Note to moderator: Read the answerline carefully. The products of this small-scale process exist in stable equilibrium on the ascending branch of a Köhler curve. Pseudomonas syringae secretes proteins that promote this small-scale process in plant cells, but can also promote it in the atmosphere. After an INP induces this process, its products rapidly grow due to a difference in vapor pressure in the Wegener–Bergeron–Findeisen process. For sulfuric acid hydrates, the presence of ammonia reduces this process’s free energy barrier, which generally decreases for clusters larger than the critical radius. At high mixing ratios above the lifting condensation level, this process occurs on CCNs. This process’s homogeneous form is unlikely, so it usually occurs on namesake “sites,” such as silver iodide seeds. For 10 points, name this process in which molecules aggregate to form a new phase. ■END■
ANSWER: nucleation [or nucleate or nucleating; accept water, droplet, or ice nucleation; accept ice-nucleating particle or ice nucleation-active proteins; prompt on condensation, freezing, crystallization, deposition, phase transitions, phase changes, descriptions of droplet formation, descriptions of cloud formation, or descriptions of the formation of precipitation, hail, snow, or rain by asking “what small-scale process is that initiated by?”] (An INP is an ice-nucleating particle. A CCN is a cloud condensation nucleus.)
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