Title: Clarifying the dominant sources and mechanisms of cirrus cloud formation

Abstract
Cirrus are high altitude clouds composed of ice crystals. They are the first tropospheric clouds that can scatter incoming solar radiation and the last which can trap outgoing terrestrial heat. Considering their extensive global coverage, estimated at between 25 and 33% of the Earth’s surface, cirrus exert a measurable climate forcing. Their global radiative influence depends on a number of properties including their altitude, ice crystal size and number density, and vertical extent. These properties in turn depend on the ability of upper tropospheric aerosol particles to initiate ice formation. Because aerosol populations, and therefore cirrus formation mechanisms, may change due to human activities, the sign of cirrus forcing (a net warming or cooling) due to anthropogenic effects is not universally agreed upon. Cirrus also play a major role in the water cycle in the tropopause region, affecting not only redistribution in the troposphere but also the abundance of vapor entering the stratosphere. Both the current lack of understanding of cirrus properties and the need to improve our ability to project changes due to human activities in the future highlight the critical need to determine the aerosol particles on which cirrus form.