In article ,
Dave Poole wrote:

Clivias are extremely susceptible to exposure from direct sun if they
have been kept shaded. Your plant's symptoms are completely typical
of one that has been severely burnt. ...

This is something that we have discussed before, but I still don't
understand the mechanisms, and so am asking for clarification and
cross-posting. Here is what I understand the situation to be.

The annual peak intensity of the sun in the UK is perhaps 70% of
the daily, clear sky, peak intensity in the tropics, perhaps less.
The ultraviolet levels are a LOT less, but I haven't been able to
find what they are - let's take them as 40%. And, of course, even
those figures apply to (typically) half a dozen days a year in the
UK - the average daily peak in summer is much lower.

My experience is that 'burning' damage is almost always caused by
those few days, and that the sun levels on a 'Phew! What a scorcher'
day might be 50% higher than on a typical 'hot' day in summer. This
is because our sun levels are primarily controlled by atmospheric
absorption, not sun angle. Is that your experience?

I believe that the problems caused by sun through glass are mainly
surface heating, because it is typically associated with slow air
movement, is much more serious close to the glass than a distance
away, and 1/2" air gap K glass double glazing does not seem to cause
the effect much. Can you confirm or deny this?

I don't have a clue what the primary 'burning' effect on plants is
(i.e. ultraviolet or surface heating) and what the 'tanning' effect
is. But they assuredly exist. Can you clarify those at all?


Regards,
Nick Maclaren.

In article ,
Janet Baraclough.. writes:
|
| The ozone layer (or lack of it) plays a significant role in screening
| UV. In Scotland the ozone layer is seriously depleted, May usually has
| clear skies, and even when the temp is only 60 to 70 F, skin can burn
| within minutes. It's not uncommon in that month for people here to burn
| seriously enough to require hospital admission. Scotland has the highest
| incidence of skincancer in the UK.

Yes, but you also have to remember that ultraviolet is also absorbed
by water vapour and water droplets, which I believe is the main cause
of the low levels in the UK (especially outside May, June and July).

It is possible that Scotland (specifically) could have very high
levels on freak days in May, if both the ozone layer were depleted
and the atmosphere was very dry, but I have difficulty in believing
that anyone with normal skin will burn 'within minutes', due to the
sunlight alone, if by that you mean a small number of them.

The requirement for hospital admission proves little, because people
will have 9 months of essentially zero exposure, and then the low
temperatures encourage them to overexpose. It is common all over
the UK. And similar effects could lead to the cancer statistics.

| A couple of years back we were in Darwin Australia in May, also clear
| skies, 90 degrees, sun much more direct, no burn sensation at all. I met
| an Australian doctor there who had just arrived back from Scotland, and
| remarked on the same thing. Her caucasian skin which never blistered at
| home in Darwin,was burned in Scotland.

I don't know Darwin, but I suspect that you are assigning the effect
to the wrong cause. I have burnt and blistered on wholly overcast
days in autumn - the combination of salt, wind and minimal sunlight
can cause exactly the same symptoms, though I don't know why. In
particular, anyone used solely to the UK (i.e. tanned by exposure
to UK sunlight) WILL burn when exposed to the sun pretty well
anywhere in the dryish tropics.

The point about the above claims is that they would place Scotland in
May as comparable to (say) Nairobi in terms of ultraviolet levels.
Yes, I am aware that they are also made by the politico-medical
stablishment, but their record for the abuse of statistics and just
plain deceit is unparalleled. That was why I tried hunting up some
figures. No joy. The few figures I have found have confirmed my
suspicions, but have been inconclusive.

The sunlight in the UK is not without its dangers, but I am not
convinced that they are due to its strength - in fact, I suspect
the converse!

| In the 80's when I was being treated for multiple malignant melanoma,
| I asked about UV transmission through glass. The reply was that single
| glazing,(car windows for example) and standard double glazing at that
| time, offered virtually no protection at all.

Yes. I was referring to the known effect by which glass 'magnifies'
the strength of sunlight, as it affects plants. I believe that it
a reradiation effect.


Regards,
Nick Maclaren.

In article ,
Janet Baraclough.. wrote:

a small number of what? I didn't mean "within minutes, skin will burn
badly enough for hospital admission".

Well, yes, I assumed that you didn't.

The fast May burn I'm talking about is the kind when skin begins to
sting within minutes of sun exposure. Red within a half hour. Painful
hot and tight during the night, maybe with watery blisters, peels off a
few days later. That's what the Dr below commented on. She didn't
recognise that the sting on her tropic-accustomed skin could be giving
her a warning after mere minutes of exposure in Scotland.

Hmm. I have heard enough reports of that to accept that it is NOT
just hysteria, but I have also seen enough contrary evidence to
suspect that it isn't quite what it is claimed to be.

I don't know Darwin, but I suspect that you are assigning the effect
to the wrong cause.

What do you think the cause is?

Dunno. I would have to investigate carefully. As I said, for reasons
I can't explain, a salt wind will do that. So will a pollen-laden
wind. But I can't rule out plain ultraviolet radiation, though I
doubt it.

So whaddya reckon accounts for the many observations by people who
weren't expecting their skin to suddenly start tingling and going red on
May mornings in Scotland....mass hysteria ? :-)

That is always possible - think of tarantism! What I feel should be
done is some proper mensuration and collection of such reports. It
could be that the phenomenon is due to freak ultraviolet levels, but
it could also be photosensitisation. Or other causes.

http://www.nas.nasa.gov/Services/Edu...radiation.html gives some research refs at the end.

Thanks.


Regards,
Nick Maclaren.

"Janet Baraclough.." wrote in
message ...
The message
from (Nick Maclaren) contains these words:

This is something that we have discussed before, but I still don't
understand the mechanisms, and so am asking for clarification and
cross-posting. Here is what I understand the situation to be.

The annual peak intensity of the sun in the UK is perhaps 70% of
the daily, clear sky, peak intensity in the tropics, perhaps less.
The ultraviolet levels are a LOT less, but I haven't been able to
find what they are - let's take them as 40%. (snip)

The ozone layer (or lack of it) plays a significant role in
screening
UV. In Scotland the ozone layer is seriously depleted, May usually
has
clear skies, and even when the temp is only 60 to 70 F, skin can
burn
within minutes. It's not uncommon in that month for people here to
burn
seriously enough to require hospital admission. Scotland has the
highest
incidence of skincancer in the UK.

A couple of years back we were in Darwin Australia in May, also
clear
skies, 90 degrees, sun much more direct, no burn sensation at all. I
met
an Australian doctor there who had just arrived back from Scotland,
and
remarked on the same thing. Her caucasian skin which never blistered
at
home in Darwin,was burned in Scotland.

I believe that the problems caused by sun through glass are mainly
surface heating, because it is typically associated with slow air
movement, is much more serious close to the glass than a distance
away, and 1/2" air gap K glass double glazing does not seem to
cause
the effect much. Can you confirm or deny this?

In the 80's when I was being treated for multiple malignant
melanoma,
I asked about UV transmission through glass. The reply was that
single
glazing,(car windows for example) and standard double glazing at
that
time, offered virtually no protection at all.

I don't believe that. Most glasses have quite a sharp cut-off just
beyond the visible blue end of the spectrum. If not, the near-UV
photodetectors which I used for decades would not have had to be
constructed with fused silica windows.

Franz

"Nick Maclaren" wrote in message
...

In article ,
Janet Baraclough.. writes:
|
| The ozone layer (or lack of it) plays a significant role in
screening
| UV. In Scotland the ozone layer is seriously depleted, May
usually has
| clear skies, and even when the temp is only 60 to 70 F, skin can
burn
| within minutes. It's not uncommon in that month for people here
to burn
| seriously enough to require hospital admission. Scotland has the
highest
| incidence of skincancer in the UK.

Yes, but you also have to remember that ultraviolet is also absorbed
by water vapour and water droplets, which I believe is the main
cause
of the low levels in the UK (especially outside May, June and July).

It is possible that Scotland (specifically) could have very high
levels on freak days in May, if both the ozone layer were depleted
and the atmosphere was very dry, but I have difficulty in believing
that anyone with normal skin will burn 'within minutes', due to the
sunlight alone, if by that you mean a small number of them.

The requirement for hospital admission proves little, because people
will have 9 months of essentially zero exposure, and then the low
temperatures encourage them to overexpose. It is common all over
the UK. And similar effects could lead to the cancer statistics.

| A couple of years back we were in Darwin Australia in May, also
clear
| skies, 90 degrees, sun much more direct, no burn sensation at
all. I met
| an Australian doctor there who had just arrived back from
Scotland, and
| remarked on the same thing. Her caucasian skin which never
blistered at
| home in Darwin,was burned in Scotland.

I don't know Darwin, but I suspect that you are assigning the effect
to the wrong cause. I have burnt and blistered on wholly overcast
days in autumn - the combination of salt, wind and minimal sunlight
can cause exactly the same symptoms, though I don't know why. In
particular, anyone used solely to the UK (i.e. tanned by exposure
to UK sunlight) WILL burn when exposed to the sun pretty well
anywhere in the dryish tropics.

The point about the above claims is that they would place Scotland
in
May as comparable to (say) Nairobi in terms of ultraviolet levels.
Yes, I am aware that they are also made by the politico-medical
stablishment, but their record for the abuse of statistics and just
plain deceit is unparalleled. That was why I tried hunting up some
figures. No joy. The few figures I have found have confirmed my
suspicions, but have been inconclusive.

The sunlight in the UK is not without its dangers, but I am not
convinced that they are due to its strength - in fact, I suspect
the converse!

| In the 80's when I was being treated for multiple malignant
melanoma,
| I asked about UV transmission through glass. The reply was that
single
| glazing,(car windows for example) and standard double glazing at
that
| time, offered virtually no protection at all.

Yes. I was referring to the known effect by which glass 'magnifies'
the strength of sunlight, as it affects plants. I believe that it
a reradiation effect.

What does that mean?
The intensity of the UV per unit wavelength increment is almost
negligible compared to that in the yellow-green region of the spectrum
of sunlight reaching the surface of the earth. If I understand
correctly what you mean by reradiation, namely absorption of UV and
reradiating at a longer wavelength, then the absorbed UV will be so
littlle that it will not resulet in a measurable increase in the
intensity of the botanically active frequencies.

Franz

wrote in message
...
On Tue, 13 Jul 2004 21:17:29 +0000 (UTC), "Franz Heymann"
wrote:


"Nick Maclaren" wrote in message
...

[snip]

[On reradiation]
No, what I mean is surface heating. This is the effect by which
the
surface of an object can become much hotter than either the body
of
the object or the air temperature.

Which of the objects under the glass will receive more reradiated
heat
from neighbouring bodies than others?
Are you remembering that the body you have chosen to receive
reradiated heat is itself also reradiating?
Surely as time passes, all the objects in the enclosure will try to
achieve the same temperature?

One point is that glass reflects
long (far) wavelength infrared well,

That is not true. It absorbs infrared radiation.

I thought you might says that :-)

and you didn't mention black body radiation.

Firstly, the glass is not a black body. {:-))
Secondly, reradiation is not reflection.
Thirdly, reradiation is not the only (or even the major?) process by
which the heat is retained in the gteenhouse. Convection is very
important.


[snip]

Franz


Franz

You are having an attack of double signatures again

My apologies.

Franz

In article ,
"Franz Heymann" writes:
|
| There is no such thing as "focus of the glass structure". Each light
| ray exits from the glass at the same angle as that a which it entered.
| At worst, it might be displaced sideways parallel to its original
| trajectory by a millimetre or two. The intensity distribution is then
| essentially the same as it would have been if there had been no glass.

I am talking about reradiation from inside and, perhaps even more
importantly, other glass. Consider a greenhouse built like:

---
/ x \

A plant at point 'x' is, in some sense, at the focus of the structure.
See below.

| Surely as time passes, all the objects in the enclosure will try to
| achieve the same temperature?
|
| Er, no. That is FAR too simplistic a model.
|
| Err, no.

I suggest that you take a few max./min. thermometers, calibrate
them against each other, and place them at various parts of a
greenhouse (properly shaded from direct sunlight). They won't all
show the same values.

| Remember that there
| is an external source of energy, and therefore the most elementary
| steady state calculations do not apply.
|
| I know that. I am almost right. The short term temperature of each
| body in it will depend essentially only on its albedo. The various
| plant leaves will have very nearly equal albedos.

Er, no. You have forgotten convection and evaporation. Those can
vary just as much as the albedo.

| I suggest you do that. Glass absorbs infrared quite strongly, which
| is why infrared lenses have to be made of rather unusual materials,
| many of which are in fact black as far as visible light is concerned.

I know that. It isn't the point, which is the RELATIVE transmission
of near and far infrared.

| You seem to be unaware of the fact that there is a relationxsship
| between the reflection coefficient and the absorption coefficient of
| any optical medium. A good absorber is a bad reflector, and glass is
| a very good absorber of infrared radiation.

In physics, as in life, things are rarely in black and white. While
what you say is true, it does NOT have an albedo of one for infrared
radiation.

| The greenhouse effect arises in fact because the glass absorbs
| essentially all the reradiated infrared quite close to the inner
| surface of the glass, whose temperature rises as a consequence. Most
| of this heat is returned to the enclosed volume by convection and
| reradiation.

Ah. Cross-purposes. Yes, that is so. I was referring to the
'greenhouse effect', where reflection is more important.

It is possible that the "plants burning under glass" effect is more
due to reradiation from hot glass that reflection of the reradiated
infrared from plants. I hadn't thought of that one. In particular,
it makes the 'focus of the structure' even more important.


Regards,
Nick Maclaren.

On Thu, 15 Jul 2004 05:52:23 +0000 (UTC), "Franz Heymann"
wrote:

That is not true. It absorbs infrared radiation.

I thought you might says that :-)

and you didn't mention black body radiation.

Firstly, the glass is not a black body. {:-))

{:-)) ---------the trademark of the real Franz.

That's probably why you didn't mention it then :-)

Secondly, reradiation is not reflection.

yes

Thirdly, reradiation is not the only (or even the major?) process by
which the heat is retained in the gteenhouse. Convection is very
important.

and insulation?



[snip]

Franz


Franz

You are having an attack of double signatures again

My apologies.

Franz


now about your lack of snipping ... :-)
--
Martin

wrote in message
...
On Thu, 15 Jul 2004 05:52:23 +0000 (UTC), "Franz Heymann"
wrote:

That is not true. It absorbs infrared radiation.

I thought you might says that :-)

and you didn't mention black body radiation.

Firstly, the glass is not a black body. {:-))

{:-)) ---------the trademark of the real Franz.

Yes. In addition I also don't talk as much rubbish as the impostor
using my name. {:-))
The "{" is a matter of wishful thinking, since I have practically no
hair left.

That's probably why you didn't mention it then :-)

Secondly, reradiation is not reflection.

yes

Thirdly, reradiation is not the only (or even the major?) process
by
which the heat is retained in the gteenhouse. Convection is very
important.

and insulation?

You have a point. The major reason why a greenhouse retains heat is
actually that it prevents the hotter interior from cooling by large
scale convection. All this stuff about the glass absorbing the
infrared radiation emitted by the contents of the greenhouse is small
beer compared with the simple mechanical shielding offered by the
structure. This is illustrated nicely by the fact that a greenhouse
cools off when all the ventilators are opened wide.

[snip]

Franz

"Nick Maclaren" wrote in message
...

In article ,
"Franz Heymann" writes:
|
| There is no such thing as "focus of the glass structure". Each
light
| ray exits from the glass at the same angle as that a which it
entered.
| At worst, it might be displaced sideways parallel to its original
| trajectory by a millimetre or two. The intensity distribution is
then
| essentially the same as it would have been if there had been no
glass.

I am talking about reradiation from inside and, perhaps even more
importantly, other glass. Consider a greenhouse built like:

---
/ x \

A plant at point 'x' is, in some sense, at the focus of the
structure.

No. The intensity of reradiation by the glass which is received at
any point inside the greenhouse is proportional to the solid angle
subtended by the glass at that point. The value of this does not vary
all that much from point to point, and it is very small in comparison
with the incident radiation.
Radiation from the glass plays a minor role compared to convective
heat transfer. Remember that the temperature of the glass is only a
fraction of a percent different from that of any other object in the
greenhouse.

See below.

| Surely as time passes, all the objects in the enclosure will
try to
| achieve the same temperature?
|
| Er, no. That is FAR too simplistic a model.
|
| Err, no.

I suggest that you take a few max./min. thermometers, calibrate
them against each other, and place them at various parts of a
greenhouse (properly shaded from direct sunlight). They won't all
show the same values.

Of course not, to the extent that warm air is lighter than cooler air.
I have actually tried it, and there is not much to choose from point
to point, except for a small increase in temperature with height above
the floor


| Remember that there
| is an external source of energy, and therefore the most
elementary
| steady state calculations do not apply.
|
| I know that. I am almost right. The short term temperature of
each
| body in it will depend essentially only on its albedo. The
various
| plant leaves will have very nearly equal albedos.

Er, no. You have forgotten convection and evaporation. Those can
vary just as much as the albedo.

Touche. I was considering only radiation effects. That was obviously
wrong.

| I suggest you do that. Glass absorbs infrared quite strongly,
which
| is why infrared lenses have to be made of rather unusual
materials,
| many of which are in fact black as far as visible light is
concerned.

I know that. It isn't the point, which is the RELATIVE transmission
of near and far infrared.

You have lost me there.

| You seem to be unaware of the fact that there is a relationxsship
| between the reflection coefficient and the absorption coefficient
of
| any optical medium. A good absorber is a bad reflector, and
glass is
| a very good absorber of infrared radiation.

In physics, as in life, things are rarely in black and white. While
what you say is true, it does NOT have an albedo of one for infrared
radiation.

I did not say, or imply that.

| The greenhouse effect arises in fact because the glass absorbs
| essentially all the reradiated infrared quite close to the inner
| surface of the glass, whose temperature rises as a consequence.
Most
| of this heat is returned to the enclosed volume by convection and
| reradiation.

Ah. Cross-purposes. Yes, that is so. I was referring to the
'greenhouse effect', where reflection is more important.

That is not correct. The greenhouse gases *absorb* the infrared
radiation, they reflect little of it. The heat exchange effect is
still primarily a convective mechanism. Do think of the fact that the
temperature of the greenhouse gases will be less than around 90% of
the temperature at the surface of the earth. Consider what that means
in terms of relative amounts of radiated power per unit area.
You might spare a moment to have a read at the Wikipedia entry under
"Greenhouse effect".

It is possible that the "plants burning under glass" effect is more
due to reradiation from hot glass that reflection of the reradiated
infrared from plants.

Not so much of this "hot glass" lark! The temperature of the glass is
unlikely to be more than a degree or two above the ambient temperature
in the greenhouse.

I hadn't thought of that one. In particular,
it makes the 'focus of the structure' even more important.

No. There is no "focus of the structure".

I have spent a couple of hours playing with a shielded minimax
thermometer in a greenhouse. There were no noticeable hot spots
anywhere in it. There was only a gentle increase of temperature with
distance, amounting to about 2 deg. C between floor highest point.

Franz

On Thu, 15 Jul 2004 19:47:56 +0000 (UTC), "Franz Heymann"
wrote:


wrote in message
.. .
On Thu, 15 Jul 2004 05:52:23 +0000 (UTC), "Franz Heymann"
wrote:

That is not true. It absorbs infrared radiation.

I thought you might says that :-)

and you didn't mention black body radiation.

Firstly, the glass is not a black body. {:-))

{:-)) ---------the trademark of the real Franz.

Yes. In addition I also don't talk as much rubbish as the impostor
using my name. {:-))
The "{" is a matter of wishful thinking, since I have practically no
hair left.

hair? I thought you had bumped your head on the greenhouse door frame
:-)

Incidentally one winter I measured the temperature at floor level and
ceiling level in the living room.
Ceiling 30DegC
Floor 14DegC

You have a point. The major reason why a greenhouse retains heat is
actually that it prevents the hotter interior from cooling by large
scale convection. All this stuff about the glass absorbing the
infrared radiation emitted by the contents of the greenhouse is small
beer compared with the simple mechanical shielding offered by the
structure. This is illustrated nicely by the fact that a greenhouse
cools off when all the ventilators are opened wide.

[snip]

To really appreciate this you have to see the Mickey mouse automated
heating control systems in the local green houses, where they grow
orchids 25,000 at a time.
Gas heated hot air blowers heat the green houses, at max temp all the
windows open automatically, hot air rushes out, temperature plummets,
all windows shut and gas comes back on again. Goodness knows how much
gas they are wasting, but I am not surprised that North Sea gas is
running out.
--
Martin

[ sci.bio.botany restored, in case anyone more knowledgable can
comment. ]

In article ,
"Franz Heymann" writes:
|
| Yes. I was referring to the known effect by which glass 'magnifies'
| the strength of sunlight, as it affects plants. I believe that it
| a reradiation effect.
|
| What does that mean?
| The intensity of the UV per unit wavelength increment is almost
| negligible compared to that in the yellow-green region of the spectrum
| of sunlight reaching the surface of the earth. If I understand
| correctly what you mean by reradiation, namely absorption of UV and
| reradiating at a longer wavelength, then the absorbed UV will be so
| littlle that it will not resulet in a measurable increase in the
| intensity of the botanically active frequencies.

Why did you think that I meant ultraviolet? I didn't. But, on that
topic, ultraviolet is as effective at damaging plant cells as it is
at damaging animal ones, and plants that grow in high ultraviolet
locations have developed protection mechanisms.

No, what I mean is surface heating. This is the effect by which the
surface of an object can become much hotter than either the body of
the object or the air temperature. One point is that glass reflects
long (far) wavelength infrared well, though it transmits short (near),
and that causes the greenhouse effect, but you can get it even with
materials that transmit uniformly.

What can happen is that an object under glass can receive the direct
radiation, and a proportion of the reflected radiation from ALL of
the objects under the glass (i.e. a focussing effect). This does
not have to be a precise focus to double or even triple the total
radiation it is receiving, and explains why the exact location is
an important factor.

Now, it might appear that this would raise the temperature of the
leaf as a whole, but it is not necessarily so. Transpiration will
keep the leaf cool, just as sweating does for humans, but that will
not stop the surface cells between the pores from getting very hot.
It is quite possible that a significant amount of human sunburn
(under dry conditions, when sweat evaporates rapidly) is due to this,
rather than purely to ultraviolet.

Now, I have no PROOF of the above, but it is the only explanation
that I can think of that matches the properties of the effect that
I know about, and of course the biology and physics.


Regards,
Nick Maclaren.

[ sci.bio.botany restored, in case anyone more knowledgable can
comment. ]

In article ,
"Franz Heymann" writes:
|
| Yes. I was referring to the known effect by which glass 'magnifies'
| the strength of sunlight, as it affects plants. I believe that it
| a reradiation effect.
|
| What does that mean?
| The intensity of the UV per unit wavelength increment is almost
| negligible compared to that in the yellow-green region of the spectrum
| of sunlight reaching the surface of the earth. If I understand
| correctly what you mean by reradiation, namely absorption of UV and
| reradiating at a longer wavelength, then the absorbed UV will be so
| littlle that it will not resulet in a measurable increase in the
| intensity of the botanically active frequencies.

Why did you think that I meant ultraviolet? I didn't. But, on that
topic, ultraviolet is as effective at damaging plant cells as it is
at damaging animal ones, and plants that grow in high ultraviolet
locations have developed protection mechanisms.

No, what I mean is surface heating. This is the effect by which the
surface of an object can become much hotter than either the body of
the object or the air temperature. One point is that glass reflects
long (far) wavelength infrared well, though it transmits short (near),
and that causes the greenhouse effect, but you can get it even with
materials that transmit uniformly.

What can happen is that an object under glass can receive the direct
radiation, and a proportion of the reflected radiation from ALL of
the objects under the glass (i.e. a focussing effect). This does
not have to be a precise focus to double or even triple the total
radiation it is receiving, and explains why the exact location is
an important factor.

Now, it might appear that this would raise the temperature of the
leaf as a whole, but it is not necessarily so. Transpiration will
keep the leaf cool, just as sweating does for humans, but that will
not stop the surface cells between the pores from getting very hot.
It is quite possible that a significant amount of human sunburn
(under dry conditions, when sweat evaporates rapidly) is due to this,
rather than purely to ultraviolet.

Now, I have no PROOF of the above, but it is the only explanation
that I can think of that matches the properties of the effect that
I know about, and of course the biology and physics.


Regards,
Nick Maclaren.

"Nick Maclaren" wrote in message
...
In article ,
Franz Heymann wrote:

[On reradiation]
No, what I mean is surface heating. This is the effect by which
the
surface of an object can become much hotter than either the body
of
the object or the air temperature.

Which of the objects under the glass will receive more reradiated
heat
from neighbouring bodies than others?

Those that are an approximation to a focus of the glass structure.

There is no such thing as "focus of the glass structure". Each light
ray exits from the glass at the same angle as that a which it entered.
At worst, it might be displaced sideways parallel to its original
trajectory by a millimetre or two. The intensity distribution is then
essentially the same as it would have been if there had been no glass.

Are you remembering that the body you have chosen to receive
reradiated heat is itself also reradiating?

Of course.

Surely as time passes, all the objects in the enclosure will try to
achieve the same temperature?

Er, no. That is FAR too simplistic a model.

Err, no.

Remember that there
is an external source of energy, and therefore the most elementary
steady state calculations do not apply.

I know that. I am almost right. The short term temperature of each
body in it will depend essentially only on its albedo. The various
plant leaves will have very nearly equal albedos.

One point is that glass reflects
long (far) wavelength infrared well,

That is not true. It absorbs infrared radiation.

Please go and look it up.

I suggest you do that. Glass absorbs infrared quite strongly, which
is why infrared lenses have to be made of rather unusual materials,
many of which are in fact black as far as visible light is concerned.

The greenhouse effect is precisely that
the short wavelength infrared emitted by the sun is transmitted,
but the long infrared emitted by the earth is reflected.

No. You misunderstand the greenhouse effect quite seriously.

Yes,
they are both absorbed, too, but everything is relative.

You seem to be unaware of the fact that there is a relationxsship
between the reflection coefficient and the absorption coefficient of
any optical medium. A good absorber is a bad reflector, and glass is
a very good absorber of infrared radiation.

The greenhouse effect arises in fact because the glass absorbs
essentially all the reradiated infrared quite close to the inner
surface of the glass, whose temperature rises as a consequence. Most
of this heat is returned to the enclosed volume by convection and
reradiation.

Franz

"Nick Maclaren" wrote in message
...

In article ,
"Franz Heymann" writes:
|
| There is no such thing as "focus of the glass structure". Each
light
| ray exits from the glass at the same angle as that a which it
entered.
| At worst, it might be displaced sideways parallel to its original
| trajectory by a millimetre or two. The intensity distribution is
then
| essentially the same as it would have been if there had been no
glass.

I am talking about reradiation from inside and, perhaps even more
importantly, other glass. Consider a greenhouse built like:

---
/ x \

A plant at point 'x' is, in some sense, at the focus of the
structure.

No. The intensity of reradiation by the glass which is received at
any point inside the greenhouse is proportional to the solid angle
subtended by the glass at that point. The value of this does not vary
all that much from point to point, and it is very small in comparison
with the incident radiation.
Radiation from the glass plays a minor role compared to convective
heat transfer. Remember that the temperature of the glass is only a
fraction of a percent different from that of any other object in the
greenhouse.

See below.

| Surely as time passes, all the objects in the enclosure will
try to
| achieve the same temperature?
|
| Er, no. That is FAR too simplistic a model.
|
| Err, no.

I suggest that you take a few max./min. thermometers, calibrate
them against each other, and place them at various parts of a
greenhouse (properly shaded from direct sunlight). They won't all
show the same values.

Of course not, to the extent that warm air is lighter than cooler air.
I have actually tried it, and there is not much to choose from point
to point, except for a small increase in temperature with height above
the floor


| Remember that there
| is an external source of energy, and therefore the most
elementary
| steady state calculations do not apply.
|
| I know that. I am almost right. The short term temperature of
each
| body in it will depend essentially only on its albedo. The
various
| plant leaves will have very nearly equal albedos.

Er, no. You have forgotten convection and evaporation. Those can
vary just as much as the albedo.

Touche. I was considering only radiation effects. That was obviously
wrong.

| I suggest you do that. Glass absorbs infrared quite strongly,
which
| is why infrared lenses have to be made of rather unusual
materials,
| many of which are in fact black as far as visible light is
concerned.

I know that. It isn't the point, which is the RELATIVE transmission
of near and far infrared.

You have lost me there.

| You seem to be unaware of the fact that there is a relationxsship
| between the reflection coefficient and the absorption coefficient
of
| any optical medium. A good absorber is a bad reflector, and
glass is
| a very good absorber of infrared radiation.

In physics, as in life, things are rarely in black and white. While
what you say is true, it does NOT have an albedo of one for infrared
radiation.

I did not say, or imply that.

| The greenhouse effect arises in fact because the glass absorbs
| essentially all the reradiated infrared quite close to the inner
| surface of the glass, whose temperature rises as a consequence.
Most
| of this heat is returned to the enclosed volume by convection and
| reradiation.

Ah. Cross-purposes. Yes, that is so. I was referring to the
'greenhouse effect', where reflection is more important.

That is not correct. The greenhouse gases *absorb* the infrared
radiation, they reflect little of it. The heat exchange effect is
still primarily a convective mechanism. Do think of the fact that the
temperature of the greenhouse gases will be less than around 90% of
the temperature at the surface of the earth. Consider what that means
in terms of relative amounts of radiated power per unit area.
You might spare a moment to have a read at the Wikipedia entry under
"Greenhouse effect".

It is possible that the "plants burning under glass" effect is more
due to reradiation from hot glass that reflection of the reradiated
infrared from plants.

Not so much of this "hot glass" lark! The temperature of the glass is
unlikely to be more than a degree or two above the ambient temperature
in the greenhouse.

I hadn't thought of that one. In particular,
it makes the 'focus of the structure' even more important.

No. There is no "focus of the structure".

I have spent a couple of hours playing with a shielded minimax
thermometer in a greenhouse. There were no noticeable hot spots
anywhere in it. There was only a gentle increase of temperature with
distance, amounting to about 2 deg. C between floor highest point.

Franz