We May Finally Understand Why Clouds Are Different Between Earth's Hemispheres

You might think that all of Earth is covered in clouds.

Scientists are familiar with the fact that the clouds in the Southern Hemisphere are more abundant and reflective than the clouds in the Northern Hemisphere.

New research sheds light on why clouds work differently in the two hemispheres, and in particular the role that updrafts play in the formation of clouds.

The longest dataset ever collected in the region was used in the study, covering three years in Germany, Cyprus, and Arena Puntas in Chile.

The Southern Hemisphere as a whole is so pristine that a high percentage of it is ocean rather than land. It means cleaner air, less aerosol particles for cloud droplets to freeze around, and brighter clouds.

In the Southern Hemisphere, cloud cover is less dense and has more liquid water at the same temperatures.

They influence the incident sunlight and the thermal radiation emitted from the Earth's surface differently than in the north.

The study found that air massed at higher altitudes was less affected by local pollution. The clouds over Punta Arenas formed ice less often than the clouds over Leipzig.

The team discovered something new, but this matches previous research. When the air is even colder, gravity waves created by westerly winds from the Pacific collide with the Andes, are an important factor.

By measuring the upward and downward winds within the clouds, we were able to detect clouds that had been influenced by these waves and filter them out of the overall statistics.

This allowed us to show that gravity waves and not the lack of ice nuclei are responsible for the excess cloud droplets.

The next question is if gravity waves are having an impact over the open ocean or if they are exclusive to the landscape of Chile. To figure out how much of the excess liquid water in the clouds is down to updrafts and how much is down to ice crystals, further measurement will be required.

The researchers say that global climate models aren't accurate enough when it comes to representing the radiation balance of the Southern Hemisphere.

To be as useful as possible, already complex climate models need to factor in regional differences, whether that is in built-up urban areas like Leipzig, areas with clearer air like Punta Arenas, or areas with a mix of human-made pollution and natural desert dust particles.

Because of travel restrictions imposed by the global Pandemic, they kept their monitoring systems in place for an additional two years, which enabled them to factor in extra influences such as the wildfire smoke drifting over from Australia during 2019/2020.

Boris Barja is an atmospheric scientist from the University of Magallanes (UMAG) in Chile.

The research has been published.