It is very challenging to observe cirrus clouds, which is a type of high-altitudinal clouds composed mostly of ice crystals below -40°C. There are two major pathways for ice crystals to form at this cold temperature range: homogeneous nucleation and heterogeneous freezing. Homogeneous nucleation generally requires a higher relative humidity threshold to be reached, in order to have liquid aerosols freeze into ice particles. On the other hand, heterogeneous freezing requires a lower threshold of relative humidity, yet it also requires the existence of specific types of ice nucleating particles (INPs).
Due to the complexity of ice crystal formation mechanisms and the difficulty of quantifying the global distributions of INPs, it is still very challenging to estimate anthropogenic influences on clouds, particularly for ice clouds, because ice crystal formation is highly sensitive to the chemical content of aerosols. Some aerosols, such as dust and metallic particles, can facilitate ice crystal formation, while others, such as organic aerosols, can inhibit such event. In particular, it is still highly uncertain how black carbon from biomass burning and aviation emission influences ice cloud formation. Here we use in-situ observations based on multiple flight campaigns to provide a statistical analysis on the ice microphysical properties of cirrus clouds. A method was developed in Diao et al.  that separate the five evolutionary phases of cirrus clouds. Comparisons on the evolution of relative humidity, ice crystal numbder concentration and mean diameter were conducted between the Northern and Southern Hemispheres [Diao et al., 2014].