Water vapor (precipitable water)

The invisible moisture held in a column of air. “Precipitable water” is how deep that water would be if you squeezed all the vapor in the column down into liquid.

Why it matters

Water vapor is the atmosphere’s most abundant greenhouse gas and the raw material for clouds and rain, so measuring it is essential for weather forecasting, storm prediction, and understanding climate feedbacks.

Where you’ll meet it

• MODIS retrieves total column water vapor from its near-infrared and infrared bands.
• AIRS on Aqua profiles water vapor at many altitudes to feed weather and climate models.
• GPM and atmospheric reanalyses use water-vapor measurements to estimate where rain will form.

In plain terms

It’s like the humidity that fogs up a bathroom mirror — you can’t see the moisture in the air, but it’s there, ready to condense.