The freezing temperatures of H2SO4/HNO3/H2O mixtures were systematically documented. Nitric acid was found to affect freezing significantly. Measurements show that nitric acid can cause substantial supercooling over a broad composition range. However, some ternary compositions, like to those in polar stratospheric clouds (PSCs), have high freezing temperatures. The freezing of PSC particles could be controlled by the temperature and vapor pressure of both nitric acid and water in a non‐linear way. Formation of polar stratospheric clouds may be forecasted on the basis of conditions of temperature and vapor contents of water and nitric acid.

Interannual variability of high‐level cloudiness (HC) is examined using global outgoing longwave radiation (OLR). Variations of HC are analyzed versus a measure of global stratospheric aerosol amount and an El Nino index. Volcanic aerosols are apparently associated with widespread increases of up to 10% in an OLR‐based HC index. The most significant effects occurred in middle latitudes and persisted for several years after major eruptions. El Nino is found to be associated with decreased cloud activity in the subtropics. This study suggests that volcanic aerosols can significantly modify global cloudiness, and that stratospheric aerosol loading can be an important variable controlling the interannual variations of high level clouds and climate.

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