Precipitable water

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.

Measurement
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.