Purdue University expert highlights the science of clouds in Earth’s sky and beyond

Alexandria Johnson does hard science on the most nebulous of subjects: clouds. As an atmospheric scientist and assistant professor of practice in Purdue University’s College of Science, she studies clouds wherever they are: in her lab, on Earth, throughout the solar system and into the galaxy.

“The coolest thing about my research is that I can see clouds every day,” Johnson said. “I can look up into our own atmosphere and watch them change and evolve. Then I can take that knowledge and apply it to other planetary bodies, both within and outside our solar system.”

The science of clouds covers a lot of ground. Her research shines light into topics ranging from the rainfall and microplastic precipitation in Indiana to the climates of moons and planets far outside the realm of human experience.

Studying clouds in their natural environments can be complex and subject to the variations of climate, weather and observation devices. Johnson’s solution is to create her own homegrown clouds to study in her lab in the Department of Earth, Atmospheric, and Planetary Sciences. She strips the systems down to their basics to get a clear understanding on how the particles that make up clouds form, develop and interact with their environment. Nothing in her lab actually looks like a cloud; there are no mists swirling picturesquely in glass bottles. It’s mostly lasers and big black boxes. But the behavior of these lab-based cloud particles mimics the behavior of cloud particles in massive sky-sweeping clouds, only in miniature.

“Of course, we don’t grow them at quite the same scale you see in an atmosphere,” Johnson said. “Instead, we can take one particle that is representative of a cloud, pump in different gases, and change the temperature and pressure of the system. We then watch as that particle grows, shrinks or changes phase with time, which are processes that happen in clouds everywhere.”

Clouds on Earth don’t often form without the aid of a nuclei, or a particle, and in some cases what would be considered a nuclei on Earth may be an exotic cloud elsewhere. The particles in Johnson’s lab, like all particles, have a charge. Johnson and her team use an electric field to levitate and contain the individual particles so that they can’t move around. These particles are then stable for extended periods of time, which enables long-term research experiments, where the pressure, temperature, electric field and laser illumination may be tweaked, and observations recorded. Other methods build upon these to allow the team to look at groups of particles and observe how they scatter and polarize light.

Using methods like these, Johnson can study how clouds form and what different cloud particle shapes and compositions can reveal, and she is able to understand the conditions that lead to different cloud types and behaviors. Like aeronautical engineers using a wind tunnel to observe how currents move around structures, Johnson uses these particles to understand the microphysics that underpin vast and complex systems.

Many scientists – climatologists, meteorologists and planetary scientists, to name a few – study clouds as part of their broader research. But Johnson is one of the few who studies the particular physics of clouds in the laboratory.

“There are not many of us who dig into the microphysics of how clouds form,” Johnson said. “Anyone who studies the atmosphere has a general sense of knowledge about clouds. But none of those systems work without the physics. We have to understand the microphysics to truly grasp the complexities and implications.”

Every cloud has a silver lining
It’s a long-running joke that the nights of notable astronomical events on Earth seem to be almost supernaturally disposed to be cloudy. That is true of other planets, too.

Using enormous, advanced, vastly powerful telescopes, astronomers can peer through miles and light-years of space just to find clouds blocking their view of the planet itself. Rather than the planet’s surface, they can only perceive the opaque atmosphere that enswathes it.

Every planetary body in the solar system that has a dense atmosphere, and many outside of it, has clouds in that atmosphere. Even bodies with thin, wispy or intermittent atmospheres – like Pluto – have particulates hanging in the atmosphere that, while not true clouds, are a haze of particles and share many of clouds’ properties.

“Clouds are a ubiquitous feature of planetary atmospheres,” Johnson said. “This is something we’ve seen from our own solar system, and when we look at exoplanet atmospheres, it’s no surprise that we find clouds there too. Unfortunately, they tend to block our view of the atmosphere that is below.”

Scientists have been able to send probes and rovers to close planetary neighbors, including Venus and Mars. But for bodies that are farther away, including exoplanets – planets in other star systems entirely – scientists must come up with clever ways to conduct science.

“The astronomers find the clouds to be an annoyance. They get in the way of the data they want, whether that’s learning about the surface of the planet or its atmospheric composition,” Johnson said. “We see it a little differently. Yes, they’re there. We can’t get rid of them. So let’s use our understanding of clouds on Earth and planetary atmospheres of our solar system to learn about these things that we can’t observe in exoplanets.”

Most of the planets Johnson studies are “cool” planets. While Earth seems balmy (with planetary temperature averages around 60 degrees Fahrenheit), it is actually chilly by planetary standards, when contrasted to large, gas giants orbiting close to their stars like hot Jupiters.

Johnson and her team accumulate information about planetary bodies in Earth’s solar system or exoplanets. Astronomers can collect spectrographic data to analyze the chemical compounds that make up the atmosphere and use mathematical models, observations and gravitation studies to determine a planet’s mass, speed and orbit. Combining that information with insights from her laboratory studies, Johnson can help astronomers determine what a planet’s atmosphere might be like and extrapolate its chance for hosting life.

“Our big questions are when, where and why do clouds form in these atmospheres?” Johnson said. “If we want to understand these enshrouded exoplanets, we need to understand the clouds. That understanding gives us insights into the atmospheric chemistry at work, atmospheric circulation and the climate. In a way we ground-truth astronomical observations.”

Both sides now
Johnson is also looking up at the clouds from below, a little closer to home. In a current study, she is examining the role microplastics play in cloud formation. Microplastics pollution, which has been found just about everywhere, including large bodies of water like the Great Lakes, may form a part of clouds or be scavenged by precipitation, then shower the landscape in rainstorms and snowfall. Those microplastics have dire implications for ecosystem health, human health and agriculture.

Understanding how they become attached to clouds, move through weather systems and impact the landscape when deposited can help Johnson and her team protect life on Earth, just as they explore the possibility of livable conditions on other planets.

“It’s the same physics,” Johnson said. “It’s the same processes, all throughout the universe, and it brings me a huge amount of wonder and joy. As an undergraduate physics major, I chose a senior research project studying how water droplets froze under varying conditions. I literally watched a droplet freeze hundreds of times to study the process and was entranced. I said, ‘This is what I want to do with my life. This is amazing. I want to study clouds.’”