Precipitable Water Calculator
Precipitable water (PW) quantifies the total moisture available in the atmosphere above a given location. It represents the depth of water that would accumulate on the surface if every water vapor molecule in a vertical column were condensed to liquid. Forecasters use PW alongside lifting mechanisms to assess heavy rainfall potential. While precise PW values require radiosonde soundings, a practical approximation using surface dewpoint provides a useful first estimate. Enter the surface dewpoint in Fahrenheit to compute the estimated precipitable water in inches and millimeters.
Precipitable water estimate formula
PW (in) = exp(0.1133 - ln(Td_C + 1) * (-1.055) + 0.0869 * Td_C) / 25.4
A simpler NWS operational approximation: PW (in) approximately equals surface dewpoint in F divided by 25.4 for dewpoints above 50 F. A more precise empirical formula by NWS Hydrometeorological Prediction Center relates PW to surface dewpoint using an exponential fit to observed sounding data. This calculator uses the exponential relationship valid for temperate latitudes.
Note: this is a surface-based estimate only. Actual PW requires integrating the moisture profile through the full atmospheric column, typically from a radiosonde sounding.
Precipitable water and heavy rain potential
- Below 0.50 in (13 mm): very dry atmosphere; heavy rain unlikely even with strong lift.
- 0.50-1.00 in (13-25 mm): typical cool-season or moderate conditions.
- 1.00-1.50 in (25-38 mm): moist; heavy rain possible with organized convection.
- Above 1.50 in (38 mm): very moist; extreme rainfall potential when combined with strong lift.
- Above 2.00 in (50 mm): exceptional; typically associated with tropical systems or monsoon moisture.
Precipitable water calculator: frequently asked questions
What is precipitable water?
Precipitable water (PW) is the total depth of liquid water that would result if all the water vapor in a vertical column of the atmosphere were condensed. It is measured in inches or millimeters and is a key parameter for forecasting extreme rainfall. High PW values indicate moist atmospheric columns with heavy rain potential.
How is precipitable water estimated from surface dewpoint?
A practical NWS approximation relates surface dewpoint to precipitable water: PW (in) is approximately 0.1 * es(Td) where es is saturation vapor pressure in mb at the surface dewpoint. A simpler empirical rule used operationally is PW (in) approximately equals the surface dewpoint in F divided by 25.4. More precise values come from radiosonde upper-air soundings.
What is a high precipitable water value?
For the continental US, values above 1.5 inches (38 mm) are considered high and indicate significant heavy rain potential. Values above 2 inches (50 mm) are exceptional and associated with extreme rainfall events. Tropical systems can have PW values of 2.5-3 inches. Values below 0.5 inches indicate a dry atmosphere.
Why is precipitable water important for flash flooding?
High precipitable water combined with strong lift and slow storm motion creates conditions for extreme rainfall rates. When storms repeatedly pass over the same area and the atmosphere contains high PW, rainfall totals can exceed anything the soil and drainage systems can handle, leading to flash floods. The NWS uses PW climatologies to identify anomalously high moisture for flash flood outlooks.
How is precipitable water measured operationally?
The primary observational source is radiosonde upper-air soundings launched twice daily at approximately 100 sites across the US. GPS integrated precipitable water (GPS-IPW) derived from ground-based GPS receivers now provides near-real-time PW estimates at hundreds of sites. Satellite-derived PW products from GOES and polar-orbiting satellites fill spatial gaps.
Official sources
- NOAA/NWS Hydrometeorological Prediction Center: Precipitable Water Products.
- NOAA/NWS: Precipitable Water Estimation.
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.