NASA Water Vapor Project (NVAP)

The NASA MEaSUREs program began in 2008 and has the goal of creating stable, community accepted Earth System Data Records (ESDRs) for a variety of geophysical time series. A reanalysis and extension of the NASA Water Vapor Project (NVAP), called NVAP-M was performed and completed in early 2013 as part of this program.


NVAP began in the early 1990s as a NASA Pathfinder project to create a record of the distribution of Earth's water vapor on a daily basis. Since its inception, there have been several extensions of NVAP, growing the dataset to include 14 years (1988-2001) of gridded total column and layered water vapor over both ocean and land. When processing is complete, NVAP-M will span 1987-2010.

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 (shown above). 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.

Project Status

August, 2013:

NVAP-M data for 1988-2009 is now available at NASA Langley Atmospheric Science Data Center (ASDC). NVAP-M is further described in Vonder Haar, T. H., J. L. Bytheway and J. M. Forsythe, 2012: "Weather and climate analyses using improved global water vapor observations." Geophys. Res. Lett., 39, L16802, doi:10.1029/2012GL052094

Data characteristics

Distinguishing features of the NVAP-M dataset:

  • Global (land and ocean) data coverage
  • Uses consistent, intercalibrated data sources
  • Uses consistent, peer-reviewed algorithms
  • Uses new data sources
  • Three-tiered production format for a variety of users

Products of NVAP-M:

Data timeline

Retrieval Algorithms

Both the microwave and infrared retrievals are optimal estimation type algorithms. The microwave retrieval is that of Elsaesser and Kummerow (2008) while the infrared retrieval is that of Engelen and Stephens (1999). The flow charts below illustrate the input, ancillary, and a priori data used in the retrievals.

SSM/I Retrieval Flowchart

HIRS Retrieval Flowchart

Sample results

Daily Products

Example of NVAP-M Climate global water vapor product for September 10, 2004:

Example of NVAP-M Weather global water vapor product for September 10, 2004:

Example of NVAP-M Ocean water vapor product of blended SSM/I data for September 10, 2004:

Example of Layered NVAP-M Climate global water vapor product, blending HIRS and radiosonde data, for September 10, 2004:

Seasonal Variability

Animation of monthly average water vapor from NVAP-M Climate 1987-2009:

Interannual Variability

References and Citations


Vonder Haar, T. H., J. L. Bytheway, and J. M. Forsythe, 2012: Weather and climate analyses using improved global water vapor observations. Geophysical Research Letters, 39, L15802, doi:10.1029/2012GL052094

Vonder Haar, T. H., D. L. Reinke, D. L. Randel, G. L. Stephens, C. L. Combs, and T. J. Greenwald, 1995: Production of a long-term global water vapor and liquid water data set using ultra-fast methods to assimilate multi-satellite and radiosonde observations. STC Technical Report 2927. Final Report, NASA Contract NASW-4715. 24 pp. Science and Technology Corporation, Hampton, Virginia.

Randel, D.L., T.H. Vonder Haar, M.A. Ringerud, G.L. Stephens, T.J. Greenwald, and C.L. Combs, 1996: A new global water vapor dataset. Bull. Amer. Meteor. Soc., 77, 1233-1246.

Vonder Haar, T. H., J. M. Forsythe, D. L. Randel, and S. Woo, 2003: Analysis of the NVAP water vapor dataset: A tool for monitoring Earth's water vapor from daily to decadal scales. Preprints, 12th AMS Conference on Satellite Meteorology and Oceanography, Long Beach, California, Feb. 2003.

Forsythe, J. M., D. L. Randel, S. Woo, D. S. McKague, and T. H. Vonder Haar, 2003: Extending the 12-year NVAP global water vapor dataset into the 21st century. Preprints, 12th AMS Conference on Satellite Meteorology and Oceanography, Long Beach, California, Feb. 2003.


coming soon

Upcoming Presentations


Q: Are there trends in the NVAP dataset? A: The NVAP team has released the following Trend Statement.

Q: Does the dataset show major interannual variability? A: Yes. For example, the impact of the 1991 Mt. Pinatubo eruption is clearly evident.

Contact us

Team leader: John Forsythe (email: John dot Forsythe at

Last updated on Aug 7, 2012