Atmospheric stability and lapse rate

Andrew Sarangan wrote:
What you described is exactly the point many people (including
myself)
have been confused about. The 2C/1000' is the average environmental
lapse rate. Adiabatic lapse rate is never 2C/1000'. It is 1C/1000' or

3C/1000'. Many FAA texts do not explain this point clearly. Since
most
pilots get their meterology knowledge from FAA texts, and are not
formally educated on the subject, it is not surprising this confusion

exists.

There is an excellent explanation of all this stuff (including how to
predict cloud bases, the presence of vertical air currents, and the
likelihood of T-storms) in Reichmann's "Streckensegelflug" (man I hope
I got that right) which is translated into English (the whole book -
you need not speak German) as "Cross Country Soaring." It includes the
use of the Stuve diagram to predict what the atmosphere is going to do.

I would bet you any money that if you took a survey of CFI's
most would not know this fact.

Sure, as long as you limit to power-only CFI's. I can't think of any
glider CFI's who have not read Reichmann, though of course anything is
possible.

Michael

"Michael" wrote in message
ups.com...
Andrew Sarangan wrote:
....
Many FAA texts do not explain this point clearly. Since most
pilots get their meterology knowledge from FAA texts, and are not
formally educated on the subject, it is not surprising this confusion
exists.
....snipped...
I would bet you any money that if you took a survey of CFI's
most would not know this fact.

Sure, as long as you limit to power-only CFI's. ...

I have no doubt that this is absolutely correct.

Not being a soarer, but I expect he/she not only knows the "conditions" that
give rise to good thermal lift, but also the meteorological situations to
look for which are conducive.

Power pilots as a group seem less interested in the meteorological
situation. Give them the ceiling and visibility numbers from the TAF and
METAR and they go on that. Nobody seems to ask WHY does the TAF lower the
ceiling after 2100Z... If the ceiling should lower two hours early at 1900Z
instead of 2100Z, many are totally lost and simply consider this a "bad
forecast".

Whoops. Typing error. the last entry should read "the average is 2°.

--

Darrell R. Schmidt
B-58 Hustler History: http://members.cox.net/dschmidt1/
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"Darrell S" wrote in message news:bdUNd.46622$bu.24635@fed1read06...
2°/1000' is "average" since air at different levels may be saturated or unsaturated and can change from one to the other at different levels. Lifted air would cool at 3°/1000' while lifting through dry air and at 1°/1000' lifting through moist air levels. So.... the average is 1°

--

Darrell R. Schmidt
B-58 Hustler History: http://members.cox.net/dschmidt1/
-

"Andrew Sarangan" wrote in message 1...
Instability produces cumulus clouds and stability produces stratus clouds.
We know that. However, since the saturated and unsaturated lapse rates are
significantly different (1C/1000' compared to 3C/1000'), it seems quite
possible to get cumulus clouds even when the atmosphere below is stable.
For instance, if the environmental lapse rate is 2C/1000', the unsaturated
air is stable. Once clouds form (how they form without vertical currents is
a different matter), the air inside the clouds will become unstable. Does
this seem reasonable?

On a related question, where does the concept of 'average' lapse rate
(2C/1000') come from? I always took this to mean 50% RH air, but it took me
a long time to learn that that was not the case. The air is saturated or it
is unsaturated. How can there be an average between saturated and
unsaturated? The standard lapse rate and standard temperature at
different elevations are all based on this 2C/1000' concept. What's the
deal with this?

"Andrew Sarangan" wrote in message
1...
Instability produces cumulus clouds and stability produces stratus clouds.
We know that. However, since the saturated and unsaturated lapse rates are
significantly different (1C/1000' compared to 3C/1000'), it seems quite
possible to get cumulus clouds even when the atmosphere below is stable.
For instance, if the environmental lapse rate is 2C/1000', the unsaturated
air is stable. Once clouds form (how they form without vertical currents
is
a different matter), the air inside the clouds will become unstable. Does
this seem reasonable?

I think there's an aspect to this that hasn't been discussed. It *does*
require instability to produce cumulus cloud, but that instability can be
very local. So you may see an average environmental lapse rate of 2
degC/1000' through the lowest 3000' of the atmosphere, but actually you've
got at least patches of surface being heated by the sun, producing higher
temperatures and local instability. In that simple example, if you heat a
thin layer at the surface by just 3 degC, you've now got instability and the
makings of vertical convection.

That's not to say that stratiform clouds can't become unstable by the
mechanism you propose, but cu can form, particularly close to the surface,
in atmospheres that start off looking stable.

Julian Scarfe