In turbulent air, droplets don't fly in straight lines: they get tossed around like dust motes in a beam of light. Physicists have built a setup that tracks each droplet, which is about a tenth of the thickness of a human hair. A camera captures their motion, and a computer reconstructs their trajectories.
The main obstacle is false tracks, when the camera mistakes a glint or reflection for a droplet. The authors devised a filter that removes such artifacts based on the angles of motion. After that, they were able to measure how often droplets come within less than one-tenth of the smallest eddy in the flow.
It turns out that the heavier a droplet is relative to the air, the more it tends to cluster. This is important for understanding how raindrops grow in clouds: they first merge by colliding in turbulence.
The new method opens the way to measurements at the smallest scales, where clouds and rain are born.
Turbulence in clouds works like a mixer: it bumps droplets together, causing them to merge into larger ones that later fall as rain.