Imagine wringing out the entire atmosphere like a sponge; how much water would fall?

That’s what precipitable water tells us. Precipitable water (often abbreviated as PWAT) is a measure of how much water vapor is available in a column of the atmosphere, from the surface all the way to the top. It tells us the total amount of moisture that would fall as rain if all the water vapor in that vertical slice of air condensed and fell to the ground.

This value is typically expressed in inches, and while it doesn’t guarantee how much rain will fall, it gives meteorologists a crucial clue about the potential for heavy precipitation.

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High precipitable water values usually mean there's a lot of moisture in the atmosphere, which is great fuel for heavy rain, thunderstorms or even flash flooding if other conditions like lift and instability are present.

For example, tropical systems and humid summer days in the southeastern U.S. often have PWAT values well above 2 inches, signaling a soggy atmosphere ready to pour.

On the flip side, in dry or desert regions, PWAT can be as low as 0.2 inches, meaning there’s very little moisture available for precipitation at all.

It’s important to note that high PWAT doesn't automatically mean it will rain; it just means the potential is there if the right triggers are in place. Meteorologists often use precipitable water in combination with other weather data and computer models to assess flooding risks and thunderstorm intensity.

So the next time you hear there's "a lot of moisture in the atmosphere," it likely means PWAT values are climbing, and the skies may be getting ready to open up.

Jennifer Gray is a weather and climate writer for weather.com. She has been covering some of the world's biggest weather and climate stories for the last two decades.