Several satellite-based water vapor datasets are examined to address variability on a variety of time and space scales. Specific questions addressed are: What is the daily, interannual, and decadal variability of water vapor? Are there decadal trends in total precipitable water? The focus will be on the NASA Water Vapor Project (NVAP) dataset, spanning the period from 1988-2001.

The NVAP record of atmospheric water vapor shows a global, annual mean of 24.5 mm with an annual cycle of about 10% of the total value. Considerable interannual variability has also been observed. During 1988-2001, atmospheric water vapor responded to both the eruption of Mount Pinatubo and El Nino events in step with the record of tropospheric temperature. A study of daily water vapor variability shows that on a percentage basis water vapor content from 300 – 700 hPa in the midlatitudes shows the largest variability. This result could be a useful comparison point for general circulation models to assess the vigor of their water vapor circulation.

Trend analysis of a 12-year record of NVAP reveals no robust statistically significant trend in global total precipitable water. There are significant regional trends, some of which are due to atmospheric processes and some of which are likely due to known time-dependent biases in NVAP.

Current and future work on impoving and extending the NVAP water vapor dataset will be discussed.