By Sid Perkins
A fresh look at old experimental data is threatening to overturn a longstanding theory about how water droplets freeze within clouds.
Suspended water droplets can remain liquid even when they and the air that surrounds them have temperatures far below the normal freezing point, says Azadeh Tabazadeh, an atmospheric chemist at NASA’s Ames Research Center in Mountain View, Calif. Data collected in recent years show that clouds as cold as –37.5C can still contain many supercooled droplets. Such droplets freeze solid almost instantly if they bump into each other or are otherwise disturbed.