The clouds occur at very high altitudes, at a temperature of about minus 40-50 degrees Celsius. They are generally seen when a warm or occluded front is approaching. They are very high in the troposphere, and generally mean that precipitation, usually rain, is approaching.
In situ observation of cirrus scattering phase functions with 22(deg) and 46(deg) halos: Cloud field study on 19 February 1998
Nov 15, 2001; ABSTRACT Observations of halos and related phenomena due to ice crystals are commonly reported from ground observations and...