"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.