Scientists have determined how turbulent air inside clouds causes rain to fall, according to a new study. The findings, published today in the journal Nature, could help meteorologists make more accurate rain predictions for various types of clouds.
For raindrops to fall from a cloud, they must grow big enough to drop swiftly through the air. First, water vapor condenses out of the surrounding air to form drops that grow slowly to diameters of approximately 20 microns. Once they reach this size, the drops grow further through a process known as coalescence--in short, they crash into one another. Air turbulence accelerates this process. Gregory Falkovich of the Weizmann Institute of Science in Israel and his colleagues designed mathematical models to describe how these air disturbances alter the rate of collision between droplets. They determined that vortices that form within the cloud act as centrifuges, which spin heavier droplets outwards. This motion creates jets of moving droplets and alters the distribution of different-sized droplets, both of which increase the collision rate. The team also described a new mechanism to explain the increased coalescence, dubbed the sling effect. It seems that the air turbulence serves to accelerate a string of droplets, which then become detached from the churning airflow before colliding with other droplets. As a result, the authors conclude that "air turbulence can substantially accelerate the appearance of large droplets that trigger rain."