The nasal cavity is essential for humidifying and warming the air before it reaches the sensitive lungs. Because humans inhabit environments that can be seen as extreme from the perspective of respiratory function, nasal cavity shape is expected to show climatic adaptation. This study examines the relationship between modern human variation in the morphology of the nasal cavity and the climatic factors of temperature and vapor pressure, and tests the hypothesis that within increasingly demanding environments (colder and drier), nasal cavities will show features that enhance turbulence and air-wall contact to improve conditioning of the air. We use three-dimensional geometric morphometrics methods and multivariate statistics to model and analyze the shape of the bony nasal cavity of 10 modern human population samples from five climatic groups. We report significant correlations between nasal cavity shape and climatic variables of both temperature and humidity. Variation in nasal cavity shape is correlated with a cline from cold–dry climates to hot–humid climates, with a separate temperature and vapor pressure effect. The bony nasal cavity appears mostly associated with temperature, and the nasopharynx with humidity. The observed climate-related shape changes are functionally consistent with an increase in contact between air and mucosal tissue in cold–dry climates through greater turbulence during inspiration and a higher surface-to-volume ratio in the upper nasal cavity. Am J Phys Anthropol 2011. © 2011Wiley-Liss, Inc.