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 In radiometry, irradiance is the radiant flux ''received'' by a ''surface'' per unit area. The SI unit of irradiance is the watt per square metre (W⋅m−2). The CGS unit erg per square centimetre per second (erg⋅cm−2⋅s−1) is often used ...
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 Beer–Lambert law, also known as Beer's law, the Lambert–Beer law, or the Beer–Lambert–Bouguer law relates the attenuation of light to the properties of the material through which the light is travelling. The law is commonly applied t ...
.
*The precipitable water can also be calculated by integration of
radiosonde
A radiosonde is a battery-powered telemetry instrument carried into the atmosphere usually by a weather balloon that measures various atmospheric parameters and transmits them by radio to a ground receiver. Modern radiosondes measure or calcula ...
data (
relative humidity
Humidity is the concentration of water vapor present in the air. Water vapor, the gaseous state of water, is generally invisible to the human eye. Humidity indicates the likelihood for precipitation, dew, or fog to be present.
Humidity depe ...
,
pressure
Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and e ...
and
temperature
Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer.
Thermometers are calibrated in various temperature scales that historically have relied o ...
) over the whole
atmosphere
An atmosphere () is a layer of gas or layers of gases that envelop a planet, and is held in place by the gravity of the planetary body. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A s ...
.
*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
The Global Positioning System (GPS), originally Navstar GPS, is a satellite-based radionavigation system owned by the United States government and operated by the United States Space Force. It is one of the global navigation satellite sy ...
have been developed.
*Some work has been performed to create empirical relationships between surface
specific humidity
Humidity is the concentration of water vapor present in the air. Water vapor, the gaseous state of water, is generally invisible to the human eye. Humidity indicates the likelihood for precipitation, dew, or fog to be present.
Humidity depe ...
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.
External links
Current global map of precipitable water*
References
Water
Atmospheric thermodynamics
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