Observation of Liquid Particles at -65° in a Polar Cirrus

It is widely accepted that pure water cannot exist as a liquid below about -40°. Theoretical and laboratory studies confirm this behavior for pure water. Liquid droplets have been seldom observed in cirrus clouds down to about -50°C. The LIDAR technique can help to find out unusual supercooled clouds, when the depolarization technique is implemented: the presence of non-depolarizing layers in a cloud is indicative of a very special scattering media: scattering particles must have a symmetry axis oriented along the laser beam. This is possible either with spherical droplets or ice plates horizontally oriented. In this work, a -65°C cold, non-depolarizing cloud observed in Finland is studied, concluding that supercooled droplets are responsible for the absence of depolarization in most of the cloud. This is the coldest supercooled cirrus ever observed