Dear Tom,
You sometimes refer to the atmosphere’s water content as, for example, 2.00 inches. What does that mean? Steve Kohnfrst, Rockford

Dear Steve,
The reference is to an atmospheric quantity known as “precipitable water.” Meteorologists describe the state of the atmosphere using a variety of atmospheric variables. Some of those variables are familiar to you: temperature, air pressure, wind direction and speed, to name four. Some, like precipitable water, are less familiar.

Precipitable water is the depth of water that would result if all the moisture in a column of air extending from the Earth’s surface upward to the “top” of the atmosphere were condensed out. It is the water available for precipitation and, indirectly, a measure of the energy available for thunderstorm development.