What is Precipitable Water?

Precipitable water (PWAT) is a measure of the total amount of water vapor contained in the atmosphere over an area. It is measured in a hypothetical vertical column of the atmosphere from the surface to a specified height. It's often expressed in millimeters or inches, representing the depth of water that would accumulate on the surface if all the water vapor in that column were to condense and fall as precipitation.

In other words, PWAT represents how much water can be “wrung” out and obtained from a hypothetical volume of the atmosphere. Imagine squeezing every drop of water from a saturated sponge, and then picture the amount of water that falls out of it. The sponge represents the atmosphere and the water vapor it contains, and the amount of water squeezed from it is the precipitable water. 

Why is it Important?

Precipitable Water (PWAT) plays a significant role in various atmospheric events, especially during the winter season. PWAT affects atmospheric rivers, snowfall intensity, and winter storm strength. PWAT values provide an idea of the potential amount of rain or snow a region can receive during a storm. High PW values indicate a greater potential for heavy rain or snow. During the winter, due to the sub-tropical origins, atmospheric rivers and the associated high PWAT values are often associated with warm air and high snow levels. 

PWAT is most often discussed about atmospheric rivers. Atmospheric rivers are characterized by high precipitable water values because of their nature and dynamics. These long, narrow bands of concentrated water vapor transport draw moisture from warm, humid regions of equatorial waters. 

Image: High PWAT values along the equator and an atmospheric river transporting these high PWAT values from near the equator toward the west coast of the US.

This moisture is integrated vertically into the atmosphere and efficiently transported courtesy of strong low-level jets from the source region to areas far way. When an atmospheric river makes landfall, especially in mountainous regions, this often results in heavy rain and snow events.

The high PWAT values in atmospheric rivers are a direct reflection of their role as primary conduits for the transport of moisture from the warm, humid regions of the oceans to the mid-latitudes.

The figure below, from January 2021, shows several atmospheric rivers impacting the western US and Canada, and the high PWAT levels associated with them. 

(Image courtesy of NASA)

These atmospheric rivers led to heavy rain, snow, flooding, and landslides.

During winter storms, the added moisture available in the atmosphere associated with higher PWAT values can lead to heavier snowfall rates. A higher PWAT can contribute to the overall intensity of the storm, leading to heavier snowfall, stronger winds, and more significant impacts.

How is it Measured?

Radiosondes are weather balloons equipped with instruments that measure various atmospheric parameters, including humidity, as they ascend through the atmosphere. By integrating the humidity values over the height of the atmosphere, we can calculate the total amount of water vapor in the column, which gives the PWAT value.

Modern satellites are equipped with sensors that can measure water vapor content in the atmosphere as well. These measurements can be used to estimate PWAT values over large areas.

Instruments like GPS receivers and microwave radiometers can also provide estimates of PWAT by measuring the delay caused by water vapor in the atmosphere.

PWAT forecasts are an important tool for predicting and understanding weather patterns and a storm's potential to bring heavy rain or snow. High PWAT values indicate a higher potential for heavy rain or snow,  aiding in forecasting precipitation and assessing flood risks.

Luke Stone
Forecaster, OpenSnow