The discovery of massive supersaturations with respect to ice in upper tropospheric cloud-free air and inside cirrus clouds calls into question our understanding of the physics of ice cloud formation. These findings represent potentially important modifications in our characterization of upper tropospheric and stratospheric water and energy budgets, with implications for cloud formation, for fluxes of water and radiation, and for atmospheric chemistry. Various hypotheses put forward as explanations will be discussed in this talk.

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