The clouds in the mid-latitudes of the southern hemisphere therefore ice up to a lesser extent and contain more liquid water at the same temperatures than their counterparts in the northern hemisphere. They are therefore lighter and reflect the incoming sunlight more, which in turn has consequences for the heat radiation of the earth.
However, the clean air is not the only explanation for the differences. A second factor also plays a role: Atmospheric gravity waves also influence cloud formation. They arise after the westerly winds have flown against and cross the Andes. At the back of the mountain range they are swirled, resulting in the gravity waves. »By measuring the updrafts and downdrafts characteristic of the waves, we were able to identify clouds that had been influenced by these waves and filter them out of the overall statistics. They show that these gravity waves and not the lack of ice nuclei are primarily responsible for the excess of droplets at minus 25 degrees Celsius,” says Radenz.
However, it is unclear whether this is only a local phenomenon or whether gravity waves also occur over the open sea. They could also occur in the New Zealand Alps or on the Antarctic Peninsula. Future measurements should determine this. In the long term, the data should improve climate models, which so far have only inadequately represented the radiation balance of the southern hemisphere.