The NASA Water Vapor Project (NVAP) total column water vapor (aka, precipitable water) data sets comprise a combination of radiosonde observations, Television and Infrared Operational Satellite (TIROS) Operational Vertical Sounders (TOVS), and Special Sensor Microwave/Imager (SSM/I) data sets. These data sets span 14 years (1988 - 2001) and contain total and layered global water vapor data.
The following is taken from: http://nvap.stcnet.com/ and http://eosweb.larc.nasa.gov/PRODOCS/nvap/readme/readme_nvap_pw_mergd_mnthly
The NVAP dataset is designed to be model-independent and relies mainly on satellite measurements; however historically, rawinsonde data has also been included. Early versions of NVAP included layered and total column water vapor on a 1 x 1 degree grid combining water vapor measurements from radiosondes, the TIROS Operational Vertical Sounder (TOVS) and Special Sensor Microwave / Imager (SSM/I), while the "next generation" dataset, NVAP-NG (2000-2001), added data from the Advanced Microwave Sounding Unit (AMSU) and Special Sensor Microwave / Temperature-2 (SSM/T2) on a 0.5 x 0.5 degree grid. Retrieved atmospheric water vapor values from each instrument were merged using a simplistic weighting scheme based on the perceived accuracy of each measurement. Gaps due to non-existent or bad data were filled using spatial and temporal averaging techniques.
Many time dependent biases exist in the heritage NVAP dataset due to algorithm changes and the addition/subtraction of instruments. Use of consistent algorithms through time should remove these. Many new sources of data have been added since the last phase of NVAP. Many existing data sources have been reanalyzed themselves, producing more stable, climate quality sources of TPW.
As companion data sets for analysis, STC-METSAT also prepared two global (over oceans only), integrated liquid water products (since the best global climate models now contain liquid water as an explicit variable). The liquid water path (LWP) product is the liquid water in any region, averaged during all-sky conditions. The cloud liquid water (CLW) is the liquid water in cloudy-only regions using a specified threshold of the liquid water retrievals. The complete data set (all three sets) has been named NVAP, an acronym for NASA Water Vapor Project. STC-METSAT developed methods to process the data at a daily time scale and 1 X 1 degree spatial resolution. More information can be found in a journal paper about NVAP (Randel et.al. 1996). This work has become an internationally accepted contribution to the World Climate Research Programme (WCRP).
National Center for Atmospheric Research Staff (Eds). Last modified 20 Aug 2013. "The Climate Data Guide: Water Vapor from NVAP." Retrieved from https://climatedataguide.ucar.edu/climate-data/water-vapor-nvap.