Air turbulence can substantially accelerate the appearance of large droplets
triggering rain by presenting a new mechanism, the "sling effect", which
increases collisions of droplets that have become detached from the airflow.
Line First, vapor condensation in cloud cores produces small droplets resembling one
(5) another in size, which then expand to raindrop size by coalescing under the
effects of air turbulence—a force thought to cause collisions of similar-sized
droplets whose radii exceed a few micrometers. Then, turbulent vortices act as
small centrifuges that spin heavy droplets out, creating concentration
heterogeneities and jets of droplets, both of which increase the mean collision
(10) rate, which in turn accelerates rain initiation.
One can conclude that rain prediction requires a quantitative description of
droplet collision in turbulence, a mechanism which helps meteorologists to
forecast rainfall, but detailed understanding of the phenomenon entails
consideration of such factors as warm and cold fronts stretching over hundreds
(15) of miles, individual clouds perhaps a mile or so across, and even, as the
mechanism illustrates, tiny eddies perhaps a few centimeters or so in size.

The primary purpose of the passage is to（）