Glaciation of a Cumulus at Approximately −4C

Abstract

A long-lived cumulus cloud off the southern coast of Australia was found to contain ice crystals, even though the coldest temperature in it was about ?4C. Two instrumented aircraft made successive passes through the cloud and continuous samplers were used to replicate cloud particles. It was found that columnar crystals were present in concentrations up to 100 liter?1. This was at least three orders of magnitude greater than the expected concentration of active ice nuclei as determined by cloud chamber measurements made under the base of the same cloud. It appears that some ice crystal multiplication mechanism was at work, though the exact nature of the process cannot be decided on available evidence.