We detail the relationships between operating environment and performance of a thin and lightweight open-cathode fuel cell based on flexible circuits that may be advantageous for unmanned air vehicle (UAV) propulsion. The open-cathode fuel cell performance is studied in a broad range of realistic atmospheric flight conditions by mounting it in a wind tunnel within an environmental chamber. The relationships between the operating environment and performance are quantitated in terms of polarization behavior and the underlying loss mechanisms are discussed qualitatively in the context of electrochemical impedance spectroscopy (EIS), DC resistance and infrared temperature measurements. The wind tunnel experiments demonstrate that open cathode operation is possible over wide ranges of ambient temperature (5-55 degrees C), relative humidity (22-90% RH), air speed (0-15.4 m s(-1)) and altitude (240-3240 m). Forced airflow is shown to improve mass transport, waste heat rejection, and water management, broadening the effective operating envelope of this open-cathode fuel cell over others that rely on free convection. Sacrificing air preconditioning renders open-cathode fuel cell performance sensitive to the conditions of the surrounding environment. The lightweight open-cathode fuel cell in this study has high specific power, exceeding 1.3 kW/kgcell in favorable ambient conditions of 5 degrees C, 50% RH at sea level. (C) 2016 The Electrochemical Society. All rights reserved.