Precipitable water is the depth of water in a column of the atmosphere, if all the water in that column were precipitated as rain. As a depth, the precipitable water is measured in millimeters or inches. Often abbreviated as "TPW", for Total Precipitable Water.
There are different measurement techniques:
- One type of measurement is based on the measurement of the solar irradiance on two wavelengths, one in a water absorption band, and the other not. The precipitable water column is determined using the irradiances in these bands and the Beer–Lambert law.
- The precipitable water can also be calculated by integration of radiosonde data (relative humidity, pressure and temperature) over the whole atmosphere.
- Data can be viewed on a Lifted-K index. The numbers represent inches of water as mentioned above for a geographical location.
- Recently, methods using the Global Positioning System have been developed.
- Some work has been performed to create empirical relationships between surface specific humidity and precipitable water based on localized measurements (generally a 2nd to 5th order polynomial).  However, this method has not received widespread use in part because humidity is a local measurement and precipitable water is a total column measurement.
- Current global map of precipitable water
- Maurellis, A. N.; Lang, R.; van der Zande, W. J.; Aben, I.; Ubachs, W. (2000). "Precipitable water column retrieval from GOME data" (PDF). Geophys. Res. Lett. 27 (6): 903–906. Bibcode:2000GeoRL..27..903M. doi:10.1029/1999GL010897. hdl:1871/23072.
- Remote Sensing of Water Vapor From GPS Receivers
- Bevis, M.; et al. (1992). "GPS Meteorology: Remote Sensing of Atmospheric Water Vapor Using the Global Positioning System". Journal of Geophysical Research. 97 (D14): 15787–15801. Bibcode:1992JGR....9715787B. doi:10.1029/92JD01517.
- Liu, Timothy (27 February 1986). "Statistical Relation between Mean Precipitable Water and Surface-Level Humidity over Global Oceans". Monthly Weather Review. 114 (8): 1591–1602. Bibcode:1986MWRv..114.1591L. doi:10.1175/1520-0493(1986)114<1591:srbmmp>2.0.co;2.
- Hsu, S.A.; Blanchard, B.W. (15 October 1989). "The Relationship Between Total Precipitable Water and Surface-Level Humidity Over the Sea Surface: A Further Evaluation". Journal of Geophysical Research. 94 (C10): 14539–14545. Bibcode:1989JGR....9414539H. doi:10.1029/JC094iC10p14539.
- Guangxiong, Peng; Jing, Li; Yunhao, Chen; Norizan, Abdul; Tay, Liphong (17 July 2006). "High-resolution Surface Relative Humidity Computation Using MODIS Image in Peninsular Malaysia" (PDF). Chinese Geographical Science. 16 (3): 260–264. doi:10.1007/s11769-006-0260-6. Retrieved 22 January 2014.