In a cloud, billions of droplets swirl around, collide, and merge: that's how rain is born. But exactly how turbulence, those invisible air swirls, helps or hinders droplet growth has been unclear. In the lab, researchers recreated a slice of a cloud and tracked every droplet.
It turns out that vortices don't always speed up droplet merging. If all droplets are roughly the same size, turbulence barely helps them collide. But once there's a mix of sizes, small and large droplets, the vortices start pushing them together, and droplets grow faster. It's like in a crowd: if everyone moves at the same speed, there are few collisions, but if some hurry and others dawdle, people bump into each other constantly.
For rain and climate forecasts, it's important to know when turbulence kicks in. The scientists suggest refining cloud models so they account for the spread in droplet sizes. Then predictions of downpours and droughts will become more accurate.
Droplets in a cloud can vary in size by a factor of a thousand — from tiny, dust-like specks to large, pea-sized drops.