Atmospheric stability and lapse rate

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?

Yes are often a combination of clouds at various levels


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?

The 2C/1000 was arbitrarily chosen as the "standard" for things like
calibrating altimeters. As you note, it has nothing to do with the real
world.

Mike
MU-2

Andrew Sarangan wrote:

Instability produces cumulus clouds and stability produces stratus
clouds.

Better to say "can result in" rather than "produces". That's because in
addition to atmospheric lapse rate characteristics, moisture content of
the air also determines when and where clouds will form. Many stable
situations actually act to dissipate 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?

Almost. The formation of a cumulus cloud is already an indication that
some localized instability has occurred. After the cloud has formed, it
is more likelly to remain unstable because the air within it is
saturated, and will cool at a slower rate as it rises than will
unsaturated air. This is due to the latent heat released by additional
droplet formation as the cloud grows.

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?

It really is just what you said at the start of your paragraph: an
average lapse rate. Observed lapse rates will vary all over the map,
sometimes greater, sometimes less than the average. Sometimes lapse
rates can go negative, which results in an inversion (warm air over
cold). What they won't do is exceed the dry adiabatic rate (the
3C/1000' you mentioned). This is because instability will result, and
convective overturning will restore the lapse rate to dry adiabatic.

An excellent introductory text explaining these issues is "Atmospheric
Science, an Introductory Survey", by Wallace and Hobbs. It's written
for a 2nd semester atmospheric science student, but is very readable.

Jim Rosinski

There was an inversion over Michigan the past few days... On both
Friday afternoon and Saturday, the temperature at 3500 feet was 5
degrees warmer than on the ramp... I didn't remember to note the
temperature change on Sunday, but the conditions were the same...

Denny

On 6 Feb 2005 21:19:53 -0600, Andrew Sarangan
wrote:

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?


Regarding the "lapse rate", I've always been curious as to how knowing
about such a thing is useful while flying. I recall reading about it
while studying for the written and wondering how on earth I'd make use
of such information **IF** I could memorize it.

Do you fly around getting temperature readings at various locations
and start working the formula or do you just keep it in the back of
your head as something interesting (or not) and stear clear of the
clouds?

Thanks, Corky Scott

It is not really useful for actually flying, but it is useful for
understanding the fundemental forces that drive weather.

Mike
MU-2


"Corky Scott" wrote in message
...
On 6 Feb 2005 21:19:53 -0600, Andrew Sarangan
wrote:

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?


Regarding the "lapse rate", I've always been curious as to how knowing
about such a thing is useful while flying. I recall reading about it
while studying for the written and wondering how on earth I'd make use
of such information **IF** I could memorize it.

Do you fly around getting temperature readings at various locations
and start working the formula or do you just keep it in the back of
your head as something interesting (or not) and stear clear of the
clouds?

Thanks, Corky Scott

Andrew Sarangan wrote:

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?

Just realized that my last response didn't fully answer your question.
Lapse rate and moisture content are only loosely related--they are not
inextricably linked. It is possible to have moist air (at or near 100%
relative humidity) in one location with a steeper lapse rate than dry
air at another location. An inversion in dry conditions is just such a
situation.

One thing that CAN be said about the relation between lapse rate and
moisture content: unsaturated air can accomodate a steeper lapse rate
(up to the dry adiabatic rate of around 3C/1000') than can saturated
air. Saturated lapse rates can only reach saturated adiabatic values,
which are less than 3C/1000'. Actual saturated adiabatic lapse rates
are nonlinear functions of temperature, with lower lapse rates for
higher temperatures. This is because warm air can hold more moisture
than cold air.

Hope this helps.

Jim Rosinski

"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?

Yes.

The most visible example of this situation is a relatively cloud-free day,
followed by severe thunderstorms along a passing cold front.

The warm air ahead of the front can be relatively "stable" and few or no
cumulus-type clouds form. Then when the cold front comes along and forces
the warm air upward, cooling it to saturation, suddenly the huge
cumulonimbus clouds erupt.... just because once saturated, the air is now
unstable.


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?



That is because the concept of "lapse rate" was taught to you badly.

There are the 4 different "lapse rates".

One answers the question: "What IS the temperature difference with height
in THIS airmass, as I travel up and down within it".

This is what you measure with your OAT as you travel. It is more correctly
known as the "environmental lapse rate"... in other words... the real lapse
rate in the real atmosphere as of right now.

Next one answers the question: What is the "average" condition of the
world's atmospheres throughout time, throughout the entire globe? Okay, not
exactly.... but if we could produce the specifications of an average like
that, then aircraft manufacturers can relate their performance specification
to "if you operate under these meteorological conditions". This became the
"ICAO Standard atmosphere", with the "standard" lapse rate of 1.98 degrees C
per 1000 feet (in the troposphere). It is a purely artificial "average"
that allows comparison against the real atmosphere, above.... if my aircraft
is supposed to perform like such in the "standard", then it will perform
like "so" in today's real life.

The third and forth lapse rates are of a totally different "type". Above,
we talked about "difference in temperature with height"... how one level is
different from another.

Now we talk about a "rate of cooling". This requires some background:
1. If we do not add or remove heat, then the temperature of air will lower
as the pressure lowers on that air.
2. Pressure lowers as we go up, as we all know, so we can translate
pressure decrease to altitude increase.
3. Condensation causes a release of heat... the reverse of evaporation
which requires an input of heat.

Therefore, if we raise a parcel of air in elevation (reduce its pressure),
and no condensation occurs, then the air will cool at some rate. If we
raise a parcel of air, and condensation DOES occur, then the released heat
will warm the air up a little and it will not cool as quickly.

Okay, back to the lapse rates:
The "dry adiabatic lapse rate" is not a difference in temperature with
height as are the "environmental" and "ICAO standard" lapse rates. It is,
instead, a rate of COOLING... the THEORETICAL change of temperature in a
parcel of air, IF that parcel were to rise. "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.

Similarly the "moist (or saturated) adiabatic lapse rate" is the THEORETICAL
change in temperature in a parcel of air, if it were to rise WHILE
CONDENSATION WAS OCCURRING (and hence some heating of the air was
occurring). This number is NOT linear, because high-dewpoint air means way
more moisture condensing and way more heat being release.... so the cooling
may be only about 1 degree per 1000 feet with 30 deg C dewpoints, but nearly
3 degrees per 1000 at -30, because at minus 30 the amount of moisture in the
air is miniscule.


Remember, actual or "standard" change in temperature with height, versus a
known theoretical "rate of cooling" of a rising parcel. Two very different
things.

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

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?