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We describe a new laser‐based hygrometer for ambient water vapor monitoring which uses a fiber‐optic coupled, near‐infrared diode laser in conjunction with high frequency wavelength modulation spectroscopy. The instrument operates unattended, uses little power, can be extremely compact, and exhibits high detection sensitivity. This instrument was flown on a KC‐135 aircraft for a period of six months and measured frost points at altitudes between 10 000 and 40 000 ft. The water detection sensitivity corresponds to a volume mixing ratio of 8 ppm V. Further improvements including the use of newly designed near‐infrared lasers are expected to realize frost points below -100 °C.