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Water vapor enjoys unique importance in Earth’s atmosphere and human environments in space. In spite of this importance, humidity measurement remains a difficult technological problem, and no single instrument is optimal for all applications. We have developed and demonstrated a high-sensitivity dewpoint hygrometer in flight tests on a small radiosonde balloon and the NASA DC8. This instrument achieves fast response to atmospheric humidity by using a surface acoustic wave (SAW) device to detect condensation with much higher sensitivity than conventional optical dew detectors. An early prototype showed more than an order of magnitude faster response than chilled-mirror hygrometers in tropospheric humidity measurements on the NASA DC8. For the radiosonde experiment, we miniaturized and integrated the SAW hygrometer into a 1 kg package that includes pressure and temperature sensors, GPS, a programmable instrument controller, a high-speed radio modem, and lithium-ion batteries.