We have developed a closed-path water vapor sensor based on optical absorption of an atomic argon emission line that is suitable for rapid response (10 Hz bandwidth) humidity measurements in the lower and middle troposphere. The sensor uses an argon emission line at 935.4 nm that can be Zeeman-split into two components, only one of which is strongly absorbed by a water absorption line at 10,687.36 cm-1 (~935.6 nm). The sensor exhibits a noise figure-of-merit of 40 ppmv Hz-1/2 at low humidities (below 0.5%); the noise level slowly rises to 60 ppmv Hz-1/2 as water vapor mixing ratios reach 2.5%. The performance of this sensor was compared to that of a chilled mirror hygrometer and a capacitance-based polymer sensor, all of which were installed on board a Twin Otter airplane during an atmospheric humidity measurement campaign conducted northeast of Hawaii. Over a range of water vapor volume mixing ratios of 0.1--3% (at altitudes of 50--3000 m), the sensors were shown to agree with one another within 5% of the measurement in regions of slowly changing humidity. However, in regions of large humidity gradients, the optical sensor (with the use of an appropriate inlet tube) was able to track water vapor features on the scale of tens of meters or less that neither the slow-response chilled mirror hygrometer nor the capacitance-based sensor could adequately reproduce. The optical sensor was also able to track humidity in the vicinity of clouds and discern fine structure within the cloud itself.