Thus, clouds in the middle latitudes of the Southern Hemisphere freeze less and contain more liquid water at the same temperatures than their counterparts in the Northern Hemisphere. So they are lighter and reflect more incoming sunlight, which in turn has consequences for Earth’s thermal radiation.

However, clean air is not the only explanation for the differences. The second factor also plays a role: atmospheric gravitational waves also affect cloud formation. It arises after the westerly winds have flown towards and crossed the Andes Mountains. At the back of the mountain range, they are moved in a circular motion, which causes gravitational waves. “By measuring the characteristics of the ascending and descending waves, we were able to identify which clouds were affected by these waves and filter them from the overall statistics. They show that it is these gravitational waves, and not the lack of ice cores, that are primarily responsible for the increase in droplets at minus 25 degrees Celsius,” Raddens says.

However, it is unclear whether this is just a local phenomenon or whether gravitational waves also occur over the open sea. It can also occur in the New Zealand Alps or the Antarctic Peninsula. Future measurements should determine this. In the long term, the data should improve climate models, which have not yet adequately represented the radiation balance in the Southern Hemisphere.