What is Water Vapor?
Over 99% of the atmospheric moisture is in the form of water vapor, and this vapor is the principal source of the atmospheric energy that drives the development of weather systems on short time scales and influences the climate on longer time scales.
Water vapor is a critical component of Earth's climate systems. It is the Earth's primary greenhouse gas, trapping more heat than carbon dioxide. Movement of water vapor, and its associated latent heat of vaporization, is also responsible for about 50% of the transport of heat from the tropics to the poles. The movement of water vapor is also important for determining the amount of precipitation a region receives.
Total column water vapor is a measure of the total gaseous water contained in a vertical column of atmosphere. It is quite different from the more familiar relative humidity, which is the amount of water vapor in air relative to the amount of water vapor the air is capable of holding. Atmospheric water vapor is the absolute amount of water dissolved in air. When measured in linear units (millimeters, mm), it is the height (or depth) the water would occupy if the vapor were condensed into liquid and spread evenly across the column. Using the density of water, we can also report water vapor in kg/m2 = 1 mm or g/cm2 = 10 mm.
Microwave Measurement of Atmospheric Water Vapor
Because of the strong water vapor absorption line near 22 GHz, within the microwave range, we can use microwave radiometers to measure columnar (atmospheric total) water vapor. This is a very accurate measurement due to the high signal-to-noise ratio for this measurement. With little diurnal variation, the measurements from different satellites at the same location often agree to within a few tenths of a millimeter.
RSS Water Vapor Products
1-degree, Monthly Total Precipitable Water
We have merged the water vapor measurements from the many radiometers in operation since 1987, including SSM/I, SSMIS, AMSR-E, WindSat, and AMSR2. These data were all processed in a consistent manner using our radiative transfer model and careful instrument intercalibration. The water vapor from these instruments are used to create a Total Precipitable Water (atmospheric water vapor) product that is best for use in climate study. This 1-degree, monthly gridded product is further described in the document Merged Monthly 1-degree Total Precipitable Water - TPW. This TPW product is available in netCDF format from the RSS ftp server and from the NASA GHRC. We have browse images of mean TPW, TPW anomalies, 12 monthly climatology maps (constructed using 1988 to 2007 data), a global trend map and a latitude-time plot.
Individual Radiometer Gridded Binary Data Files
In addition, water vapor is one of the measurements we derive from each satellite. For detailed product information and data access, see the Mission page for each instrument.
| Instrument | Data Range | Version |
| SSM/I | 1987 - present | V7 |
| SSMIS | 2003 - present | V7 |
| TMI | 1997 - 2015 | V7.1 |
| AMSR-E | 2002 - 2011 | V7 |
| WindSat | 2003 - present | V7.0.1 |
| AMSR2 | 2012 - present | V7.2 |
| GMI | 2014 - present | V8.1 |