Atmospheric stability - Winds/Temps aloft

[MrManH]

Hi everyone,

First post here and I'd like to start off with a weather question.

Looking at this chart
http://en.citizendium.org/images/b/b3/Lapse_rates.png

It looks like the steeper the temperature gradient and the more unstable the atmosphere. However I would kind of except the opposite because if a parcel of air was rising, it would have to climb much higher to cool down to ambient temperature if the lapse rate was low. Since atmospheric stability is defined as the ability to resist vertical movement, wouldn't a low lapse rate encourage vertical movement?

Thanks!

 

[MAKG1]

Good question!

A parcel of air when lifted will always follow the (dry) adiabatic lapse rate until it saturates.

But that doesn't mean the atmosphere is structured that way.

If you lift a parcel of air, it cools according to the adiabatic lapse rate. If the surrounding air happens to be still cooler, your parcel will continue to rise. The cooler the surrounding air is, the more buoyant force your parcel feels.

When people plot the "lapse rate," they plot the surrounding air, not your parcel. So, the steeper IT is, the more your parcel wants to rise.

At night, the lapse rate can easily get inverted near the ground. That is, you lift a parcel of air, and it cools according to the adiabatic lapse rate as it always does. But the surrounding air is warmer than the ground! So, the buoyant force is downward and your parcel goes back down. Those of us who live in mountain valleys can see tens of degrees of inversion in an early morning, often with fog at the bottom, 'cause of all the cold air getting forced down with no solar heating.

 

[MrManH]

Thank you for your reply!

Thanks for making the distinction between the two lapse rates, here's how I understand it now.

The parcel of air will cool at the adiabatic lapse rate (wet or dry depending on moisture content).

The atmosphere itself (surrounding air) follows the environmental lapse rate. If that lapse rate is greater than the adiabatic lapse rate of the rising parcel, the atmosphere will cool faster than the rising parcel of air can, causing it to climb higher as it doesn't cool "fast enough".

If this is the right understanding, this leads me to my next question. Is the Winds/Temps Aloft chart a good tool to derive atmospheric stability?

 

[Clark1961]

check out: http://rucsoundings.noaa.gov/

(use the html5 plot)

 

[MAKG1]

If this is the right understanding, this leads me to my next question. Is the Winds/Temps Aloft chart a good tool to derive atmospheric stability?

If only….

It's good enough at the point they specify, and as long as no unstable layer is narrower than 3000 feet (unfortunately, they can be).

Unfortunately, they happen to be rather widely spaced. There is no appropriate point for the entire Sierra Nevada, for instance. Fresno may be close, but the weather is distinctly different uphill. Yesterday, weather was clear and a bit windy with 20 mile visibility all over the Valley (including Fresno), but there were some nasty looking anvils hanging over the high mountains.

The skew-T plots (rucsoundings.noaa.gov) gives much better samples, but it's still not Gospel. The atmosphere does what it does, not what any forecast says it should be doing. You have to learn how to read them, but it's not too bad. They do let you very easily pick out cloud layers and stability.

Honestly, though, for real VFR flights, you can learn a lot by looking at clouds. Those show you where air is cooling, usually because it is rising. At the tops, it isn't rising anymore.

If there are fair-weather cumulus clouds hanging over every ridgetop, you may have a bumpy ride. Thin haze layers are a sign of smooth air. And so on.

 

[N801BH]

Good question!

A parcel of air when lifted will always follow the (dry) adiabatic lapse rate until it saturates.

But that doesn't mean the atmosphere is structured that way.

If you lift a parcel of air, it cools according to the adiabatic lapse rate. If the surrounding air happens to be still cooler, your parcel will continue to rise. The cooler the surrounding air is, the more buoyant force your parcel feels.

When people plot the "lapse rate," they plot the surrounding air, not your parcel. So, the steeper IT is, the more your parcel wants to rise.

At night, the lapse rate can easily get inverted near the ground. That is, you lift a parcel of air, and it cools according to the adiabatic lapse rate as it always does. But the surrounding air is warmer than the ground! So, the buoyant force is downward and your parcel goes back down. Those of us who live in mountain valleys can see tens of degrees of inversion in an early morning, often with fog at the bottom, 'cause of all the cold air getting forced down with no solar heating.

Great explanation sir......

 

[bobmrg]

Thank you for your reply!

Thanks for making the distinction between the two lapse rates, here's how I understand it now.

The parcel of air will cool at the adiabatic lapse rate (wet or dry depending on moisture content).

The atmosphere itself (surrounding air) follows the environmental lapse rate. If that lapse rate is greater than the adiabatic lapse rate of the rising parcel, the atmosphere will cool faster than the rising parcel of air can, causing it to climb higher as it doesn't cool "fast enough".

If this is the right understanding, this leads me to my next question. Is the Winds/Temps Aloft chart a good tool to derive atmospheric stability?

No. It is a good tool to answer one of the five questions on the knowledge test that refer to that chart. Just memorize those answers for the test, then do as others have suggested and learn to use the Skew-T chart. Scott Dennstaedt has a killer CD-rom program for sale at www.avwxworkshops.com. (Lots of free goodies there, too.)

Bob Gardner

 

[MrManH]

Thank you very much everyone, and Scott this was a great read. I bookmarked it. As you say in your article, "This discussion far exceeds what most pilots care to know about the weather", I tried to not go too in depth in weather theory during my PPL training but now that I'm working on my commercial license, I'm trying to patch a few holes in my general understanding of weather. I will take a closer look at Scott's website as well.