Forecasters use several ways to determine the amount of moisture that is in the air. From relative humidity to the dew point, it’s all about knowing how much water vapor the air in a location contains.
Precipitable water, often abbreviated as PWAT, is another such measurement. It’s defined as the amount of water vapor contained in a column of air if it were to be condensed and collected. It’s expressed in inches.
PWAT is used to help meteorologists determine a number of things, but the one it is most often used for is forecasting the amount of precipitation that will fall. However, the precipitable water value does not translate into the exact amount of rain that will fall. Instead, it’s meant to give forecasters an idea of the amount of moisture that is available to fall as precipitation. If PWAT values are high, there is more moisture available that could become precipitation. If PWAT values are low, the opposite is true.
In general, a precipitable water value of an inch or less is considered low and a value of 2 inches or more is considered very high.
PWAT can also be used to help forecasters determine flooding potential, lightning density, the severity of storms and snowfall potential.