Back in the forties of the last century, scientists conducted an experiment during which they were able to cause a rainstorm by dropping pieces of dry ice onto cumulus clouds. But until now the mechanism of this phenomenon remains unclear. Now Prasanth Prabhakaran from the Institute for Dynamics and Self-Organization. Max Planck and his colleagues created an artificial cloud and found that the fall of just one drop of cold water triggers a chain reaction with the formation of new drops.
In real clouds, water droplets usually grow adhering to the smallest aerosol particles, which are dust, ice crystals, and even bacteria. But Prabhakaran and his colleagues were able to model a system in which high-pressure liquid droplets can grow on their own without the need for any aerosols. This greatly simplified the task of making a small cloud in the laboratory.
For the experiment, the scientists used a box with a heated bottom and a cooled top lid. This was necessary to create a temperature difference between the bottom and top layers of four degrees.
In addition, to recreate atmospheric processes on earth, the researchers performed a series of chemical manipulations. In particular, helium gas played the role of air inside the box, and sulfur hexafluoride was taken instead of water. Like atmospheric moisture, this substance can be both liquid and vapor, but forms droplets at a lower temperature and pressure than water.
As a result, a model of the water cycle in nature was recreated inside the box. A small pool of liquid sulfur hexafluoride appeared at the bottom, which partially evaporated and floated as a cloud in gaseous helium. On the cold lid, sulfur hexafluoride condensed into drops that broke off and fell down as they flew through the gas.
Observations of the created model showed that the falling droplets cooled the warmer environment and caused the formation of a large number of microdroplets. This happened according to the same principle as the appearance of condensation on a cold surface in contact with warm air.
According to scientists, this model, described in the journal Physical Review Letters, can explain some atmospheric phenomena, when the downpour begins in a matter of seconds, as if someone turned on a tap in the shower. Under certain conditions, the smallest drops of water in the clouds can merge into fairly large formations. If such droplets enter the ascending air currents, they rise to colder layers and then break down, provoking the massive creation of new droplets.
However, experts believe that atmospheric processes are so complex and diverse that experimentally modeled conditions in nature are quite rare. But perhaps the next time you unexpectedly get caught in the rain, it’s not the weather forecasters to blame, but a few occasional cold drops.
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