The message
from (Nick Maclaren) contains these words:


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.


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.

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

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.

| 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.

What do you think the cause is?

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.

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 ? :-)

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

Janet.

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

The message
from (Nick Maclaren) contains these words:

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!

Thinking of tarantism is enough to make me join the Wee Frees.

Janet

The message
from "Franz Heymann" contains these words:

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.

I've been burned through car windows, and I see many parents
protecting their children from that risk with stick-on screens.

Janet.

Getting back on topic....

Here is what I understand the situation to be.

The annual peak intensity of the sun in the UK is perhaps 70% of
Snip
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?

No, my experience is that the leaves will burn, regardless of whether
it is a 'scorcher'. I find that it tends to coincide with direct
overhead sun and is far worse on plants that have not been very
gradually 'hardened' to increased light levels. Clivias can cope with
full sun in the U.K. but they need very gradual hardening and placing
them suddenly out of doors as indicated by the O.P. is a recipe for
the bleached out look. I hasten to add, that here in the far south at
least, non-burnt, fully exposed plants do not grow as well as when
given some shade and typically have short, broad leaves with a slight
yellow caste. I've experimented with 3 year old seedlings many times
and there's no doubt that growth rates at full exposure are as much a
50% lower than those given light to medium (20 - 40%) shade.

My own mature plants get some full sun later in the day - ironically
when heat levels are at their highest which is usually from
mid-afternoon onwards. There is a difference between exposure to sun
at a high angle (noon) and at a much lower angle (late afternoon).
Quite a few shade loving plants can cope with the latter, but not the
former.

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?

Well having grown them in very large glasshouses where they were never
closer than 3 feet and often as much as 10 feet away from the glass
and still got burnt, I do not think proximity to glass is the complete
answer.

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?

Well, it is a bit of a poser, I'll agree. If surface heating was a
contributory factor, there would be little or no damage at low
temperatures. Unfortunately burning can occur when air temperatures
are only 15 or 16C. The burning appears to be a bleaching of plant
cells with cell contents being oxidized. I think suddenly increased
UV exposure may be part of the problem and if I were to 'hang my hat'
on a reason, I suspect it would be that.

Dave Poole
Torquay, Coastal South Devon UK
Winter min -2°C. Summer max 34°C.
Growing season: March - November

[ 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.

In article ,
Dave Poole writes:
|
| No, my experience is that the leaves will burn, regardless of whether
| it is a 'scorcher'. I find that it tends to coincide with direct
| overhead sun and is far worse on plants that have not been very
| gradually 'hardened' to increased light levels. Clivias can cope with
| full sun in the U.K. but they need very gradual hardening and placing
| them suddenly out of doors as indicated by the O.P. is a recipe for
| the bleached out look. I hasten to add, that here in the far south at
| least, non-burnt, fully exposed plants do not grow as well as when
| given some shade and typically have short, broad leaves with a slight
| yellow caste. I've experimented with 3 year old seedlings many times
| and there's no doubt that growth rates at full exposure are as much a
| 50% lower than those given light to medium (20 - 40%) shade.

Thanks again. That could well account for my C. nobilis - while
the leaves are very dark green, the other symptoms are similar.

| My own mature plants get some full sun later in the day - ironically
| when heat levels are at their highest which is usually from
| mid-afternoon onwards. There is a difference between exposure to sun
| at a high angle (noon) and at a much lower angle (late afternoon).
| Quite a few shade loving plants can cope with the latter, but not the
| former.

I am surprised that your maximum heat is so late, but that effect
does not surprise me. The light path through the atmosphere gets
rapidly longer after mid-afternoon. That is the main reason that
the ultraviolet levels are dangerously low in the UK winter.

| 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?
|
| Well having grown them in very large glasshouses where they were never
| closer than 3 feet and often as much as 10 feet away from the glass
| and still got burnt, I do not think proximity to glass is the complete
| answer.

Oh, no, but I have certainly seen it be heavily dependent on the
distance - such as 6" away burning badly and 2' away not burning.
My other posting explains what I think that is happening, and it
could equally occur in large buildings (after all, the area of the
'mirror' goes up as the distances do).

| 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?
|
| Well, it is a bit of a poser, I'll agree. If surface heating was a
| contributory factor, there would be little or no damage at low
| temperatures. Unfortunately burning can occur when air temperatures
| are only 15 or 16C. The burning appears to be a bleaching of plant
| cells with cell contents being oxidized. I think suddenly increased
| UV exposure may be part of the problem and if I were to 'hang my hat'
| on a reason, I suspect it would be that.

As I explained there, the first conclusion doesn't necessarily
follow. You could be right that it is ultraviolet rather than
pure heating in the 'glass' effect, but I hold another view.
What is certain is that both ultraviolet and surface heating
cause cell damage, in plants just as much as in humans.


Regards,
Nick Maclaren.

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.

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

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


Regards,
Nick Maclaren.

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.


[snip]

Franz


Franz

You are having an attack of double signatures again

--
Martin

"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