Atmospheric stability and lapse rate

Icebound's post is well-written and almost exactly accurate. Just one
quibble:

Icebound wrote:

"dry adiabatic" mean no heat
added and no condensation occurring. This number has been
experimentally determined... it is an almost straight line
value of approximately 3 degrees C per 1000 feet.

The dry adiabatic lapse rate is not an experimentally determined
number. It can be derived from equations, and turns out to be defined
by the remarkably simple expression: g/Cp, where g is gravity (9.8 m/s)
and Cp is the specific heat of dry air (1004 J/kg). Perhaps the
experimental nature you're referring to has to do with an "average"
moisture content which acts to change Cp slightly?

Jim Rosinski

"jim rosinski" wrote in message
ups.com...
Icebound's post is well-written and almost exactly accurate. Just one
quibble:

Icebound wrote:

"dry adiabatic" mean no heat
added and no condensation occurring. This number has been
experimentally determined... it is an almost straight line
value of approximately 3 degrees C per 1000 feet.

The dry adiabatic lapse rate is not an experimentally determined
number.

I simply meant that it is a number which, one way or another, is
more-or-less known. It was a bit of a typo... "experimentally" should not
have been there, but I didn't want to say "theoretically", because I worried
pilots might think that the number is some kind of guess and not really
known.

Saying "Experimentally" didn't alter the gist of the post for pilots... only
for research-meteorologists! :-)

Icebound wrote:

Saying "Experimentally" didn't alter the gist of the post for
pilots... only
for research-meteorologists! :-)

Agreed. I really am pleased to see this much interest, and frankly
knowledge, displayed by pilots for a subject I've spent a good part of
my life studying.

Jim Rosinski

On 7 Feb 2005 20:06:38 -0800, "jim rosinski"
wrote:

Agreed. I really am pleased to see this much interest, and frankly
knowledge, displayed by pilots for a subject I've spent a good part of
my life studying.

Jim Rosinski

Jim, to me this is kind of the point regarding this subject and the
FAA's insistance on it being a part of the written examination. How
does knowing this information help the average pilot in his task of
flying safely from one point to another.

Does any pilot (besides yourself) actually think about this while
flying? If so when? Under what circumstances?

After a while don't pilots kind of get to understand when clouds begin
forming due to warming? And when the clouds do form, don't we (VHF
guys) normally just avoid them?

Thanks, Corky Scott

"Corky Scott" wrote in message
...

Does any pilot (besides yourself) actually think about this while
flying? If so when? Under what circumstances?

After a while don't pilots kind of get to understand when clouds begin
forming due to warming? And when the clouds do form, don't we (VHF
guys) normally just avoid them?


Perhaps there are parts of the country where that is true, but in the
4-seasons part of the world....
yes, you should be thinking about these things BEFORE flying, not just WHILE
flying.

Now I DO agree that knowing the "fact" that the
dry-adiabatic-lapse-rate-is-3-degrees-Celsius-per-thousand-feet and being
able to check off the correct multiple-choice-box on the FAA or
Transport-Canada exam... is somewhat irrelevant if we do not take that fact
and understand it within the context of the rest of our weather environment.

And I have this belief that some of our instructors are concentrating on
ensuring we pass the exam by knowing these "facts", just as they did,
without really understanding nor properly communicating the broader subject
of aviation meteorology to us.
Therefore, it is left to US to obtain that understanding somehow. We should
not cancel our willingness, hell, our *obligation* to learn, once we walk
out of that ground-school session.


As has been often recorded in these newsgroups, TAFs are often "wrong".
Sometimes even METAR observations are less than perfect, especially from
AUTO sites.

If we had a real good understanding of all aspects of meteorology, we could
recognize the situations in which forecasting should be relatively "easy",
and the situation is which it is more "difficult".... therefore the
situation in which we can take the TAF as gospel, and the situation in which
it is likely to be suspect.

We would recognize not only the "actual" forecast for your area, but also
the "potential" of what the other possibilities were. This works both
ways.... we would recognize the potential for good weather when the TAF said
no, and we would recognize the potential for bad weather when the TAF said
go.

We would recognize whether the formation of an unexpected cloud bank is
potentially dangerous or benign. We would recognize whether an unexpected
clearing is real (and may be bad timing on the part of the TAF), or just a
sucker-hole.

We would understand the "thinking behind the TAF" and we would be in a
position to do our own "now-casting" if the
underlying-conditions-to-that-thinking have changed.... because we would
understand what "underlying conditions" to look for, and what their
implications are.

Knowing more about the underlying meteorology of your current situation will
not only help us avoid current BAD weather... it will help us understand
when GOOD-weather-going-bad is a possibility, and it will help us to
understand the difference between MARGINAL-weather-getting-good and
marginal-weather-getting-bad.

I have a real fear that the new generation of in-cockpit tools to
"upload-the-weather" will further deteriorate our desire to learn. If we
are going to use those tools only to "avoid the bright spots on the map",
then I am afraid that they will not increase our safety factor one bit.

I am certain (well okay: hopeful, anyway) that a very large segment of the
pilot population was well taught, understand meteorology very well and are
doing all they can to learn more and learn correctly. I do fear, however,
that some of us were not only poorly taught, but have accepted that as the
"norm" to be passed on to the next generation. And we now treat meteorology
as just one more check-mark on the exam to be forgotten-about, once passed.

(Pardon me for cross-posting to r.a.s, where this really belongs.)

On Tue, 8 Feb 2005 12:25:34 -0500, "Icebound"
wrote:

If we had a real good understanding of all aspects of meteorology, we could
recognize the situations in which forecasting should be relatively "easy",
and the situation is which it is more "difficult".... therefore the
situation in which we can take the TAF as gospel, and the situation in which
it is likely to be suspect.

Can you give me a realistic example of how knowing exactly what the
definition of lapse rate is would help a pilot flying from point A to
point B?

Forgive me, I'm just trying to understand why the FAA considers this
so important that it is put on the written. I've never heard of
anyone calculating the lapse rate for a flight. Even if they did, it
seems to me that this rate could easily be different from one point to
the next throughout the flight.

I understand the need to be thorough when flight planning but I don't
get how to use this particular knowledge.

Thanks, Corky Scott

"Corky Scott" wrote in message
...
On Tue, 8 Feb 2005 12:25:34 -0500, "Icebound"
wrote:

If we had a real good understanding of all aspects of meteorology, we
could
recognize the situations in which forecasting should be relatively "easy",
and the situation is which it is more "difficult".... therefore the
situation in which we can take the TAF as gospel, and the situation in
which
it is likely to be suspect.

Can you give me a realistic example of how knowing exactly what the
definition of lapse rate is would help a pilot flying from point A to
point B?

....


If a pilot does not know the definition of lapse rate, then it is pretty
difficult for him to recognize the conditions which lead to atmospheric
buoyancy or to atmospheric stability.

I get to the airport on a clear summer morning. 7 AM. Sun is up. Not a
cloud in the sky. Temperature is 20C, dewpoint is 15. TAF for my airport
for later is PROB30 TSRA. TAF For airport 40 mi south for a similar time
period is TEMPO TSRA. Okay, only a risk of thunderstorms for me, more
definite for the guy down south.

As a pilot, I am interest in the possibilities of the TAF being "wrong". I
take off, fly up through the early-morning inversion and find the
temperature starts to fall off at about 4000 feet, peaking there at, say,
about 20 to 22 C.

*If I know* (maybe get a PIREP) that the 10,000 foot temperature in my area
is rather warm, say 14-15... that's maybe 1.5 degrees per 1000, pretty
stable in that mid layer and I'm pretty confident that even with good summer
heating only a few are likely to pop and I pretty much trust the TAF. I
know that it is "pretty stable" because I understand the concept of
adiabatic lapse rates and therefore know more-or-less how much colder a
rising air parcel will be than its environment.... and hence not buoyant.

But if I find out that the 10,000 foot temperature is closer to, say 6 or 8,
that's becoming 2.5 per 1000, and I am starting to consider the possibility
that my airmass maybe just as unstable as the guy down south and
Thunderstorms are a distinct possibility and something more than a "risk".

Am I likely to do exactly that kind of calculation in flight?

Maybe, maybe not..... If I did, would it affect my go-nogo decision? Maybe,
maybe not. But if I *did* go, I know what I would be looking out for, and I
would treat the first appearance of a Towering Cumulus quite differently in
those two situations. In the first case, I'm not panicking until I see a
few more.... in the second, I'm thinking seriously about heading for an
airport.

My rant was about knowledge of meteorological concepts in general, not lapse
rates in particular. Lapse rates were an easy target example because of the
obvious misunderstanding of adiabatic lapse rates in that particular post.
I agree that in a great many cases, knowing the actual lapse rate may not
help you much. You might not have the information, or it may be a situation
where it is not important. (And FAA or TC exams that test "knowledge" are a
whole other issue :-) )

But knowing the complete concept... how adiabatic lapse rates affect the
temperature of rising parcels... how that relates to the difference in
temperature between the environment and a rising air bubble... how that
difference in temperature affects buoyancy... how the degree of buoyancy
affects convection....

Knowledge of these concepts may just help you to understand the TAF, your
own observations, and how to reconcile a busted forecast.

Corky Scott wrote:

Jim, to me this is kind of the point regarding this subject and the
FAA's insistance on it being a part of the written examination. How
does knowing this information help the average pilot in his task of
flying safely from one point to another.

I agree the lapse rate stuff and implications for stability/instability
aren't of much practical value while flying. The main things are being
able to look at the sky and make some assessment of whether flying is a
good idea, and knowing what aspects of meteorological data might
warrant alarm. For example:

o Lenticulars over the mountains = won't be flying in the mountains
today (high winds).
o Smog over Denver = inversion, might be some bumps at the inversion
but no reason not to go flying.
o Temperature-dewpoint spread dropping toward zero = uh-oh, fog might
form. Don't stray too far.
o Low clouds forming east of the Rockies = upslope, bad weather moving
in. Maybe IMC soon.

Most pilots know these things, which I think are more important for
safe flying than some of the more esoteric aspects of atmospheric
science. And layman-level understanding of local meteorological warning
signs (I've given a few for the Denver area above) is really money in
the bank.

Jim Rosinski

On Tue, 08 Feb 2005 10:01:35 -0500, Corky Scott
wrote:

Jim, to me this is kind of the point regarding this subject and the
FAA's insistance on it being a part of the written examination. How
does knowing this information help the average pilot in his task of
flying safely from one point to another.

Does any pilot (besides yourself) actually think about this while
flying? If so when? Under what circumstances?

GA pilots might not use it much. For glider pilots it can be very
valuable, as a predictor of when and where thermals will form. Also
ultralights, since it's also a good predictor of how smooth the air
will be (thermic days can be uncomfortable if not downright dangerous
in the extreme low end of ultralights like powered paragliders).

-Dana
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I wrote:

remarkably simple expression: g/Cp, where g is gravity (9.8 m/s)

This should have read 9.8 m/s^2, not 9.8 m/s, for those who care.

Jim Rosinski