Just bought a new (Met 01) Stevenson Screen. The interior is black instead
of white as before. Does anyone know the scientific principle behind this -
why should a black interior be superior to the usual white? It seems a bit
of a contradiction as the black surface is shiny.

Ian Bingham
Inchmarlo, Aberdeenshire.

On Monday, June 9, 2014 12:50:49 PM UTC+1, Ian Bingham wrote:
Just bought a new (Met 01) Stevenson Screen. The interior is black instead
of white as before. Does anyone know the scientific principle behind this -
why should a black interior be superior to the usual white? It seems a bit
of a contradiction as the black surface is shiny.

Ian Bingham

Inchmarlo, Aberdeenshire.

You may be interested in these comments by Steven Burt on COL forum

"The rationale behind the black interior is indeed a bit of a puzzle, one Ian Strangeways and myself have spent several years trying to get to the bottom of, with rather ambiguous results. Tests run by the Met Office in 2000/01 showed that black-interior screens had slightly lower maxima than white interior versions in strong sunshine, and thus were presumably more effective at blocking solar radiation. Other tests have been run by other authorities, most recently one by Campbell Scientific in the UK, and the black screens do seem to offer slightly better performance than the white interior versions. (If you'd like to see copies of the reports, please drop me an e-mail and I'll send them to you; I've also covered the topic in my forthcoming book on weather observing, which will be published in July - http://www.cambridge.org/9781107662285 )

While accepting that the tests were rigorous and the reported differences are genuine, I'm not 100% convinced that the difference is due simply to the black plastic material used having different (more opaque) radiative properties in the infra-red than the equivalent white plastic, rather than their colour in the visible spectrum. A black-interior plastic screen is also a poor choice for a site where the screen is opened several times a day, as it would have been before most sites moved to remote-reading and/or logged equipment, as the temperature does rise quickly in sunshine when the door is opened, more so than a white interior model. I keep mine firmly shut during the day!

I've compared the observed max and min temperatures in the two screens carefully over the last 6 months, and find the differences small - the Metspec screen appears to be slighly more responsive, probably because it is smaller than the large wooden screen and thus has less thermal inertia. However, the differences are within calibration tolerances, so I'm happy that swapping the wooden one for the plastic one isn't going to disrupt the continuity of my records."

Graham
Penzance (still with a trad white wood screen)

"Graham Easterling" wrote in message
...
On Monday, June 9, 2014 12:50:49 PM UTC+1, Ian Bingham wrote:
Just bought a new (Met 01) Stevenson Screen. The interior is black
instead
of white as before. Does anyone know the scientific principle behind
this -
why should a black interior be superior to the usual white? It seems a
bit
of a contradiction as the black surface is shiny.

Ian Bingham

Inchmarlo, Aberdeenshire.

You may be interested in these comments by Steven Burt on COL forum

"The rationale behind the black interior is indeed a bit of a puzzle, one
Ian Strangeways and myself have spent several years trying to get to the
bottom of, with rather ambiguous results. Tests run by the Met Office in
2000/01 showed that black-interior screens had slightly lower maxima than
white interior versions in strong sunshine, and thus were presumably more
effective at blocking solar radiation. Other tests have been run by other
authorities, most recently one by Campbell Scientific in the UK, and the
black screens do seem to offer slightly better performance than the white
interior versions. (If you'd like to see copies of the reports, please drop
me an e-mail and I'll send them to you; I've also covered the topic in my
forthcoming book on weather observing, which will be published in July -
http://www.cambridge.org/9781107662285 )

While accepting that the tests were rigorous and the reported differences
are genuine, I'm not 100% convinced that the difference is due simply to the
black plastic material used having different (more opaque) radiative
properties in the infra-red than the equivalent white plastic, rather than
their colour in the visible spectrum. A black-interior plastic screen is
also a poor choice for a site where the screen is opened several times a
day, as it would have been before most sites moved to remote-reading and/or
logged equipment, as the temperature does rise quickly in sunshine when the
door is opened, more so than a white interior model. I keep mine firmly shut
during the day!

I've compared the observed max and min temperatures in the two screens
carefully over the last 6 months, and find the differences small - the
Metspec screen appears to be slighly more responsive, probably because it is
smaller than the large wooden screen and thus has less thermal inertia.
However, the differences are within calibration tolerances, so I'm happy
that swapping the wooden one for the plastic one isn't going to disrupt the
continuity of my records."

Graham
Penzance (still with a trad white wood screen)
=======================

I've had a black interior MetSpec screen since 2004 and the best bit about
it is there is no need to paint it! Just a clean now and again and even that
is easier. They aren't cheap though, I paid £450 (without stand) in 2004 - a
lot of money to house max/min thermometers and even they cost £100 each!
The screen maxes tend to be close to my £700 Davis AWS (at same height) but
the mins are lower as the AWS is more exposed to small air currents.

Will
--
http://www.lyneside.demon.co.uk/Hayt...antage_Pro.htm
Will Hand (Haytor, Devon, 1017 feet asl)
---------------------------------------------

"Ian Bingham" wrote in message
...

Just bought a new (Met 01) Stevenson Screen. The interior is black instead
of white as before. Does anyone know the scientific principle behind this -
why should a black interior be superior to the usual white? It seems a bit
of a contradiction as the black surface is shiny.

Ian Bingham
Inchmarlo, Aberdeenshire.

====================================

Thanks, Graham and Will. Most interesting.

Ian.

On 09/06/2014 12:50, Ian Bingham wrote:

Just bought a new (Met 01) Stevenson Screen. The interior is black
instead of white as before. Does anyone know the scientific principle
behind this - why should a black interior be superior to the usual
white? It seems a bit of a contradiction as the black surface is shiny.

ISTR they have determined empirically that it works better to prevent
ingress of solar heat when the unit is in the sun.

In long wave thermal band IR anything that isn't a shiny polished
metallic surface is "black" to a very good approximation.

It is possibly due to the amount of scattered light getting in but these
things are incredibly finely balanced. I'd also consider that making
the slats out of the foil backed polystyrene foil facing inwards might
also reduce the thermal radiation getting into the enclosure.

Smooth mirror finishes are poor radiation emitters which probably
explains the interior gloss finish.

Observatory domes are now painted a semimetallic grey white to prevent
supercooling at night which is a problem with pure whites. The cold air
then drips into the dome aperture and creates turbulence damaging image
quality. Modern observatories are actively cooled during daytime.

I'd hazard a guess that super cooled air also runs down the side of a
Stephenson screen at night and slightly biases the readings lower
overnight. I guess for a weather station the thermal load in sunshine is
a much more serious consideration.

--
Regards,
Martin Brown

"Martin Brown" wrote in message ...

On 09/06/2014 12:50, Ian Bingham wrote:

Just bought a new (Met 01) Stevenson Screen. The interior is black
instead of white as before. Does anyone know the scientific principle
behind this - why should a black interior be superior to the usual
white? It seems a bit of a contradiction as the black surface is shiny.

ISTR they have determined empirically that it works better to prevent
ingress of solar heat when the unit is in the sun.

In long wave thermal band IR anything that isn't a shiny polished
metallic surface is "black" to a very good approximation.

It is possibly due to the amount of scattered light getting in but these
things are incredibly finely balanced. I'd also consider that making
the slats out of the foil backed polystyrene foil facing inwards might
also reduce the thermal radiation getting into the enclosure.

Smooth mirror finishes are poor radiation emitters which probably
explains the interior gloss finish.

Observatory domes are now painted a semimetallic grey white to prevent
supercooling at night which is a problem with pure whites. The cold air
then drips into the dome aperture and creates turbulence damaging image
quality. Modern observatories are actively cooled during daytime.

I'd hazard a guess that super cooled air also runs down the side of a
Stephenson screen at night and slightly biases the readings lower
overnight. I guess for a weather station the thermal load in sunshine is
a much more serious consideration.

--
Regards,
Martin Brown

Thanks for that Martin; it's certainly food for thought. I will be
comparing my temperature and humidity readings before and after the new
screen, and with neighbouring stations to see if I can spot any slight
difference the new screen might have made.

Ian.

If the wooden screen wasn't maintained properly, painted regularly, then the wood would absorb water, thus cooling the screen by evaporation. Obviously, this doesn't happen with the plastic version. I have noticed that the shiny white on the plastic does grow dull (almost matt) with age. I'm sure any trials have been conducted with nice new screens so what happens over time has probably never been looked at.

On 10/06/2014 10:09, Desperate Dan wrote:
If the wooden screen wasn't maintained properly, painted regularly, then the wood would absorb water, thus cooling the screen by evaporation. Obviously, this doesn't happen with the plastic version. I have noticed that the shiny white on the plastic does grow dull (almost matt) with age. I'm sure any trials have been conducted with nice new screens so what happens over time has probably never been looked at.


I would not have thought the gloss vs matt finish on the white outside
would matter so much although matt is probably better there.

It is an interesting question as to what an ideal air temperature
measurement enclosure should be like. The classic Stephenson screen used
by meteorologists almost certainly introduces small systematic errors in
strong sunlight and also with a clear night sky.

And also depending on its surface wetness and windspeed at the time.

OTOH all historical data is recorded with this methodology and so any
radical change would put a discontinuity in the records.

I expect it could be important though at tropical latitudes where the
sun can get right overhead and beat down on the roof of the enclosure.

--
Regards,
Martin Brown

and"Desperate Dan" wrote in message
...

If the wooden screen wasn't maintained properly, painted regularly, then the
wood would absorb water, thus cooling the screen by evaporation. Obviously,
this doesn't happen with the plastic version. I have noticed that the shiny
white on the plastic does grow dull (almost matt) with age. I'm sure any
trials have been conducted with nice new screens so what happens over time
has probably never been looked at.

==============================

OTOH with a poorly maintained screen, the paint would dull and peel and
algae grow on it, reducing its reflective properties, and so it might tend
to OVERread! Like so many things in Meteorology, infinitely complicated.

Ian.

On 11/06/2014 10:10, Ian Bingham wrote:
and"Desperate Dan" wrote in message
...

If the wooden screen wasn't maintained properly, painted regularly, then
the wood would absorb water, thus cooling the screen by evaporation.
Obviously, this doesn't happen with the plastic version. I have noticed
that the shiny white on the plastic does grow dull (almost matt) with
age. I'm sure any trials have been conducted with nice new screens so
what happens over time has probably never been looked at.

==============================

OTOH with a poorly maintained screen, the paint would dull and peel and
algae grow on it, reducing its reflective properties, and so it might
tend to OVERread! Like so many things in Meteorology, infinitely
complicated.

Ian.

There was a conjecture that when they switched from what was just an
inorganic lime whitewash to TiO2 acrylic resin based paints.

Whilst I don't consider Wattsupwiththat a sound reference site he has at
least done some experiments on the topic (assume cherry picked data):

http://wattsupwiththat.com/2008/01/1...en-paint-test/

I am genuinely curious as to why whitewash surface appears to come down
faster from the local maximum temperature. The bare wood version isn't
all that different from the painted one and the acrylic paint tracks the
night minimum slightly better (if his data are to be believed).

--
Regards,
Martin Brown