool Super-Cooled Southern Ocean Clouds

We hope by now that you will agree with us in thinking that Southern Ocean clouds are supercool! Their importance for the Southern Ocean, as well as for the entire Earth, has been recognized by scientists for a long time. However, we still have a lot of work to do to make sure we understand these clouds and model them properly. The new research expeditions will go a long way in helping with this effort. Maybe one day you can help too!

Glossary

Cloud Condensation Nuclei: ↑ Tiny particles in the air that water condenses on to form cloud droplets.

Water Vapor: ↑ Water in the air that is in the gas phase.

Super-cooled Liquid Water: ↑ Water that exists as a liquid below 0°C.

Ice Nucleating Particles: ↑ Tiny particles in the air that help water turn from liquid to ice.

Weather: ↑ The state of the atmosphere at a particular place and time.

Climate: ↑ The long-term, average conditions of the atmosphere.

Climate Models: ↑ Mathematical models of the Earth’s atmosphere over long time scales. We use these models for predicting the Earth’s future climate.

Light Detection and Ranging (LiDAR): ↑ A technique that sends out a laser beam and measures how long it takes to bounce off an object, telling you about the distance, shape and size of the object.

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Acknowledgments

This authors received grant funding from the Australian Government as part of the Antarctic Science Collaboration Initiative program, under the Australian Antarctic Program Partnership, ASCI000002. The authors would like to acknowledge the Australian Antarctic Program Partnership for supporting this work.