"C Kingsbury" wrote in message
om...
"Icebound" wrote in message
...

3. I've tried to learn weather interpretation beyond simple
METAR-reading. Since you seem to know something about this, what would
you say are the points about lapse rate that we as GA pilots might
want to actually look for in terms of flight planning?



Before I answer that....

In this post, let me show you a tool that might help explain adiabatic lapse
rates a little more. In the next one, I will try to deal with some
specifics.

---

Note this diagram:

http://satellite.usask.ca/mcidas/fram32.gif (I hope they don't change it
before you get there :-)

(This is one of those inventions for meteorology that has a similar sublime
genius as does the E6-B computer for aviation)

Forget all the yellow stuff for the time being... that is specific
information for the particular station, in this case Corpus Christi. Ignore
all that, and draw your attention to the lines and number in the background.

This diagram is known as a tephigram, and it depicts all of the adiabatic
"laws" that we have been talking about. Print a copy off... (better still
would be to find a big original somewhere)... and you don't have to be
guessing as to how a lifted parcel is going to behave.



1. Height in metres goes up the left side.

This has already been converted from the original observed pressure (reduced
pressure upwards equals increasing height, of course). Unfortunately, the
tick marks are not any particular "scale". These are just the levels where
the temperature or dewpoint trend made a significant deviation from its
previous trend, in this particular sounding. The good news is that they
chose the pressure scale very carefully, so that when converted to height,
the height scale turns out to be very nearly linear, and you can
interpolate.


2. The temperature lines are slanted to the right, in grey.

The highest values are at the bottom right, and the coldest at the top left.
Note the value number along the right side and the top. (The lines are
slanted because it keeps real-world plots more vertically on the page.)

3. The green lines represent the behaviour of the dewpoint.

If I start a parcel at some level, and lift it, the dewpoint value will
decrease parallel to the green lines. For example, if a parcel of air, with
a dewpoint of 20 deg C, is somehow lifted to the 3000 metre level, the
dewpoint will decrease to about 15, (providing no condensation takes
place). If the parcel started with a dewpoint of 0 it will decrease to
about -5. ... just about the 0.5 per thousand feet that has been discussed.

The values on the green lines tells you the amount of water, in grams per
kilogram (g/kg) of dry air, for that particular dewpoint and pressure. A
dewpoint of 20 at 1500 metres represents about 17 or 18 g/kg. If that
starts to condense, its a lot of water. But a dewpoint of 0 at 1500 metres
represents only about one-quarter as much water, about 4.5 g/kg, and
at -30, just over one-quarter of a g/kg.

4. The slightly-curved grey lines slanted *backward* represent the rate of
cooling of unsaturated air...

....should it be lifted. This is the *dry adiabatic lapse rate*.

You find the height (pressure) and *temperature* of an air-parcel. Then
*if* it were somehow lifted, its temperature would drop at a rate parallel
to those grey lines.

For example, a parcel at 30 deg at 0 ASL, lifted to 3000 metres, would drop
from 30 to about -5, providing that it was dry enough so that it never
became saturated and no condensation took place.

5. The purple lines represent rate of cooling of a saturated air parcel...

....one where condensation would have to occur as the parcel rises. This is
the *wet adiabatic lapse rate*.

Note that these lines curve, because the rate of cooling changes (for a
saturated parcel) depending on the temperature...much slower at high
temperatures, and very quickly (almost as quickly as the dry rate) in the
cold minus-30 temperatures.


Now a typical scenario:

Using our diagram with an air-parcel whose Temperature is 30, dewpoint 20,
at 0 ASL.

*If* such a parcel were to be lifted, it would cool from 30 to about 18
around 11-1200 metres. In that same 1200 metres, the dewpoint would reduce
from 20 also to 18 ...at the same point.. Now the parcel would be saturated
and any further lifting would cool parallel to the purple lines...say it
lifted to 3000 metres, it would cool only about another 9 degrees to about
plus-9 deg C. But now note the green lines. Our parcel would have started
out with about 15 g/kg of moisture (20 deg dewpoint at 0 ASL), but now would
have only about 10 g/kg. The other 5 would have condensed into cloud.


At this point we have said nothing about the actual, environmental
conditions, the *environmental lapse rate". The adiabatic lapse rates, as
shown by this diagram, are "what if" tools, used to see what *could* happen
if a parcel got lifted independently of its environment.