The moisture in clouds eventually condenses into droplets that grow into raindrops and fall. Some steps in this process are well understood, but others are not. In particular, scientists have struggled with the problem of how droplets grow from about 30 microns to 80 microns, where they’re big enough to start falling and merging.
Laboratory experiments and numerical simulations (below) have shown that turbulence can help drive small water drops together. When droplets are tiny and light, they simply follow the air flow. But when they’re a little heavier, turbulent eddies (seen in orange below) act like miniature centrifuges, flinging larger water droplets (shown in cyan below) out into clusters, where they’re more likely to collide with one another.
Although this effect has been seen in experiments and simulation, it’s been difficult to capture in clouds themselves. But a new set of test flights (above) confirms that this mechanism is present in the wild as well! (Image credit: UCAR/NCAR Earth Observing Laboratory, P. Ireland et al., source; research credits: M. Larsen et al., P. Ireland et al.; via APS Physics; submitted by Kam-Yung Soh)