How does sun heat the air?
sne...@aol.com
the things on the ground get hot, and then the hot things on the
ground heat the air?
Sun light passing through the air does not heat the air at all? Heats
the air some but not much?
If the air is transparent to solar energy, how is the air able to
pick up so much energy from hot objects on the ground?
Bill
6W
Jim Logajan
Try reading the content of this web page for some basic answers to all your
questions:
Eric Greenwell
It's a good article by a good source, but robably more than he wants to
know, as it's not directly concerned with thermal production.
The thermals we like so much use air heated primarily by conduction from
the aforementioned hot objects, but I believe some "micro" (my term)
convection near the ground is important for making the warm layer near
the ground thicker than conduction alone would provide.
--
Eric Greenwell - Washington State, USA
* Change "netto" to "net" to email me directly
Tim Taylor
Bill,
Back to the basics, heat is transfered in general by three methods
conduction, convection and radiation (http://www.mansfieldct.org/
schools/MMS/staff/hand/convcondrad.htm).
Radiation is not radioactive but transfer from a warm body (the sun
in
our case) to a cooler body (the earth) by waves. As you stated the
air is mostly transparent to the waves so most of the energy passes
through and hits the surface below. As you stated some of the energy
is absorbed by the atmosphere (air, water vapor, dust, smog, etc),
some is scattered (bent so it goes off at an angle and and some is
reflected back to space.
The radiative energy that reaches the earth is transfered into heat
or
water vapor generally. The warmed earth then transfers the heat to
the air above it by a combination of conduction (the act of
transferring energy from one molecule to the next by vibrational
energy and direct contact) or convection (the transfer of energy from
the flow of molecules over a warm surface).
In general thermals are generated by a combination of conduction and
convection and you have probably noticed that the best soaring days
generally have at least a slight wind to help induce more heat
transfer by causing more convection rather than by just conduction
alone. The heat transfer from the earth to the air is driven by the
driving force of the difference in temperature between the air and
the
ground. If there is not airflow the layer of air above the ground
quickly heats up and nearly matches the ground temperature so the
heat
transfer slows down. Having a little convection as Eric said helps
mix the air up and keeps cooler air near the hotter ground to allow
greater amount of heat to be transfered to the air and therefore more
energy that can by turned into thermals.
T8
Tim, Eric... think you guys need to check your sources.
Principle thermal input to lower atmosphere is long wave infrared from
the earth. Over 50% iirc, but don't have an authoritative ref handy.
Conduction in air is notoriously poor....
regards,
Evan Ludeman / T8
John Smith
As shown by devices like pullovers etc.
T8
Tim Taylor
Evan,
While the long wave does have a greater input back into the total
atmosphere and to radiation back into space I think in the boundary
layer conduction and convection are still the primary sources of heat
transfer. Since the air is primarily oxygen and nitrogen that are
both opaque to long waves as well it it mostly the other sources that
generate thermals.
From: http://www.auf.asn.au/meteorology/section1b.html#atmospheric_temperature
1.7.4 Tropospheric transport of surface heating and cooling
The means by which surface heating or cooling is transported to the
lower troposphere are:
by conduction — air molecules coming into contact with the heated (or
cooled) surface are themselves heated (or cooled) and have the same
effect on adjacent molecules; thus an air layer only a few centimetres
thick becomes less (or more) dense than the air above
by convective mixing — occurs when the heated air layer tries to rise
and the denser layer above tries to sink. Thus small turbulent eddies
build and the heated layer expands from a few centimetres to a layer
hundreds, or thousands, of feet deep depending on the intensity of
solar heating; refer to section 3.3.1. Convective mixing is more
important than mechanical mixing for heating air, and is usually
dominant during daylight hours. In hot, dry areas of Australia the
convective mixing layer can extend beyond 10 000 feet
by mechanical mixing — where wind flow creates frictional turbulence;
refer to section 3.3.2. Mechanical mixing dominates nocturnally when
surface cooling and conduction create a cooler, denser layer above the
surface — thus stopping convective mixing. If there is no wind
mechanical mixing cannot occur, refer to section 3.4.
The term (planetary) boundary layer is used to describe the lowest
layer of the atmosphere, roughly 1000 to 6000 feet thick, in which the
influence of surface friction on air motion is important. It is also
referred to as the friction layer or the mixed layer. The boundary
layer will equate with the mechanical mixing layer if the air is
stable and with the convective mixing layer if the air is unstable.
The term surface boundary layer or surface layer is applied to the
thin layer immediately adjacent to the surface, and part of the
planetary boundary layer. Within this layer the friction effects are
more or less constant throughout, rather than decreasing with height,
and the effects of daytime heating and night-time cooling are at a
maximum. The layer is roughly 50 feet deep, and varies with conditions.
T8
>
> Evan,
>
> While the long wave does have a greater input back into the total
> atmosphere and to radiation back into space I think in the boundary
> layer conduction and convection are still the primary sources of heat
> transfer. Since the air is primarily oxygen and nitrogen that are
> both opaque to long waves as well it it mostly the other sources that
> generate thermals.
Hey Tim,
I think you'll find if you dig into this that it's long wave
radiation, plus convection (thermals + wind) and evaporation/
condensation. Conduction is a bit player.
See second two answers here, though I cannot vouch for numbers
http://www.newton.dep.anl.gov/askasci/wea00/wea00082.htm -- the first
response given is in error. There must be something better on the
web, but I can't find it quickly.
What's missing here is useful info on absorption coefficients and heat
flux vs wavelength and water vapor content. But what I am given to
understand here is that most of the heat energy that drives thermals
we can use comes from long wave infrared transmitted from the heated
surface and absorbed in the first few feet / tens of feet / hundreds
of feet. I cheerfully admit I don't have the time to go get the
details to back up my case
Best,
Evan Ludeman / T8
Frank
Bill,
Solar (electromagnetic) energy in the visible light spectrum doesn't
couple very well to dry air. However, energy in the visible light
spectrum *does* couple nicely to darker objects like a dark farm field
or a parking lot, etc. The visible light energy heats the ground,
thereby converting visible light energy to much lower frequency
(longer wavelength) IR energy, and energy in the IR band *does* couple
to dry air in close contact with the hot objects. However, dry air is
a great insulator, so only the air a few centimeters/meters above the
ground gets heated directly, forming a mat of heated air underneath
colder air above. This is an unstable configuration, and thermals are
nature's mechanism for re-establishing equilibrium. This mechanism is
closely analogous to heating a pot of water on the stove. The water
immediately adjacent to the bottom of the pot gets hot, and this sets
up circulation currents to re-establish equilibrium in the rest of the
pot. If the heating is fast enough, local vaporization takes place
causing bubbles to form (i.e. the pot boils).
Just as an evolutionary side note, eyes evolved to see in the visible
spectrum *because* the air is transparent (i.e. very low coupling
coefficient) to that band of energy. If our atmosphere happened to be
only transparent to what we call infrared, then we'd all see in the
infrared, not the visible range, and thermals would be much easier to
"see" ;-).
Regards,
TA
sne...@aol.com
Still air, like in a double pane window or fiberglass insulation,
seems to block heat tranfer. Moving air, like a car radiator or a
breeze, seems to heat the air and carry away heat. Neither the long
wave radition or conduction seems to explain how air is heated. As
you appoach a hot object with your hand, you feel radiation from the
hot object, not hot air. It seems that air velocity and mixing is the
key to heating air ? Dry air seems to heat much faster than damp
air?
Bill Snead
mattm
Think of the shimmer when looking over hot ground in the summer. This
is the heating process in action. The shimmer is caused because the
first foot or two of air adjacent to the ground is much warmer than
the air
above it, and because of the temperature difference it has a different
refractive index (just like the pencil sticking out of a glass of
water looks bent).
Some amount of wind is helpful because
it will push the warmer air up against a wick of some type, which will
break the surface tension and allow the heated air to travel upwards
(e.g. cell phone tower, ridge, line of trees, even a body of water).
I think the issue with damp air is really damp ground. Water has a
much
higher heat content factor, i.e. it takes more energy to raise the
temperature
of water a degree than it does dry ground. Moist air is actually more
buoyant
because H2O molecules are lighter than N2 or O2 molecules but still
take
up the same room. Normally this is only a factor when other lifting
mechanisms are at work (because of damp ground not heating as well)
but it has a huge effect in creating storms.
-- Matt
sne...@aol.com
>
> - Show quoted text -
Maybe the solar energy primarly reacts with (is absorbed by ) liquid
water. If there is a lot of water (the ocean a big lake), the mass of
water takes the energy but does not heat the air. However, if the
ground is what we think of dry, maybe the limited water grabs the
solar energy in a way that causes the air to heat quickly. In central
Texas in August typically the earth gets dry enough to wilt the
plants. As the plants turn brown, the maximum temperature will jump
from about 95 to maybe 102 F. The thermals will increase in strength
from about 350 feet per minute to over 500 feet per minute. In our
quarry the average moisture content of the crushed rock is about 6
percent. In the dry spells the moisture content of the rock never
falls below about 3 percent. So there is still a lot of water around
even though the place looks dry.
Bill Snead
sis2...@gmail.com
2G
Tom
jasnoo...@stemhigh.org
Duster
Jim White
>Not exactly transparent. The sky appears blue because air molecules and
take
>a stab.
>
The downward energy radiated by the sun is short wave and not absorbed by
the atmosphere. The upward energy radiated by the ground is long wave which
is.
Tango Whisky
The energy radiated upward is in the 10 micron region, and the only constituet of the atmosphere which absorbs infrared is CO2. Which accounts to 400 ppm.
So, it's rather that the grounds heats the adjacent air by heat conduction, not by radiation.
WaltWX
Here's a very complete, up to date and recently published one that you can read on-line or download the pdf for free!
If you are an engineer, you'll love this book because it explains meteorology with equations and examples... no derivations.
"Practical Meteorology - An Algebra-based Survey of Atmospheric Science" (www.eosas.ubc.ca/book/Practical_Meteorology). I haven't reviewed it, but it does have a very comprehensive 35 page chapter on Boundary Layer Meteorology written by someone who writes well and knows what he's talking about.
I met Dr Stull from the University of British Columbia Vancouver last January and he suggested his book. Dr Stull published in 1986 the classic book on Boundary Layer meteorology. He tried to publish this recent text book... but gave up and made it freely available.
Walt Rogers WX
Paul B
Sounds interesting, however the site cannot be reached. Could you please check the URL and repost?
Thanks
Paul
danlj
Dan Marotta
away, nonetheless.
On 4/1/2017 11:58 PM, danlj wrote:
> https://www.eoas.ubc.ca/books/Practical_Meteorology/
--
Dan, 5J
rpol...@kcsd96.org
"....At the surface this effective flux is entirely due
to molecular conduction, and above about 5 mm altitude
the effective flux is mostly due to turbulence."
fracta...@gmail.com
Air is a near perfect insulator (it has a conduction value of near zero; 0.024 no units) so the thought heat energy is transferred this way cannot be so .
Add to that: air is assumed to (with the exception of about 1% greenhouse gases) not absorb or emit any IR radiation: this is a contradiction to Radiation theory, and quantum mechanics where all matter above absolute 0 Kelvin radiates IR.
So something is wrong, because the air does increase, and decrease in temperature, and does so quickly.
My hypothesis is O2 and N2 (99% of the dry atmosphere) do absorb and emit IR radiation: I have the quantum theory to show where they do, the experimental instrument to show it does, and the application proof to show it does.
I am currently writing up my work - it is a massive undertaking; I predict the biggest upset is science of our time: greenhouse theory will collapse. Special greenhouse atmosphere theory is pre quantum mechanics 19th Century science; it has not been updated with 20th Century knowledge - yet.
Steve Koerner
https://en.wikipedia.org/wiki/Electromagnetic_absorption_by_water
Dan Marotta
turned "climate change denier" found his wiki entries deleted almost as
quickly as he would post them. Are they, too, bearers of the banner?
Dan, 5J
lharri...@gmail.com
So sunlight can and does heat the atmosphere through direct absorption; absorbing aerosols also play a role.
Nonetheless, as is everybody's direct experience -- a lot of light gets down to the ground on a cloud-free day.
2G
Tom
Tango Whisky
Paul Ruskin
> Sigh ... there's a great deal of misinformation here. A correct answer is long and complicated, ...
Any chance of putting some numbers on it? At least approximately?
Where does the energy end up, and by which mechanism?
Paul
twofiv...@gmail.com
Dan Daly
> Agreed on the Albedo rules observation. I experienced low albedo recently but found that zinc, exercise, and weight loss brought it right back up. Changed my life.
Darryl Ramm
Eric Greenwell
on a soaring day, and you can see the thin super-adiabatic layer near the surface,
then the much thicker adiabatic lay above to the inversion level. We all know some
surface areas get hotter than others and produce thermals, while any direct
heating of the atmosphere is uniform, and unlikely to produce thermals.
--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to email me)
- "A Guide to Self-Launching Sailplane Operation"
https://sites.google.com/site/motorgliders/publications/download-the-guide-1
- "Transponders in Sailplanes - Dec 2014a" also ADS-B, PCAS, Flarm
http://soaringsafety.org/prevention/Guide-to-transponders-in-sailplanes-2014A.pdf
lharri...@gmail.com
* UV is absorbed in the stratosphere; as far as total energy is concerned there is not much variability in what gets to the ground.
* 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 perhaps 5% of total energy flux and not as variable as people might think
* H2O and CO2 (and some others) at wavelengths > 1.4 microns lead to substantial extinctions in the 1.4 -> 3 micron wavelength domains -- but our human eyes don't see that, the sun's solar output is decreasing at longer wavelengths, nor do silicon solar cells get energy from these wavelengths.
When you look at energy balance commonly more than 20% of the sun's radiation 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 of total energy) so people just don't pay too much attention to it, for purposes like soaring.
This energy ends up as heat, distributed non-uniformly through the atmospheric column. The most blatantly obvious effect is that we have a stratosphere; there are also climatically-important consequences to this in the troposphere. Most of this heat is deposited at altitudes where we don't fly.
The dominant issues that effect lower-boundary layer heating rates are pretty obvious: clouds! Yes, surface-albedo ... and then a very large factor not 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 heat is used to evaporate water.
Deserts are good for soaring because most of the captured radiation does go to sensible heat.
A "secret" most western pilots don't know -- the best soaring season in the 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 ... and a subtle point is that deciduous trees flux more water than conifers ... there are easily-observable differences in Bowen-ratio from deciduous vs conifer forests.
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 flows 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 (everywhere) is that water runs off it; the trees are almost always water-stress limited and shifted to species (conifers) that do that better. A water-stressed tree keeps its stomata closed: doesn't flux water but also cannot photosynthesize.
Plants do change the surface albedo, usually lower it. Plants look green because 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 effect on albedo is usually less important that their water flux ... if they aren'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 farmer), but also spatially across the terrain ... in places (not our western deserts) where there's enough water for plants to grow generously.
Hey Eric Greenwell? You still flying around Richland Washington? I flew with you, and towed you years ago when I worked at PNL. It's pretty arid out 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, conifers better than growing hay or corn UNLESS the farmers are complaining about a dry spell, say out of the river bottoms and anywhere with deciduous trees, worse yet willows.
Eric Greenwell
> Hey Eric Greenwell? You still flying around Richland Washington? I flew with you, and towed you years ago when I worked at PNL. It's pretty arid out there ... but nobody finds good thermals coming up off the big irrigated crop circles. In desert terrains stay away from green like the plague.
Michael Opitz
>The people who really care about this stuff are climatologists (and
issue as far
that is
>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
>
>* the big H2O band at 940 nm is very important climatologically ..
but is
>* H2O and CO2 (and some others) at wavelengths > 1.4 microns
lead to
our
>When you look at energy balance commonly more than 20% of the
sun's
fraction
for
>
>This energy ends up as heat, distributed non-uniformly through the
the
don't fly.
>=20
>The dominant issues that effect lower-boundary layer heating rates
are
factor
the ratio
the
>
>Deserts are good for soaring because most of the captured
radiation does
> to sensible heat. =20
>A "secret" most western pilots don't know -- the best soaring
season in
After
nuisance ...
conifers ...
deciduous vs
>nifer forests. =20
>More subtly there is a second "good" period in the fall when the
trees
great.
stream
and
>
>One of several reasons "the high ground" is usually better soaring
stress
cannot
>synthesize.=20
>Plants do change the surface albedo, usually lower it. Plants look
green
avoid
this
they
>
>Soil moisture can be measured (sort-of) by remote sensing in the
(ask any
>Hey Eric Greenwell? You still flying around Richland Washington? I
flew
pretty arid
irrigated
plague.
>
>In the northeast the hierarchy is plowed-fields better than conifers,
complaining
deciduous
>es, worse yet willows. =20
>
Thanks for a great post!
RO
cliff...@gmail.com
CH
Dave Nadler
Great, Thanks!
Now, about those beaver-pond thermals...
lharri...@gmail.com
> lharri...@gmail.com wrote on 12/19/2017 6:28 PM:
> > Hey Eric Greenwell? You still flying around Richland Washington? I flew with you, and towed you years ago when I worked at PNL. It's pretty arid out there ... but nobody finds good thermals coming up off the big irrigated crop circles. In desert terrains stay away from green like the plague.
>
> Yes, I am! Retired for a long time now, but still flying, but with motorgliders.
My family is still in Seattle, I like flying out on the lee side of the Cascades ... if I can persuade Annie maybe we'll even move back ... but even if I don't do that I'll come out to Ephrata one of these days...
Cheers,
Lee
lharri...@gmail.com
> 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
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!”
Bruce Hoult
> 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.
Charlie M. (UH & 002 owner/pilot)
Fall can suck since the Great Lakes are not frozen, so you have to deal with lake effect snow (talk to Karl and others.....)....
We are normally far enough away that this is not an issue unless doing long flights.
Michael Opitz
>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
>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.
>
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
Michael Opitz
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
>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.
>
BobW
> At 17:42 20 December 2017, lharri...@gmail.com 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.
>
> 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
> same old stuff....... Others who don't understand the mechanisms just get
> frustrated a lot....
>
> RO
conditions tending toward creation of continuing mental interest/engagement
for someone sufficiently savvy to've begun the process of "sussing conditions
out," and "a lot of frustration" for those "not yet there," a coupla thoughts...
1) I suspect "interesting and challenging" is true (in U.S. latitudes, anyway)
wherever mountains poke up into moving airmasses. It's sure true along
Colorado's Front Range in any event! (Really motivated readers/SSA members can
find in "Soaring" mag's archives an article elaborating on one person's [my]
Front Range soaring knowledge "awakening" centered on this very point.)
2) Mental airmass models matter - a lot!!!
Bob W.
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