Under certain conditions, water can exist in a liquid state well below its freezing point, but reliably measuring the temperature of such ‘supercooled’ water has proved challenging. Physicists now report a technique that allowed them to make exceptionally accurate measurements.
Robert Grisenti at the GSI Helmholtz Centre for Heavy Ion Research in Darmstadt, Germany, and his colleagues sprayed micrometre-sized droplets of water into a vacuum. Molecules of water evaporated from the surface of the droplets, making them smaller and colder.
The droplets shrink proportionally as they cool, which enabled the researchers to determine the water’s temperature by measuring droplet size with a laser. The team achieved this with 10-nanometre precision, and calculated the water’s temperature to be –42.6 °C.
Because supercooled water occurs naturally in Earth’s upper atmosphere, gaining a better understanding of its properties, as well as how and when it transforms into ice, could help researchers to develop more-reliable climate models, say the authors.
An earlier version of this story incorrectly stated that the summarized paper included observations of "a record low temperature" for liquid water.