“Afflictions of the Tropics’ Brink: Medicine, Meteorology, and the Cultivation of Place in the Antebellum Gulf South” provides a new perspective on the United States’ expansion into the southeastern borderlands. It reveals that in these newly acquired territories, medical and atmospheric knowledge were entangled with U.S. imperial anxieties and ambitions. This dissertation argues that at the heart of these ideologies were fraught imaginaries of the tropics, which was especially stark for a region that was climatically linked to the tropical Caribbean but politically bound to a predominantly temperate nation. In this milieu, climatic knowledge guided the actions of individuals, shaped expert and lay orientation toward the environment, and drove ambitions to reform places and the people that inhabited them. Guided by the overlapping practices of medical geography and climatology, Southern planters, physicians, and regional boosters relentlessly observed the weather. They chronicled wind patterns, temperature fluctuations, and other atmospheric phenomena across hours, days, months, and years. They used this knowledge in numerous ways, such as how to recruit white settlers to contested land, shape the built environment of slave plantations, and nurture popular fantasies about the commercial and medical benefits of a tropical America.