On the rocky slopes above the lush pine forests and mountain lakes, a group of researchers has just finished setting up their equipment.

Even though the weather at sea level is a comfortable 21 degrees, the researchers need their warm clothes. The temperature rarely rises above 10 degrees at the peak at this time of year.

The mountain is Mt. Helmos on the Greek island of Peloponnese. The rocky peak is already home to an observatory, but in the past two weeks new scientific equipment has begun claiming more and more space up there.

What researchers hope to do is solve the mystery of cloud formation and changes cloud properties. In trying to better understand how Earth’s climate works and is shaped by humans, it is essential to know exactly how clouds form and how they respond to human activities.

But until today, a lot of details about the clouds are unknown, explains Ulas Im, senior researcher at the Department of Environmental Sciences at Aarhus University.

“Understanding cloud formation is essential to making projections like the one thousand scientists make every five to seven years when a new IPCC report comes out. Clouds represent the largest uncertainty in the climate projections in these reports. That’s why we try to understand them better,” he says.

Together with colleagues from all over Europe, Ulas Im is at the center of a huge research project called Clean Cloud. The idea is to learn more about how clouds and particles in the atmosphere interact, using ground and satellite observations, laboratory experiments and models.

To do this, large field experiments are planned in the European climate hotspots: the Arctic and the Mediterranean.

“We have completed our experiments in northern Greenland and are now shifting our focus to the Mediterranean with field experiments in Greece. The Arctic and the Mediterranean are the two regions in Europe most affected by climate change. By studying the clouds in these areas, we hope to better anticipate future climate change,” says Ulas Im.

A big international project

The research project is called Clean Cloud and is a collaboration with researchers from 20 institutions in 12 European countries.

The researchers aim to gain new knowledge about cloud formation and their interactions with particles over the next four years.

Ulas Im is the project coordinator.

Shrouded in clouds all winter

There’s a reason Ulas Im and his colleagues chose Mt. Helmos. During winter, the mountain is shrouded in clouds most of the time, and this allows researchers to study the cloud from within.

“Most cloud research has been done from below or from airplanes or satellites overhead. We hope that this unique approach will give us more extensive knowledge about the inner workings of clouds,” says Ulas Im.

In 2021, a smaller field research campaign took place on Mount Helmos. The data from those experiments were quite encouraging, and that’s why the researchers are doing an even bigger project there.

“In 2021, there were fewer instruments and the campaign only lasted a few weeks. Now we’re back with a lot more equipment and ready to do a long-term experiment,” he says.

The plan is for the expensive instruments to remain on top of the mountain until spring. To do this, many researchers need to take care of their equipment on a regular basis. And that’s expensive.

“We’re part of a big project … and that’s what makes these expensive research setups possible. But we hope to learn a lot more about clouds during the project,” he says.

No clouds no particles

The instruments on top of the mountain will soon be submerged in the clouds. This allows Ulas Im and his colleagues to study how small particles in the air affect cloud formation.

Because, he explains, particles are the key to understanding clouds. Without particles like dust, pollen, bacteria and exhaust from cars and factories there would be no clouds.

“The basic idea is that you will always have water vapor in the atmosphere. Dust or other particles provide a surface for water vapor to condense on. When water condenses on these particles, they form cloud droplets,” he says.

But not all particles produce new clouds. Instead, some particles change the characteristics of clouds.

“Clouds have liquid droplets or ice. Some clouds are just liquid droplets. Most clouds are both. Mineral dust or biological particles such as pollen are effective in making cloud ice. Without them, the temperature required for the droplets to freeze would be much lower.

“With more ice crystals, clouds become transparent to sunlight, which then reaches the Earth’s surface. However, with global warmingwe move from ice to more liquid clouds. This is important because clouds with more liquid droplets reflect more of the sunlight back into space, and this could affect the temperature on Earth.”

Ulas Im and his colleagues, however, expect this to change in the near future.

“Thanks to reductions in human-made emissions thanks to international agreements, we expect fewer man-made particles in the atmosphere. This suggests that the relative importance of natural aerosols may be more important in the future, which we call post-fossil.”

“Therefore, we need to better understand and anticipate how this competition will work in the future.”

How humans shape clouds

It is not only naturally occurring particles that affect clouds. Particles from human activities also play an important role.

“Above the oceans, ships make plumes, releasing particles. This actually changes the properties of the local clouds. The same is true with airplanes. They have a similar effect. Or large power plants that burn fossil fuels. All of this changes the environment. at the local level”, says Ulas Im.

In the northernmost part of Greenland, where the first part of the project was carried out, very few people pass. Some particles blow north from Europe, Russia and the US, but mostly in winter. In the summer, researchers mostly measure particles that occur naturally there.

This is not the case with Greece.

Dust from the Sahara combined with biological particles from European forests coupled with small particles from human activities such as industry, residential heating and the many ships and planes passing through the Mediterranean create a very different chemical cocktail in the atmosphere.

“Man-made particles generally have a cooling effect on the climate. But since they are issued together with CO₂which has a warming effect and stays much longer in the atmosphere, is still CO₂ who leads climate change.”

“However, particles mask some of this warming by directly scattering solar radiation, but also indirectly by affecting clouds. It is therefore important that our climate models accurately represent this and understand the human impact on global warming.”

Ulas Im and his colleagues hope to publish the first research results within a year.