Environmental Context. Meteorological weather—temperature, pressure, wind direction—is familiar to all, and contrasts with meteorological climate in short-term (weather) versus long-term (climate) influence. From the atmospheric chemistry side, the focus has largely been on the chemical climate, the long-term mean concentrations of important trace gases and aerosols. An emerging new focus of study is the chemical weather—the tremendous short-term variability of the atmospheric chemical composition, resulting from the strong influence of meteorological variability, chemical complexity, and regionally and temporally varying emissions.