Weather Terminology Unmasked

At the NWS office in Louisville, I noticed their overnight Area Forecast Discussion included the term, isentropic lift. Often, I see this term used in their discussions. While I had no idea about an exact definition of the term, isentropic, I knew it was related to precipitation.

First, here's a brief excerpt from the discussion I'm talking about...

"As a weak shortwave ejects from the longwave western trough late
tonight, increasing isentropic lift associated with the rightentrance region of a strengthening jet will bring in a second round
of widespread rain which will overspread our entire forecast area by
the dawn Monday morning."

Thankfully, the NWS provides a glossary for these terms that are underlined. In fact, by clicking on the term, isentropic lift, here is the official definition...

"Lifting of air that is traveling along an upward-sloping isentropic surface.

Isentropic lift often is referred to erroneously as overrunning, but more accurately describes the physical process by which the lifting occurs. Situations involving isentropic lift often are characterized by widespread stratiform clouds and precipitation, but may include elevated convection in the form of embedded thunderstorms."

That's a nice way to explain away the process in general terms. However, isentropic??? Me and my inquiring mind needed to know.

The word itself, isentropic, needed a little bit more explanation. Well, if the term would have been called isoentropic (iso+entropic), I could have figured out a partial explanation. Just as isobars mean lines of equal barometric pressure, isoentropic would mean equal entropy.

And what is entropy? A quantitative measure of disorder, chaos, in a system or process. Sounds a lot like our weather. It's a significant player in the law of thermodynamics that involves heat transfer.

Well, I found an excellent read about the isentropic lifting process HERE.

Here is a partial explanation from the site I just linked you guys to...

"NWS refer to isentropic lift or isentropic upglide simply as the word, "overrunning" in that it is an event defined as relatively warmer, moister air that rides up and over a thin, relatively cooler, drier shallow layer at the surface over a particular region. Such air that is forced to rise upwards when it is moist or when it becomes moist, forces the air to cool and condense into clouds and then water droplets which finally fall as rain, if the isentropic lift is strong enough to bring this about."

However, he describes the process nicely afterward. Give it a read. You might learn something.

MS