Revisiting lapse rates (From: How high is that cloud?)

"C Kingsbury" wrote in message
om...
"Icebound" wrote in message
...
I have been following the thread about "How high is that cloud", and
quite a
few of the posters seems to have some misconceptions about lapse rate.

lot of interesting stuff snipped

....

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?


Now draw your attention to the yellow plot on the tephigram

http://satellite.usask.ca/mcidas/fram32.gif

This is the vertical sounding of temperature (solid line) together with
dewpoint (dashed line). The sounding starts at the surface, and goes up
until the balloon expanded too much and broke, somewhere near 35,000 metres.

[Aside: These soundings are done twice a day, nominally 0000Z and 1200Z
(typically takes something over an hour to complete). The goal is to have
stations every 250 km, but the actual density is somewhat variable,
especially in the Arctic. I estimate there are somewhere around 200 such
sites in North America...I couldn't find a definitive number quickly... most
in the USA, about 30 in Canada. The soundings for USA, Canada and the world
are accessible at

http://www-frd.fsl.noaa.gov/mab/soundings/

but it is not a very user-friendly site... and the list of stations do not
all have soundings, but they don't tell you which ones.... I keep looking
for a better public site within NOAA and/or Environment Canada to find them,
but have not yet been able to do so.... maybe somebody knows. ]




This particular sounding was Corpus Christi 1200Z morning of Nov 3rd. You
can see that the environmental lapse rate is all over the place:

Right near the surface we have an early morning inversion.
Then around 200 to 500 metres, a 3 degree drop from 10 to 7.
Then a rise back up to 12... another, thicker inversion.
Above 3000 metres a steep drop from 10 to zero.
....etc.

Certainly nothing near the 2-degree per 1000 feet that we are told is
"average".


In mid afternoon, the lower levels may have received sufficient sun heat to
raise the surface temperature to, say, 25. If we took just the surface and
the 3200 metre temperature, we would say, oh the lapse rate is 25 minus 0
equals 25 deg in 3200 metres equivalent to about 25 deg in 10.5 thousand
feet, about 2.4 deg per 1000, pretty close to 2, right? Even more than 2,
so somewhat unstable, right?

But it does not describe that very stable layer around 1500-2000 metres.
Nor does it describe the *very* unstable layer between about 2500 and 3200
metres.
Nor does it describe the extreme dryness of the layer immediately above 1000
metres, dewpoint down to minus 10.


****
For this reason, I am very loath to interpret *anything* about the lapse
rate from isolated temperature readings such as I might get from the surface
plus FD forecasts at 6000 and 9000 feet. Even with a "sounding" as I might
get from my OAT sensor... a) ...pretty much has to be a continuous plot, not
just a few points when I happened to look away from my other piloting
duties, and b) ...is a lot more interested if I had the dewpoint along with
the temperature.
****


If I could get a tephigram plots such as this for my area, even within a
couple of hundred kilometres, then we could use those dry and wet adiabatic
laws to play some "what if" games. And we can also use your real-time
temperature measurements to understand how the sounding is changing and what
that may imply:

In the morning, the small dewpoint spread suggests there may have been a
thin layer of fog or low cloud, capped by that inversion at about 200
metres. It was stable above that to at least 2000 metres, and then fairly
unstable (temperature drops rapidly from plus 10 to zero, for just a few
thousand metres to about 3300, etc...

If we showed up ready to go flying early that afternoon, and the surface
temperature/dewpoint was now 30/10....then...

....You could see right away that any parcels lifted from the surface would
reach saturation about 1800-2000 metres or so, but would be about 5 degrees
colder than the environment at that level. So it might be bumpy in the
lower 1500 metres, but would be get stable again between 1500 and 2000
metres, thus limiting any Cumulus formation. We could see that to get
significant instability the surface dewpoint would have to increase to
more than about 14 degrees, and the temperature to around 33. Is that what
they are forecasting for a high?

etc...

If we looked at a surface chart for that date, it would not surprise me if
Corpus Christi was in some kind of high pressure system. Those thick dry
layers (the big dewpoint spreads) are typical of a gently-subsiding airmass.

Did you get a pirep that the 3300 metre temperature has decreased? If so
(ie: the environment is cooling), this increases the chances of instability
because the environment will not be as much warmer as a lifted parcel... as
compared to this morning. Especially if the temperature/dewpoint *do*
increase to about 33/14.... If a saturated parcel can get past that warm
bulge at 3300 metres (parallel to the purple lines) it may top out at 8 or 9
thousand metres. Not huge thunderstorms, but pretty good Towering Cumulus.

And so on...