Water vapor measurements made by the Halogen Occultation Experiment (HALOE) from 1991 to 1997 are compared with ground-based observations by the Water Vapor Millimeter-wave Spectrometers (WVMS) taken from 1992 to 1997 at Table Mountain, California (34.4°N, 242.3°E), and at Lauder, New Zealand (45.0°S, 169.7°E). The HALOE measurements show that an upward trend in middle atmospheric water vapor is present at all latitudes. The average trend in the HALOE water vapor retrievals at all latitudes in the 40–60 km range is 0.129 ppmv/yr, while the average trend observed by the WVMS instruments in this altitude range is 0.148 ppmv/yr. This trend is occurring below the altitude where changes in Lyman α associated with solar cycle variations should produce a significant increase in water vapor during this period, and is much larger than the ∼0.02 ppmv/yr trend in water vapor associated with increases in methane entering the stratosphere. In addition to the water vapor increase, HALOE measurements show that there is a temporal decrease in methane at altitudes between 40 and 70 km. This indicates an increase in the conversion of the available methane to water vapor, thus contributing to the observed increase in water vapor. The increase in water vapor observed by both instruments is larger than that which would be expected from the sum of all of the above effects. We therefore conclude that there has been a significant increase in the amount of water vapor entering the middle atmosphere. A temperature increase of ∼0.1 K/yr in regions of stratosphere-troposphere exchange could increase the saturation mixing ratio of water vapor by an amount consistent with the observed increase.
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