the systematic study of the composition, structure, and dynamics of the Earth's atmosphere. The atmospheric sciences are traditionally divided into three topical areas: meteorology, climatology, and aeronomy. Meteorology is the study of the daily or even hourly changes in weather within the troposphere and lower stratosphere. Climatology, by contrast, focuses on statistical descriptions of the weather in the same layers of the atmosphere over longer periods of time ranging from a month to millions of years. Aeronomy is concerned with the physical state and dominant processes of the upper atmosphere—i.e., those regions above the lower stratosphere. See climatology; meteorology.