How does sun heat the air?

At 02:28 20 December 2017, wrote:
The people who really care about this stuff are climatologists (and
climate=
modelers), and increasingly the solar power community. The
issue as far
a=
s the soaring community is concerned is that most of the sunlight
that is
l=
ost to absorption in the atmosphere is mostly-always lost=20

* UV is absorbed in the stratosphere; as far as total energy is
concerned
=
there is not much variability in what gets to the ground. =20

* Chappuis-band O3 absorption (in the visible red) can change the
surface
=
heat flux a few percent, no more

* the big H2O band at 940 nm is very important climatologically ..
but is
p=
erhaps 5% of total energy flux and not as variable as people might
think=20

* H2O and CO2 (and some others) at wavelengths 1.4 microns
lead to
substa=
ntial extinctions in the 1.4 - 3 micron wavelength domains -- but
our
huma=
n eyes don't see that, the sun's solar output is decreasing at longer
wavel=
engths, nor do silicon solar cells get energy from these
wavelengths. =20

When you look at energy balance commonly more than 20% of the
sun's
radiati=
on is absorbed in the atmosphere, and this is very important to the
thermal=
structure of the atmosphere, but it isn't highly variable (as a
fraction
o=
f total energy) so people just don't pay too much attention to it,
for
purp=
oses like soaring.

This energy ends up as heat, distributed non-uniformly through the
atmosphe=
ric column. The most blatantly obvious effect is that we have a
stratosphe=
re; there are also climatically-important consequences to this in
the
trop=
osphere. Most of this heat is deposited at altitudes where we
don't fly.
=
=20

The dominant issues that effect lower-boundary layer heating rates
are
pret=
ty obvious: clouds! Yes, surface-albedo ... and then a very large
factor
n=
ot discussed here is what meteorologists call the "Bowen ratio:"
the ratio
=
of the latent-to-sensible heat flux from the surface ... how much of
the
he=
at is used to evaporate water.

Deserts are good for soaring because most of the captured
radiation does
go=
to sensible heat. =20

A "secret" most western pilots don't know -- the best soaring
season in
th=
e northeast is spring, before the trees leaf out. It's our desert.
After
=
they leaf out ... then every damned tree is a water-sucking
nuisance ...
an=
d a subtle point is that deciduous trees flux more water than
conifers ...
=
there are easily-observable differences in Bowen-ratio from
deciduous vs
co=
nifer forests. =20

More subtly there is a second "good" period in the fall when the
trees
lose=
their leaves, although with the declining sunlight it's not really
great.
=
But since the time of Benjamin Franklin naturalists noted that
stream
flow=
s in the northeast jump after the trees lose their leaves in the fall,
and
=
correctly attributed the reason for this.

One of several reasons "the high ground" is usually better soaring
(everywh=
ere) is that water runs off it; the trees are almost always water-
stress
li=
mited and shifted to species (conifers) that do that better. A
water-stres=
sed tree keeps its stomata closed: doesn't flux water but also
cannot
photo=
synthesize.=20

Plants do change the surface albedo, usually lower it. Plants look
green
b=
ecause they don't use green light, and they want to reflect it to
avoid
its=
heat. They also increase the albedo at longer wavelengths. But
this
eff=
ect on albedo is usually less important that their water flux ... if
they
a=
ren't water stressed.

Soil moisture can be measured (sort-of) by remote sensing in the
microwave,=
and there are very large variations in soil moisture temporally
(ask any
f=
armer), but also spatially across the terrain ... in places (not our
wester=
n deserts) where there's enough water for plants to grow
generously. =20

Hey Eric Greenwell? You still flying around Richland Washington? I
flew
w=
ith you, and towed you years ago when I worked at PNL. It's
pretty arid
ou=
t there ... but nobody finds good thermals coming up off the big
irrigated
=
crop circles. In desert terrains stay away from green like the
plague.

In the northeast the hierarchy is plowed-fields better than conifers,
conif=
ers better than growing hay or corn UNLESS the farmers are
complaining
abou=
t a dry spell, say out of the river bottoms and anywhere with
deciduous
tre=
es, worse yet willows. =20


Thanks for a great post!

RO

Yes thanks for a great post! I find it interesting that the "dry" river beds, and the "wet" cattle tanks out west seem to produce great thermals. I have always attributed that to a "low" area allowing the heat bubble to pool to a larger size before being ripped off due to the wind. The late 1V, Carl Herold, always commented that you cant get good thermal days without a good 10-15 mph of wind to keep them releasing.

CH

On Wednesday, December 20, 2017 at 9:42:19 AM UTC-5, wrote:
Yes thanks for a great post! I find it interesting that the "dry" river beds, and the "wet" cattle tanks out west seem to produce great thermals. I have always attributed that to a "low" area allowing the heat bubble to pool to a larger size before being ripped off due to the wind. The late 1V, Carl Herold, always commented that you cant get good thermal days without a good 10-15 mph of wind to keep them releasing.

CH

Wet places that produce thermals are the bane of meteorologist/sailplane pilots. They can drive you nuts. When I flew out west I felt I really understood the soaring boundary layer ... there was rarely anything going on that I couldn't confidently explain.

Starting to fly in the Northeast, flying from Saratoga (5B2) there's all kinds of WTF! stuff. We have some "persistent mysteries" about house thermals within 2 miles of the field.

One REALLY important thing to remember though is that all the really good soaring days happen after a cold front booms through. This is particularly true in the non-desert parts of the country -- basically the only time you can get even "decent" conditions for long flights.

After a cold front a lot of the heat isn't solar (particularly around us) .... it's stored heat in the surface. A good cold front (at jet-stream latitudes) can easily give you -30 °F drop in surface air temperatures (and a nicely near-adiabatic lapse rate) ... and the stored heat available from that can give you a lot of lift.

We have uncommon fall days in the NE that no western pilot "gets" -- days when the skies are gelid overcast at 10,000 ft, a bitter cold wind at the surface, and shockingly good lift ... 8 - 10 kts (that's really good lift out here). All of that is being driven by the extracted surface heat stored up through the previous warm-sector passage.

And in those conditions ... shallow water bodies are often great. Water has high heat capacity AND a little wind-stress can stir it, so the heat can be "mined" out of the top meter or so much more quickly than this heat could conduct up (soil has poor thermal conductivity).

What may be counterintuitive until you think about it is that this works best when the temperature is cold, near freezing. Under those conditions the Bowen ratio will be favorable -- little water can evaporate -- sensible heat flux is maximized.

Flying downwind of the Adirondacks also produces bafflingly complex "wave" phenomena and a great many cases where thermal & wave systems coexist, and also conditions where one can climb in clear air up alongside convective clouds. A lot of this is very hard to explain classically.

My take on some of this is that it is not wave really, instead it is convergence due to the Mohawk/Hudson drainage convergence ... but without a lot of data I can't get, hard to know.

But as Dr. Suess said: “From there to here, from here to there, funny things are everywhere!�

On Wednesday, December 20, 2017 at 8:42:22 PM UTC+3, wrote:
One REALLY important thing to remember though is that all the really good soaring days happen after a cold front booms through. This is particularly true in the non-desert parts of the country -- basically the only time you can get even "decent" conditions for long flights.

Yes. Or soaring the front of the front as it arrives! There is awesome lift in front of that wall of cloud. Sometimes I've done that for a couple of hours with a front that is moving at only 5 or 10 km/h. When it gets within 5 km or so of the airfield all the gliders dash back, hangar land, get everything quickly inside and close the doors. There's nothing like just getting a cup of coffee made in the clubhouse as it starts getting pounded with wind and rain five minutes after you locked up the hangar.

At 17:42 20 December 2017, wrote:

Flying downwind of the Adirondacks also produces bafflingly
complex "wave" phenomena and a great many cases where thermal
& wave systems coexist, and also conditions where one can climb in
clear air up alongside convective clouds. A lot of this is very hard
to explain classically. =20

My take on some of this is that it is not wave really, instead it is
convergence due to the Mohawk/Hudson drainage convergence ...
but without a lot of data I can't get, hard to know.


You should come down and visit us in Freehold, NY to experience
the Hudson/Catskill convergence, plus flying in the nothern
Catskills. I find it interesting and challenging, so that I'm not
often bored with the same old stuff....... Others who don't
understand the mechanisms just get frustrated a lot....

RO

At 17:42 20 December 2017, wrote:

Flying downwind of the Adirondacks also produces bafflingly
complex "wave" phenomena and a great many cases where thermal
& wave systems coexist, and also conditions where one can climb in
clear air up alongside convective clouds. A lot of this is very hard
to explain classically. =20

My take on some of this is that it is not wave really, instead it is
convergence due to the Mohawk/Hudson drainage convergence ...
but without a lot of data I can't get, hard to know.


You should come down and visit us in Freehold, NY to experience
the Hudson/Catskill convergence, plus flying in the nothern
Catskills. I find it interesting and challenging, so that I'm not
often bored with the same old stuff....... Others who don't
understand the mechanisms just get frustrated a lot....

RO

On Tuesday, December 19, 2017 at 9:28:55 PM UTC-5, wrote:

Great, Thanks!
Now, about those beaver-pond thermals...