The nitrate radical abundance is often measured by making use of its strong absorption of visible radiation in a band near 662 nm. We show that this NO3 absorption feature has strong negative correlation with water vapor, which requires that water vapor absorption be explicitly considered in attempts to measure NO3 from the ground. Concurrent observation of a second, weaker NO3 feature near 623 nm provides an independent means of confirming and quantifying the accuracy of the measurement. These considerations are included in the analysis of a full year of nighttime stratospheric NO3 data at 40¿N. The observations are in good general agreement with theoretical predictions and do not support the existence of a stratospheric scavenger for NO3. ¿ American Geophysical Union 1989