  When cloud particles are small enough, they move with the turbulent air in
the cloud. On the other hand, as particles become larger their inertia affects
their motions, and they move differently than the air. These inertial dynamics
impact cloud evolution and ultimately climate prediction, since clouds govern
the earth's energy balances. Yet we lack a simple description of the dynamics.
Falkovich et al. describes theoretically a new dynamical mechanism called the
"sling effect" by which extreme events in the turbulent air cause idealized
inertial cloud particles to break free from the airflow (Falkovich G, Fouxon A,
Stepanov MG 2002 Nature 419, 151). The sling effect thereafter causes particle
trajectories to cross each other within isolated pockets in the flow, which
increases the chance of collisions that form larger particles. We combined
experimental techniques that allow for precise control of a turbulent flow with
three-dimensional tracking of multiple particles at unprecedented resolution.
In this way, we could observe both the sling effect and crossing trajectories
between real particles. We isolated the inertial sling dynamics from those
caused by turbulent advection by conditionally averaging the data. We found the
dynamics to be universal in terms of a local Stokes number that quantifies the
local particle velocity gradients. We measured the probability density of this
quantity, which shows that sharp gradients become more frequent as the global
Stokes number increases. We observed that sharp compressive gradients in the
airflow initiated the sling effect, and that thereafter gradients in the
particle flow ran away and steepened in a way that produced singularities in
the flow in finite time. During this process both the fluid motions and gravity
became unimportant. The results underpin a framework for describing a crucial
aspect of inertial particle dynamics.
