Meteorology has long been central to the development of American commerce. The Weather Bureau aided shipping and agricultural interests in the late 19th and early 20th centuries by forecasting cold waves and monitoring the movement of cyclones that endangered shipping, especially on the Great Lakes. After World War I, however, the air transport industry became meteorology's most influential constituency. The emerging airlines used meteorological knowledge to set corporate strategy, manage daily operations, and market flying as a safe and efficient activity. To operate reliably and profitably, airlines required both more detailed weather forecasts and sophisticated new knowledge about the dynamics of the upper atmosphere--the causes of turbulence, squall lines, and ice formation within clouds, for instance. These needs reshaped the Weather Bureau during the 1920s and 1930s. But the Weather Bureau was not a research organization, nor responsive enough to the urgent demands of aviators. Airlines like Western Air Express (later re-organized as TWA) turned to fledgling academic meteorology departments for help. Airlines provided reliable employment to these program's graduates, especially during the Great Depression, while supporting meteorological research. The needs of aeronautics in turn rewarded meteorologists' attention to the physics and thermodynamics of the upper air, helping fashion American meteorology as a research-based scientific discipline built around college degrees and three-dimensional geophysical models of the atmosphere. Meteorological knowledge thus made aviation commercially viable while airlines reshaped what it meant to be a meteorologist in the United States.