Rain falls from warm clouds that contain no ice crystals at all — yet no one has been able to explain exactly how. A Max Planck team has now caught the process in action, using a helium-filled kite balloon to peer inside a cloud with unprecedented resolution.
The platform, called CloudKite, carries two custom optical instruments: a holographic imager that reconstructs the 3D positions and sizes of individual droplets 75 times per second, and the first airborne particle-image velocimetry system to measure turbulence. Drifting at just 10 metres per second, the balloon can sample the cloud every 12 centimetres — roughly 250 times more often than research aircraft ever have.
Inside a 55-metre section of a shallow cumulus cloud, the researchers found that droplets are not evenly mixed at all. Instead, they bunch into 'hotspots' barely a metre across, where droplets sit close enough together that collisions — and the merging into larger, falling drops — become far more likely. 'These localized hotspots may therefore represent the places where rain starts in shallow cumulus clouds,' says first author Birte Thiede.
The finding overturns the long-standing assumption that droplet clustering inside clouds is weak and uniform. Warm clouds produce much of the Earth's rainfall, especially in the tropics, and how efficiently their droplets grow into raindrops is one of the largest uncertainties in climate projections. With future campaigns planned in Amazonia, the Baltic Sea and northern Finland, the team hopes the hidden anatomy of clouds will soon sharpen both weather forecasts and climate models.
